Digestive System Anatomy and Physiology PDF

Digestive System Anatomy and Physiology Functions of the Digestive System The functions of the digestive system are: 1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion. 2. Propulsion. If foo...

Digestive System Anatomy and Physiology Functions of the Digestive System The functions of the digestive system are: 1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion. 2. Propulsion. If foods are to be processed by more than one digestive organ, they must be propelled from one organ to the next; swallowing is one example of food movement that depends largely on the propulsive process called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall). 3. Food breakdown: mechanical digestion. Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and examples of mechanical digestion are the mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine. 4. Food breakdown: chemical digestion. The sequence of steps in which large food molecules are broken down into their building blocks by enzymes is called chemical digestion. 5. Absorption. Transport of digested end products from the lumen of the GI tract to the blood or lymph is absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or passive transport processes. 6. Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces Anatomy of the Digestive System The organs of the digestive system can be separated into two main groups: those forming the alimentary canal and the accessory digestive organs Organs of the Alimentary Canal The alimentary canal, also called the gastrointestinal tract, is a continuous, hollow muscular tube that winds through the ventral body cavity and is open at both ends. Its organs include the following: Mouth Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity. Lips. The lips (labia) protect its anterior opening. Cheeks. The cheeks form its lateral walls. Palate. The hard palate forms its anterior roof, and the soft palate forms its posterior roof. Uvula. The uvula is a fleshy finger-like projection of the soft palate, which extends inferiorly from the posterior edge of the soft palate. Vestibule. The space between the lips and the cheeks externally and the teeth and gums internally is the vestibule.  Oral cavity proper. The area contained by the teeth is the oral cavity proper. Tongue. The muscular tongue occupies the floor of the mouth and has several bony attachments- two of these are to the hyoid bone and the styloid processes of the skull. Lingual frenulum. The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements. Palatine tonsils. At the posterior end of the oral cavity are paired masses of lymphatic tissue, the palatine tonsils. Lingual tonsil. The lingual tonsils cover the base of the tongue just beyond. Pharynx From the mouth, food passes posteriorly into the oropharynx and laryngopharynx. Oropharynx. The oropharynx is posterior to the oral cavity. Laryngopharynx. The laryngopharynx is continuous with the esophagus below; both of which are common passageways for food, fluids, and air. Esophagus The esophagus or gullet, runs from the pharynx through the diaphragm to the stomach. Size and function. About 25 cm (10 inches) long, it is essentially a passageway that conducts food by peristalsis to the stomach. Structure. The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same four basic tissue layers or tunics. Mucosa. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ; it consists primarily of surface epithelium, plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer. Submucosa. The submucosa is found just beneath the mucosa; it is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels. Muscularis externa. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.  Serosa. The serosa is the outermost layer of the wall that consists of a single layer of flat serous fluid-producing cells, the visceral peritoneum. Intrinsic nerve plexuses. The alimentary canal wall contains two important intrinsic nerve plexuses- the submucosal nerve plexus and the myenteric nerve plexus, both of which are networks of nerve fibers that are actually part of the autonomic nervous system and help regulate the mobility and secretory activity of the GI tract organs Stomach Location. The C-shaped stomach is on the left side of the abdominal cavity, nearly hidden by the liver and the diaphragm. Function. The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown. Cardiac region. The cardiac region surrounds the cardioesophageal sphincter, through which food enters the stomach from the esophagus. Fundus. The fundus is the expanded part of the stomach lateral to the cardiac region.  Body. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum, and then the funnel-shaped pylorus. Pylorus. The pylorus is the terminal part of the stomach and it is continuous with the small intestine through the pyloric sphincter or valve. Size. The stomach varies from 15 to 25 cm in length, but its diameter and volume depend on how much food it contains; when it is full, it can hold about 4 liters (1 gallon) of food, but when it is empty it collapses inward on itself. Rugae. The mucosa of the stomach is thrown into large folds called rugae when it is empty. Greater curvature. The convex lateral surface of the stomach is the greater curvature. Lesser curvature. The concave medial surface is the lesser curvature. Lesser omentum. The lesser omentum, a double layer of peritoneum, extends from the liver to the greater curvature. Greater omentum. The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall, and is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs. Stomach mucosa. The mucosa of the stomach is a simple columnar epithelium composed entirely of mucous cells that produce a protective layer of bicarbonate-rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes. Gastric glands. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands that secrete the solution called gastric juice. Intrinsic factor. Some stomach cells produce intrinsic factor, a substance needed for the absorption of vitamin b12 from the small intestine. Chief cells. The chief cells produce protein-digesting enzymes, mostly pepsinogens. Parietal cells. The parietal cells produce corrosive hydrochloric acid, which makes the stomach contents acidic and activates the enzymes. Enteroendocrine cells. The enteroendocrine cells produce local hormones such as gastrin, that are important to the digestive activities of the stomach. Chyme. After food has been processed, it resembles heavy cream and is called chyme. Small Intestine - The small intestine is the body’s major digestive organ. Location. The small intestine is a muscular tube extending from the pyloric sphincter to the large intestine. Size. It is the longest section of the alimentary tube, with an average length of 2.5 to 7 m (8 to 20 feet) in a living person. Subdivisions. The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum, which contribute 5 percent, nearly 40 percent, and almost 60 percent of the small intestine, respectively. Ileocecal valve. The ileum meets the large intestine at the ileocecal valve, which joins the large and small intestine. Hepatopancreatic ampulla. The main pancreatic and bile ducts join at the duodenum to form the flasklike hepatopancreatic ampulla, literally, the ” liver-pacreatic-enlargement”. Duodenal papilla. From there, the bile and pancreatic juice travel through the duodenal papilla and enter the duodenum together. Microvilli. Microvilli are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border; the plasma membranes bear enzymes (brush border enzymes) that complete the digestion of proteins and carbohydrates in the small intestine. Villi. Villi are fingerlike projections of the mucosa that give it a velvety appearance and feel, much like the soft nap of a towel.  Lacteal. Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal. Circular folds. Circular folds, also called plicae circulares, are deep folds of both mucosa and submucosa layers, and they do not disappear when food fills the small intestine. Peyer’s patches. In contrast, local collections of lymphatic tissue found in the submucosa increase in number toward the end of the small intestine. Large Intestine - The large intestine is much larger in diameter than the small intestine but shorter in length. Size. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus. Functions. Its major functions are to dry out indigestible food residue by absorbing water and to eliminate these residues from the body as feces. Subdivisions. It frames the small intestines on three sides and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal. Cecum. The saclike cecum is the first part of the large intestine. Appendix. Hanging from the cecum is the wormlike appendix, a potential trouble spot because it is an ideal location for bacteria to accumulate and multiply. Ascending colon. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity. Transverse colon. The ascending colon makes a turn and continuous to be the transverse colon as it travels across the abdominal cavity.  Descending colon. It then turns again at the left colic (or splenic) flexure, and continues down the left side as the descending colon. Sigmoid colon. The intestine then enters the pelvis, where it becomes the S-shaped sigmoid colon. Anal canal. The anal canal ends at the anus which opens to the exterior. External anal sphincter. The anal canal has an external voluntary sphincter, the external anal sphincter, composed of skeletal muscle. Internal involuntary sphincter. The internal involuntary sphincter is formed by smooth muscles. Accessory Digestive Organ - Other than the intestines and the stomach, the following are also part of the digestive system: Teeth The role the teeth play in food processing needs little introduction; we masticate, or chew, by opening and closing our jaws and moving them from side to side which continuously using our tongue to move the food between our teeth  Function. The teeth tear and grind the food, breaking it down into smaller fragments. Deciduous teeth. The first set of teeth is the deciduous teeth, also called baby teeth or milk teeth, and they begin to erupt around 6 months, and a baby has a full set (20 teeth) by the age of 2 years. Permanent teeth. As the second set of teeth, the deeper permanent teeth, enlarge and develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 to 12 years they loosen and fall out. Incisors. The chisel-shaped incisors are adapted for cutting. Canines. The fanglike canines are for tearing and piercing. Premolars and molars. Premolars (bicuspids) and molars have broad crowns with round cusps ( tips) and are best suited for grinding. Crown. The enamel-covered crown is the exposed part of the tooth above the gingiva or gum. Enamel. Enamel is the hardest substance in the body and is fairly brittle because it is heavily mineralized with calcium salts.  Root. The outer surface of the root is covered by a substance called cementum, which attaches the tooth to the periodontal membrane (ligament). Dentin. Dentin, a bonelike material, underlies the enamel and forms the bulk of the tooth. Pulp cavity. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called the pulp. Root canal. Where the pulp cavity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth. Salivary Glands - Three pairs of salivary glands empty their secretions into the mouth. Parotid glands. The large parotid glands lie anterior to the ears and empty their secretions into the mouth. Submandibular and sublingual glands. The submandibular and sublingual glands empty their secretions into the floor of the mouth through tiny ducts. Saliva. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids. Salivary amylase. The clear serous portion contains an enzyme, salivary amylase, in a bicarbonate-rich juice that begins the process of starch digestion in the mouth. Pancreas - Only the pancreas produces enzymes that break down all categories of digestible foods.  Location. The pancreas is a soft, pink triangular gland that extends across the abdomen from the spleen to the duodenum; but most of the pancreas lies posterior to the parietal peritoneum, hence its location is referred to as retroperitoneal. Pancreatic enzymes. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the acidic chyme coming in from the stomach. Endocrine function. The pancreas also has an endocrine function; it produces hormones insulin and glucagon. Liver - The liver is the largest gland in the body. Location. Located under the diaphragm, more to the right side of the body, it overlies and almost completely covers the stomach. Falciform ligament. The liver has four lobes and is suspended from the diaphragm and abdominal wall by a delicate mesentery cord, the falciform ligament. Function. The liver’s digestive function is to produce bile. Bile. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments, cholesterol, phospholipids, and a variety of electrolytes. Bile salts. Bile does not contain enzymes but its bile salts emulsify fats by physically breaking large fat globules into smaller ones, thus providing more surface area for the fat-digesting enzymes to work on. Gallbladder - While in the gallbladder, bile is concentrated by the removal of water. Location. The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver. Cystic duct. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored. Physiology of the Digestive System Specifically, the digestive system takes in food (ingests it), breaks it down physically and chemically into nutrient molecules (digests it), and absorbs the nutrients into the bloodstream, then, it rids the body of indigestible remains (defecates). Activities Occurring in the Mouth, Pharynx, and Esophagus The activities that occur in the mouth, pharynx, and esophagus are food ingestion, food breakdown, and food propulsion. Food Ingestion and Breakdown Once food is placed in the mouth, both mechanical and chemical digestion begin. Physical breakdown. First, the food is physically broken down into smaller particles by chewing. Chemical breakdown. Then, as the food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose. Stimulation of saliva. When food enters the mouth, much larger amounts of saliva pour out; however, the simple pressure of anything put into the mouth and chewed will also stimulate the release of saliva. Passageways. The pharynx and the esophagus have no digestive function; they simply provide passageways to carry food to the next processing site, the stomach. Food Propulsion – Swallowing and Peristalsis For food to be sent on its way to the mouth, it must first be swallowed. Deglutition. Deglutition, or swallowing, is a complex process that involves the coordinated activity of several structures (tongue, soft palate, pharynx, and esophagus).  Buccal phase of deglutition. The first phase, the voluntary buccal phase, occurs in the mouth; once the food has been chewed and well mixed with saliva, the bolus (food mass) is forced into the pharynx by the tongue. Pharyngeal-esophageal phase. The second phase, the involuntary pharyngeal-esophageal phase, transports food through the pharynx and esophagus; the parasympathetic division of the autonomic nervous system controls this phase and promotes the mobility of the digestive organs from this point on. Food routes. All routes that the food may take, except the desired route distal into the digestive tract, are blocked off; the tongue blocks off the mouth; the soft palate closes off the nasal passages; the larynx rises so that its opening is covered by the flaplike epiglottis. Stomach entrance. Once food reaches the distal end of the esophagus, it presses against the cardioesophageal sphincter, causing it to open, and food enters the stomach. Activities of the Stomach The activities of the stomach involve food breakdown and food propulsion. Food Breakdown The sight, smell, and taste of food stimulate parasympathetic nervous system reflexes, which increase the secretion of gastric juice by the stomach glands Gastric juice. Secretion of gastric juice is regulated by both neural and hormonal factors. Gastrin. The presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin, which prods the stomach glands to produce still more of the protein-digesting enzymes (pepsinogen), mucus, and hydrochloric acid. Pepsinogen. The extremely acidic environment that hydrochloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme. Rennin. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily on milk protein and converts it to a substance that looks like sour milk. Food entry. As food enters and fills the stomach, its wall begins to stretch (at the same time as the gastric juices are being secreted). Stomach wall activation. Then the three muscle layers of the stomach wall become active; they compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme-containing gastric juice so that the semifluid chyme is formed. Food Propulsion Peristalsis is responsible for the movement of food towards the digestive site until the intestines. Peristalsis. Once the food has been well mixed, rippling peristalsis begins in the upper half of the stomach, and the contractions increase in force as the food approaches the pyloric valve. Pyloric passage. The pylorus of the stomach, which holds about 30 ml of chyme, acts like a meter that allows only liquids and very small particles to pass through the pyloric sphincter; and because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3 ml or less of chyme into the small intestine. Enterogastric reflex. When the duodenum is filled with chyme and its wall is stretched, a nervous reflex, the enterogastric reflex, occurs; this reflex “puts the brakes on” gastric activity and slows the emptying of the stomach by inhibiting the vagus nerves and tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up. Activities of the Small Intestine The activities of the small intestine are food breakdown and absorption and food propulsion. Food Breakdown and Absorption Food reaching the small intestine is only partially digested. Digestion. Food reaching the small intestine is only partially digested; carbohydrate and protein digestion has begun, but virtually no fats have been digested up to this point. Brush border enzymes. The microvilli of small intestine cells bears a few important enzymes, the so-called brush border enzymes, that break down double sugars into simple sugars and complete protein digestion. Pancreatic juice. Foods entering the small intestine are literally deluged with enzyme-rich pancreatic juice ducted in from the pancreas, as well as bile from the liver; pancreatic juice contains enzymes that, along with brush border enzymes, complete the digestion of starch, carry out about half of the protein digestion, and are totally responsible for fat digestion and digestion of nucleic acids. Chyme stimulation. When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones; two of these are secretin  and cholecystokinin which influence the release of pancreatic juice and bile. Absorption. Absorption of water and of the end products of digestion occurs all along the length of the small intestine; most substances are absorbed through the intestinal cell plasma membranes by the process of active transport. Diffusion. Lipids or fats are absorbed passively by the process of diffusion. Debris. At the end of the ileum, all that remains are some water, indigestible food materials, and large amounts of bacteria; this debris enters the large intestine through the ileocecal valve. Food Propulsion Peristalsis is the major means of propelling food through the digestive tract. Peristalsis. The net effect is that the food is moved through the small intestine in much the same way that toothpaste is squeezed from the tube. Constrictions. Rhythmic segmental movements produce local constrictions of the intestine that mix the chyme with the digestive juices, and help to propel food through the intestine. Activities of the Large Intestine The activities of the large intestine are food breakdown and absorption and defecation. Food Breakdown and Absorption What is finally delivered to the large intestine contains few nutrients, but that residue still has 12 to 24 hours more to spend there. Metabolism. The “resident” bacteria that live in its lumen metabolize some of the remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to the odor of feces. Flatus. About 50 ml of gas (flatus) is produced each day, much more when certain carbohydrate-rich foods are eaten. Absorption. Absorption by the large intestine is limited to the absorption of vitamin K, some B vitamins, some ions, and most of the remaining water. Feces. Feces, the more or less solid product delivered to the rectum, contains undigested food residues, mucus, millions of bacteria, and just enough water to allow their smooth passage. Propulsion of the Residue and Defecation When presented with residue, the colon becomes mobile, but its contractions are sluggish or short-lived. Haustral contractions. The movements most seen in the colon are haustral contractions, slow segmenting movements lasting about one minute that occur every 30 minutes or so. Propulsion. As the haustrum fills with food residue, the distension stimulates its muscle to contract, which propels the luminal contents into the next haustrum. Mass movements. Mass movements are long, slow-moving, but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum. Rectum. The rectum is generally empty, but when feces are forced into it by mass movements and its wall is stretched, the defecation reflex is initiated. Defecation reflex. The defecation reflex is a spinal (sacral region) reflex that causes the walls of the sigmoid colon and the rectum to contract and anal sphincters to relax. Impulses. As the feces is forced into the anal canal, messages reach the brain giving us time to make a decision as to whether the external voluntary sphincter should remain open or be constricted to stop passage of feces. Relaxation. Within a few seconds, the reflex contractions end and rectal walls relax; with the next mass movement, the defecation reflex is initiated again. Age-Related Physiological Changes in the Gastrointestinal System Age-related changes in the gastrointestinal system include reduced saliva, decreased esophageal and stomach motility, decreased stomach emptying time, decreased production of intrinsic factor, and decreased intestinal absorption, motility, and blood flow. In addition, tooth enamel becomes harder and more brittle, making teeth more susceptible to fractures. Urinary System Anatomy and Physiology The function of the kidneys are as follows: 1. Filter. Every day, the kidneys filter gallons of fluid from the bloodstream. 2. Waste processing. The kidneys then process this filtrate, allowing wastes and excess ions to leave the body in urine while returning needed substances to the blood in just the right proportions. 3. Elimination. Although the lungs and the skin also play roles in excretion, the kidneys bear the major responsibility for eliminating nitrogenous wastes, toxins, and drugs from the body. 4. Regulation. The kidneys also regulate the blood’s volume and chemical makeup so that the proper balance between water and salts and between acids and bases is maintained. 5. Other regulatory functions. By producing the enzyme renin, they help regulate blood pressure, and their hormone erythropoietin stimulates red blood cell production in the bone marrow. 6. Conversion. Kidney cells also convert vitamin D to its active form. Anatomy of the Urinary System The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra. The kidneys alone perform the functions just described and manufacture urine in the process, while the other organs of the urinary system provide temporary storage reservoirs for urine or serve as transportation channels to carry it from one body region to another. The Kidneys The kidneys, which maintain the purity and constancy of our internal fluids, are perfect examples of homeostatic organs. Location. These small, dark red organs with a kidney-bean shape lie against the dorsal body wall in a retroperitoneal position (beneath the parietal peritoneum) in the superior lumbar region; they extend from the T12 to the L3 vertebra, thus they receive protection from the lower part of the rib cage. Positioning. Because it is crowded by the liver, the right kidney is positioned slightly lower than the left. Size. An adult kidney is about 12 cm (5 inches) long, 6 cm (2.5 inches) wide, and 3 cm (1 inch) thick, about the size of a large bar of soap. Adrenal gland. Atop each kidney is an adrenal gland, which is part of the endocrine system is a distinctly separate organ functionally. Fibrous capsule. A transparent fibrous capsule encloses each kidney and gives a fresh kidney a glistening appearance.  Perirenal fat capsule. A fatty mass, the perirenal fat capsule, surrounds each kidney and acts to cushion it against blows. Renal fascia. The renal fascia, the outermost capsule, anchors the kidney and helps hold it in place against the muscles of the trunk wall. Renal cortex. The outer region, which is light in color, is the renal cortex. Renal medulla. Deep to the cortex is a darker, reddish-brown area, the renal medulla. Renal pyramids. The medulla has many basically triangular regions with a striped appearance, the renal, or medullary pyramids; the broader base of each pyramid faces toward the cortex while its tip, the apex, points toward the inner region of the kidney. Renal columns. The pyramids are separated by extensions of cortex-like tissue, the renal columns. Renal pelvis. Medial to the hilum is a flat, basinlike cavity, the renal pelvis, which is continuous with the ureter leaving the hilum. Calyces. Extensions of the pelvis, calyces, form cup-shaped areas that enclose the tips of the pyramid and collect urine, which continuously drains from the tips of the pyramids into the renal pelvis. Renal artery. The arterial supply of each kidney is the renal artery, which divides into segmental arteries as it approaches the hilum, and each segmental artery gives off several branches called interlobar arteries. Arcuate arteries. At the cortex-medulla junction, interlobar arteries give off arcuate arteries, which curve over the medullary pyramids. Cortical radiate arteries. Small cortical radiate arteries then branch off the arcuate arteries and run outward to supply the cortical tissue Nephrons Nephrons are the structural and functional units of the kidneys.  Nephrons. Each kidney contains over a million tiny structures called nephrons, and they are responsible for forming urine. Glomerulus. One of the main structures of a nephron, a glomerulus is a knot of capillaries. Renal tubule. Another one of the main structures in a nephron is the renal tubule. Bowman’s capsule. The closed end of the renal tubule is enlarged and cup-shaped and completely surrounds the glomerulus, and it is called the glomerular or Bowman’s capsule. Podocytes. The inner layer of the capsule is made up of highly modified octopus-like cells called podocytes. Foot processes. Podocytes have long branching processes called foot processes that intertwine with one another and cling to the glomerulus. Collecting duct. As the tubule extends from the glomerular capsule, it coils and twists before forming a hairpin loop and then again becomes coiled and twisted before entering a collecting tubule called the collecting duct, which receives urine from many nephrons. Proximal convoluted tubule. This is the part of the tubule that is near to the glomerular capsule. Loop of Henle. The loop of Henle is the hairpin loop following the proximal convoluted tubule.  Distal convoluted tubule. After the loop of Henle, the tubule continues to coil and twist before the collecting duct, and this part is called the distal convoluted tubule. Cortical nephrons. Most nephrons are called cortical nephrons because they are located almost entirely within the cortex. Juxtamedullary nephrons. In a few cases, the nephrons are called juxtamedullary nephrons because they are situated next to the cortex-medullary junction, and their loops of Henle dip deep into the medulla. Afferent arteriole. The afferent arteriole, which arises from a cortical radiate artery, is the “feeder vessel”. Efferent arteriole. The efferent arteriole receives blood that has passed through the glomerulus. Peritubular capillaries. They arise from the efferent arteriole that drains the glomerulus. Ureters The ureters do play an active role in urine transport. Size. The ureters are two slender tubes each 25 to 30 cm (10 to 12 inches) long and 6 mm (1/4 inch) in diameter. Location. Each ureter runs behind the peritoneum from the renal hilum to the posterior aspect of the bladder, which it enters at a slight angle. Function. Essentially, the ureters are passageways that carry urine from the kidneys to the bladder through contraction of the smooth muscle layers in their walls that propel urine into the bladder by peristalsis and is prevented from flowing back by small valve-like folds of bladder mucosa that flap over the ureter openings. Urinary Bladder The urinary bladder is a smooth, collapsible, muscular sac that stores urine temporarily. Location. It is located retroperitoneally in the pelvis just posterior to the symphysis pubis. Function. The detrusor muscles and the transitional epithelium both make the bladder uniquely suited for its function of urine storage. Trigone. The smooth triangular region of the bladder base outlined by these three openings is called the trigone, where infections tend to persist. Detrusor muscles. The bladder wall contains three layers of smooth muscle, collectively called the detrusor muscle, and its mucosa is a special type of epithelium, transitional epithelium. Urethra The urethra is a thin-walled tube that carries urine by peristalsis from the bladder to the outside of the body. Internal urethral sphincter. At the bladder-urethral junction, a thickening of the smooth muscle forms the internal urethral sphincter, an involuntary sphincter that keeps the urethra closed when the urine is not being passed. External urethral sphincter. A second sphincter, the external urethral sphincter, is fashioned by skeletal muscle as the urethra passes through the pelvic floor and is voluntarily controlled. Female urethra. The female urethra is about 3 to 4 cm (1 1/2 inches) long, and its external orifice, or opening, lies anteriorly to the vaginal opening. Male urethra. In me, the urethra is approximately 20 cm (8 inches) long and has three named regions: the prostatic, membranous, and spongy (penile) urethrae; it opens at the tip of the penis after traveling down its length. Physiology of the Urinary System Every day, the kidneys filter gallons of fluid from the bloodstream. The normal physiology that takes place in the urinary system is as follows: Urine Formation Urine formation is a result of three processes:  Glomerular filtration. Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule. Tubular reabsorption. Water, glucose, amino acids, and needed ions are transported out of the filtrate into the tubule cells and then enter the capillary blood. Tubular secretion. Hydrogen, potassium, creatinine, and drugs are removed from the peritubular blood and secreted by the tubule cells into the filtrate. Characteristics of Urine In 24 hours, the marvelously complex kidneys filter some 150 to 180 liters of blood plasma through their glomeruli into the tubules. Daily volume. In 24 hours, only about 1.0 to 1.8 liters of urine are produced. Components. Urine contains nitrogenous wastes and unneeded substances. Color. Freshly voided urine is generally clear and pale to deep yellow. Odor. When formed, urine is sterile and slightly aromatic, but if allowed to stand, it takes on an ammonia odor caused by the action of bacteria on the urine solutes.  pH. Urine pH is usually slightly acidic (around 6), but changes in body metabolism and certain foods may cause it to be much more acidic or basic. Specific gravity. Whereas the specific gravity of pure water is 1.0, the specific gravity of urine usually ranges from 1.001 to 1.035. Solutes. Solutes normally found in urine include sodium and potassium ions, urea, uric acid, creatinine, ammonia, bicarbonate ions, and various other ions. Micturition Micturition or voiding is the act of emptying the bladder. Accumulation. Ordinarily, the bladder continues to collect urine until about 200 ml have accumulated. Activation. At about this point, stretching of the bladder wall activates stretch receptors. Transmission. Impulses transmitted to the sacral region of the spinal cord and then back to the bladder via the pelvic splanchnic nerves cause the bladder to go into reflex contractions. Passage. As the contractions become stronger, stored urine is forced past the internal urethral sphincter into the upper part of the urethra. External sphincter. Because the lower external sphincter is skeletal muscle and voluntarily controlled, we can choose to keep it closed or it can be relaxed so that urine is flushed from the body. Age-Related Physiological Changes in the Urinary System The function of the kidney decreases with age but is still able to carry out excretory functions unless a disease process intervenes. Waste products may be filtered and excreted more slowly. Therefore, nurses must include in their responsibility the effect of drugs that older people take to their kidneys. Functions of the Endocrine System Despite the huge variety of hormones, there are really only two mechanisms by which hormones trigger changes in cells. 1. Water equilibrium. The endocrine system controls water equilibrium by regulating the solute concentration of the blood. 2. Growth, metabolism, and tissue maturation. The endocrine system controls the growth of many tissues, like the bone and muscle, and the degree of metabolism of various tissues, which aids in the maintenance of the normal body temperature and normal mental functions. Maturation of tissues, which  appears in the development of adult features and adult behavior, are also determined by the endocrine system. 3. Heart rate and blood pressure management. The endocrine system assists in managing the heart rate and blood pressure and aids in preparing the body for physical motion. 4. Immune system control. The endocrine system helps regulate the production and functions of immune cells. 5. Reproductive function controls. The endocrine system regulates the development and the functions of the reproductive systems in males and females. 6. Uterine contractions and milk release. The endocrine system controls uterine contractions throughout the delivery of the newborn and stimulates milk release from the breasts in lactating females. 7. Ion management. The endocrine system regulates Na+, K+, and Ca2+ concentrations in the blood. 8. Blood glucose regulator. The endocrine system controls blood glucose levels and other nutrient levels in the blood. 9. Direct gene activation. Being lipid-soluble molecules, the steroid hormones can diffuse through plasma membranes of their target cells; once inside, the steroid hormone enters the nucleus and binds to a specific receptor protein there; then, the hormone-receptor complex binds to specific sites on the cell’s DNA, activating certain genes to transcribe messenger RNA; the mRNA then is translated in the cytoplasm, resulting in the synthesis of new proteins. 10. Second messenger system. Water-soluble, nonsteroidal hormones-protein, and peptide hormones- are unable to enter the target cells, so instead, they bind to receptors situated on the target cell’s plasma membrane and utilize a second messenger system. Anatomy of the Endocrine System Compared to other organs of the body, the organs of the endocrine system are small and unimpressive, however, functionally the endocrine organs are very impressive, and when their role in maintaining body homeostasis is considered, they are true giants. Hypothalamus The major endocrine organs of the body include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands, the pancreas, and the gonads. Hypothalamus. The hypothalamus, which is part of the nervous system, is also considered a major endocrine organ because it produces several hormones. It is an important autonomic nervous system and endocrine control center of the brain located inferior to the thalamus. Mixed functions. Although the function of some hormone-producing glands is purely endocrine, the function of others (pancreas and gonads) is mixed- both endocrine and exocrine. Pituitary Gland The pituitary gland is approximately the size of a pea.  Location. The pituitary gland hangs by a stalk from the inferior surface of the hypothalamus of the brain, where it is snugly surrounded by the “Turk’s saddle” of the sphenoid bone. Lobes. It has two functional lobes- the anterior pituitary (glandular tissue) and the posterior pituitary (nervous tissue). Hormones of the Anterior Pituitary There are several hormones of the anterior pituitary hormones that affect many body organs. Growth hormone (GH). Growth hormone is a general metabolic hormone, however, its major effects are directed to the growth of skeletal muscles and long bones of the body; it is a protein-sparing and anabolic hormone that causes amino acids to be built into proteins and stimulates most target cells to grow in size and divide.  Prolactin (PRL). Prolactin is a protein hormone structurally similar to growth hormone; its only known target in humans is the breast because, after childbirth, it stimulates and maintains milk production by the mother’s breast. Adrenocorticotropic hormone (ACTH). ACTH regulates the endocrine activity of the cortex portion of the adrenal gland. Thyroid-stimulating hormone (TSH). TSH, also called thyrotropin hormone influences the growth and activity of the thyroid gland. Gonadotropic hormones. The gonadotropic hormones regulate the hormonal activity of gonads (ovaries and testes). Follicles-stimulating hormone (FSH). FSH stimulates follicle development in the ovaries; as the follicles mature, they produce estrogen and eggs that are readied for ovulation; in men, FSH stimulates sperm development by the testes. Luteinizing hormone (LH). LH triggers the ovulation of an egg from the ovary and causes the ruptured follicle to produce progesterone and some estrogen; in men, LH stimulates testosterone production by the interstitial cells of the testes. Hormones of the Posterior Pituitary The posterior pituitary is not an endocrine gland in the strict sense because it does not make the peptide hormones it releases, but it simply acts as a storage area for hormones made by hypothalamic neurons. Oxytocin. Oxytocin is released in significant amounts only during childbirth and in nursing women; it stimulates powerful contractions of the uterine muscle during labor, during sexual relations, and during breastfeeding and also causes milk ejection (let-down reflex) in a nursing woman. Antidiuretic hormone (ADH). ADH causes the kidneys to reabsorb more water from the forming of urine; as a result, urine volume decreases and blood volume increases; in larger amounts, ADH also increases blood pressure by causing constriction of the arterioles, so it is sometimes referred to as vasopressin. Thyroid Gland The thyroid gland is a hormone-producing gland that is familiar to most people primarily because many obese individuals blame their overweight condition on their “glands” (thyroid).  Location. The thyroid gland is located at the base of the throat, just inferior to the Adam’s apple, where it is easily palpated during a physical examination. Lobes. It is a fairly large gland consisting of two lobes joined by a central mass, or isthmus. Composition. Internally, the thyroid gland is composed of hollow structures called follicles, which store a sticky colloidal material. Types of thyroid hormones. Thyroid hormone often referred to as the body’s major metabolic hormone, is actually two active, iodine-containing hormones, thyroxine or T4, and triiodothyronine or T3. Thyroxine. Thyroxine is the major hormone secreted by the thyroid follicles. Triiodothyronine. Most triiodothyronine is formed at the target tissues by conversion of the thyroxine to triiodothyronine. Function. Thyroid hormone controls the rate at which glucose is “burned” oxidized, and converted to body heat and chemical energy; it is also important for normal tissue growth and development.  Calcitonin. Calcitonin decreases blood calcium levels by causing calcium to be deposited in the bones; calcitonin is made by the so-called parafollicular cells found in the connective tissues between the follicles. Parathyroid Glands The parathyroid glands are mostly tiny masses of glandular tissue. Location. The parathyroid glands are located on the posterior surface of the thyroid gland. Parathormone. The parathyroids secrete parathyroid hormone (PTH) or parathormone, which is the most important regulator of calcium ion homeostasis of the blood; PTH is a hypercalcemic hormone (that is, it acts to increase blood levels of calcium), whereas calcitonin is a hypocalcemic hormone.; PTH also stimulates the kidneys and intestines to absorb more calcium. Adrenal Glands Although the adrenal gland looks like a single organ, it is structurally and functionally two endocrine organs in one. Hormones of the Adrenal Cortex The adrenal cortex produces three major groups of steroid hormones, which are collectively called corticosteroids– mineralocorticoids, glucocorticoids, and sex hormones. Mineralocorticoids. The mineralocorticoids, primarily aldosterone, are produced by the outermost adrenal cortex cell layer; mineralocorticoids are important in regulating the mineral (or salt) content of the blood, particularly the concentrations of sodium and potassium ions and they also help in regulating the water and electrolyte balance in the body. Renin. Renin, am enzyme produced by the kidneys when the blood pressure drops, also cause the release of aldosterone by triggering a series of reactions that form angiotensin II, a potent stimulator of aldosterone release. Atrial natriuretic peptide (ANP). ANP prevents aldosterone release, its goal being to reduce blood volume and blood pressure. Glucocorticoids. The middle cortical layer mainly produces glucocorticoids, which include cortisone and cortisol; glucocorticoids promote normal cell metabolism and help the body to resist long-term stressors, primarily by increasing blood glucose levels, thus it is said to be a hyperglycemic hormone;  it also reduce pain and inflammation by inhibiting some pain-causing molecules called prostaglandins. Sex hormones. Both male and female sex hormones are produced by the adrenal cortex throughout life in relatively small amounts; although the bulk of sex hormones produced by the innermost cortex layer are androgens (male sex hormones), some estrogens (female sex hormones), are also formed. Hormones of the Adrenal Medulla The adrenal medulla, like the posterior pituitary, develops from a knot of nervous tissue. Catecholamines. When the medulla is stimulated by sympathetic nervous system neurons, its cells release two similar hormones, epinephrine, also called adrenaline, and norepinephrine (noradrenaline), into the bloodstream; collectively, these hormones are referred to as catecholamines. Function. Basically, the Catecholamines increase heart rate, blood pressure, and blood glucose levels and dilate the small passageways of the lungs; the catecholamines of the adrenal medulla prepare the body to cope with a brief or short-term stressful situation and cause the so-called alarm stage of the stress response. Pancreatic Islets - The pancreas, located close to the stomach in the abdominal cavity, is a mixed gland.  Islets of Langerhans.The islets of Langerhans also called pancreatic islets, are little masses of hormone-producing tissue that are scattered among the enzyme-producing acinar tissue of the pancreas. Hormones. Two important hormones produced by the islet cells are insulin and glucagon. Islet cells. Islet cells act as fuel sensors, secreting insulin, and glucagon appropriately during fed and fasting states. Beta cells. High levels of glucose in the blood stimulate the release of insulin from the beta cells of the islets. Alpha cells. Glucagon’s release by the alpha cells of the islets is stimulated by low blood glucose levels.  Insulin. Insulin acts on just about all the body cells and increases their ability to transport glucose across their plasma membranes; because insulin sweeps glucose out of the blood, its effect is said to be hypoglycemic. Glucagon. Glucagon acts as an antagonist of insulin; that is, it helps to regulate blood glucose levels but in a way opposite that of insulin; its action is basically hyperglycemic and its primary target organ is the liver, which it stimulates to break down stored glycogen into glucose and release the glucose into the blood. Pineal Gland The pineal gland, also called the pineal body, is a small cone-shaped gland. Location. The pineal gland hangs from the roof of the third ventricle of the brain. Melatonin. Melatonin is the only hormone that appears to be secreted in substantial amounts by the pineal gland; the levels of melatonin rise and fall during the course of the day and night; peak levels occur at night and make us drowsy as melatonin is believed to be the “sleep trigger” that plays an important role in establishing the body’s day-night cycle. Thymus Gland The thymus gland is large in infants and children and decreases in size throughout adulthood. Location. The thymus gland is located in the upper thorax, posterior to the sternum. Thymosin. The thymus produces a hormone called thymosin and others that appear to be essential for normal development of a special group of white blood cells (T-lymphocytes, or T cells) and the immune response. Gonads The female and male gonads produce sex hormones that are identical to those produced by adrenal cortex cells; the major difference are the source and relative amount produced. Hormones of the Ovaries The female gonads or ovaries are a pair of almond-sized organs. Location. The female gonads are located in the pelvic cavity. Steroid hormones. Besides producing female sex cells, ovaries produce two groups of steroid hormones, estrogen, and progesterone.  Estrogen. Alone, the estrogens are responsible for the development of sex characteristics in women at puberty; acting with progesterone, estrogens promote breast development and cyclic changes in the uterine lining (menstrual cycle). Progesterone. Progesterone acts with estrogen to bring about the menstrual cycle; during pregnancy, it quiets the muscles of the uterus so that an implanted embryo will not be aborted and helps prepare breast tissue for lactation. Hormones of the Testes The testes of the male are paired oval organs in a sac. Location. The testes are suspended in a sac, the scrotum, outside the pelvic cavity. Male sex hormones. In addition to male sex cells, or sperm, the testes also produce male sex hormones, or androgens, of which testosterone is the most important. Testosterone. At puberty, testosterone promotes the growth and maturation of the reproductive system organs to prepare the young man for reproduction; it also causes the male’s secondary sex characteristics to appear and stimulates male sex drive; Testosterone is also necessary for the continuous production of sperm. Other Hormone-Producing Tissues and Organs Besides the major endocrine organs, pockets of hormone-producing cells are found in fatty tissue and in the walls of the small intestine, stomach, kidneys, and heart- organs whose chief functions have little to do with hormone production. Placenta The placenta is a remarkable organ formed temporarily in the uterus of pregnant women. Function. In addition to its roles as the respiratory, excretory, and nutrition delivery systems for the fetus, it also produces several proteins and steroid hormones that help to maintain the pregnancy and pave the way for delivery of the baby. Human chorionic gonadotropin. During very early pregnancy, a hormone called human chorionic gonadotropin (hCG) is produced by the developing embryo and then by the fetal part of the placenta; hCG stimulates the ovaries to continue producing estrogen and progesterone so that the lining of the uterus is not sloughed off in the menses.  Human placental lactogen (hPL). hPL works cooperatively with estrogen and progesterone in preparing the breasts for lactation. Relaxin. Relaxin, another placental hormone, causes the mother’s pelvic ligaments and the pubic symphysis to relax and become more flexible, which eases birth passage. Physiology of the Endocrine System Although hormones have widespread effects, the major processes they control are reproduction, growth, and development; mobilizing the body’s defenses against stressors; maintaining electrolyte, water, and nutrient balance of the blood; and regulating cellular metabolism and energy balance. The Chemistry of Hormones The key to the incredible power of the endocrine glands is the hormones they produce and secrete. Hormones. Hormones may be defined as chemical substances that are secreted by endocrine cells into the extracellular fluids and regulate the metabolic activity of other cells in the body. Classification. Although many different hormones are produced, nearly all of them can be classified chemically as either amino acid-based molecules (including proteins, peptides, and amines) or steroids. Steroid hormones. Steroid hormones (made from cholesterol) include the sex hormones made by the gonads and hormones produced by the adrenal cortex. Amino acid-based hormones. All the others are nonsteroidal amino acid derivatives. Mechanisms of Hormone Action Although the blood-borne hormones circulate to all the organs of the body, a given hormone affects only certain tissue cells or organs. Target cells. For a target cell to respond to the hormone, specific protein receptors must be present on its plasma membrane or in its interior to which that hormone can attach; only when this binding occurs can the hormone influence the workings of cells. The function of hormones. The hormones bring about their effects on, the body cells primarily by altering cellular activity- that is, by increasing or decreasing the rate of a normal, or usual, metabolic process rather than stimulating a new one.  Changes in hormone binding. The precise changes that follow hormone binding depend on the specific hormone and the target cell type, but typically one or more of the following occurs: 1. Changes in plasma membrane permeability or electrical state. 2. Synthesis of protein or certain regulatory molecules (such as enzymes) in the cell.’ 3. Activation or inactivation of enzymes. 4. Stimulation of mitosis. 5. Promotion of secretory activity. Control of Hormone Release What prompts the endocrine glands to release or not release their hormones? Negative feedback mechanisms. Negative feedback mechanisms are the chief means of regulating blood levels of nearly all hormones. Endocrine gland stimuli. The stimuli that activate the endocrine organs fall into three major categories- hormonal, humoral, and neural. Hormonal stimuli. The most common stimulus is a hormonal stimulus, in which the endocrine organs are prodded into action by other hormones; for example, hypothalamic hormones stimulate the anterior pituitary gland to secrete its hormones, and many anterior pituitary hormones stimulate other endocrine organs to release their hormones into the blood. Humoral stimuli. Changing blood levels of certain ions and nutrients may also stimulate hormone release, and this is referred to as humoral stimuli; for example, the release of parathyroid hormone (PTH) by cells of the parathyroid glands is prompted by decreasing blood calcium levels. Neural stimuli. In isolated cases, nerve fibers stimulate hormone release, and the target cells are said to respond to neural stimuli; a classic example is sympathetic nervous system stimulation of the adrenal medulla to release norepinephrine and epinephrine during periods of stress. Hormones: 9 Chemicals at Play Nurses Need to Know Originally published by Marianne Belleza R.N. It is amazing how smoothly the functions of our body flow. It is like a well-oiled machinery that would produce and produce when it is in an excellent condition. All of us are curious as to what is behind the masterpiece that is our body, and the principal stars are tiny, teeny hormones that are playing inside us. Each and every hormone has their own function and designation, so let’s meet and greet these little ones and see what they got for us.  1. The Women’s Hormone: Estrogen You might be familiar with this tiny, lady-like hormone. Let’s call her a ‘she’ because she is the main sex hormone in females. If you are curious as to what maneuvers our puberty stages, then she is the one you are looking for. Estrogen does not only spearhead the development of females, but it also prepares their body for pregnancy. In addition to that, it is responsible for regulating the menstrual cycle. All these tasks for a minuscule element! Everything that the estrogen does is for the development of every woman’s being; hence, it is always identified as the women’s hormone. 2. The Mother’s Hormone: Progesterone Here comes estrogen’s best friend. This hormone is also a ‘she’ because, like estrogen, it gives a hand to the development of females’ body. Yet progesterone’s main goal is its role in pregnancy. It assists in the menstrual cycle, and when it recognizes that the woman is pregnant, it immediately imposes itself against estrogen, causing a decrease in estrogen and an increase in progesterone. Without progesterone, women could not become full-fledged mothers and bear adorable babies. 3. The Men’s Hormone: Testosterone The counterpart of the women’s hormone, testosterone is responsible for the masculinity of males. Some people would tell a man that he is oozing with testosterone when what they really mean is he is very macho or exudes a very masculine appeal. The primary task of this male sex hormone is to cause puberty. The deepening of the voice, the increase in facial hair growth, and the muscle mass growth for males is all thanks to this sturdy, tiny hormone. You know who to thank or blame about your physical attributes guys! 4. The Stress Hormone: Cortisol Almost all of us must be hunting for the one responsible for the infamous stress we are always feeling during complicated situations. Be glad that you read on, because you have finally found the culprit. Cortisol assists our body in responding to stress, so we would not be able to feel stress without this tricky hormone. However, this is not the only function for cortisol. It is also responsible for the regulation of our blood pressure and blood glucose. We have to be friendly with cortisol if we don’t want to suffer the deadly diseases of hypertension and hypercholesterolemia. As some would say, keep your enemies closer than your friends. 5. The Sleep Hormone: Melatonin Wake those sleepy heads up for they will benefit from this part of the topic. Ever wondered why you are so sleepy most of the time, or you are having a hard time sleeping? Well, well, we already got the hormone in question. The only hormone that could knock us out is melatonin. Depending on its levels, we are able to feel sleepiness or hardly feel any sleepiness at all. Its levels increase as the dark approaches, explaining why we feel sleepy as the night approaches. Stock up on your melatonin if you want to sleep for a longer time, guys! 6. Grow Up or Grow Down: Growth Hormone You could not forever blame your parents’ genes for not achieving the height that you have always wanted. Well, it is partly one of the causes, but aside from that you divert that piercing look of yours to the silent but somehow powerful hormones from the anterior pituitary gland. Each one of us has our own growth hormone to nurture, and these are secreted in great amounts during our sleep. So never forget to tell your younger siblings about the story of our growth hormones if they really want to become basketball players or supermodels someday. Call on up your melatonin levels too! 7. The Milk Hormone: Prolactin One of the most amazing miracles manifested by human females is being able to feed their young. The miracle of breastfeeding is really heartwarming and tender to behold, especially for the mothers who are participating in this act. Let me introduce you to the bearer of this miracle, prolactin. This is the official milk stimulator of the body which enables lactating mothers to produce more milk for their babies. It also plays a role in the sexual behavior of females. Without this hormone, the most important food for babies, breast milk, would never be possible. 8. The Bitter Hormone: Gastrin Have you ever wondered why vomiting brings a bitter taste to your mouth? The moment all your stomach’s contents are emptied out because you are vomiting, what would follow next is a bitter, colorless substance coming out of your mouth. This is all due to the hormone gastrin which promotes the secretion of acid in your stomach. Acid assists in the breakdown of your food molecules once it hits the stomach, but too much of it could be bad for your gastrointestinal system. Befriend your gastrin hormone by avoiding too many spicy foods and taking your meals on time. You would not want to taste it in your mouth, right? 9. The Bloody Hormone: Erythropoietin Red blood cells do not just burst out of nowhere. They are formed in our bone marrow, and there is an outstanding event organizer that makes everything possible for red blood cells. Meet erythropoietin, the trusty caregiver of red blood cells. They stimulate its development in the bone marrow, and without it, anemia would surely hit us up. These are just a few of the important hormones that play all around our bodies. They may be tiny and not visible to the naked eye, but they have loads of critical functions that enable our bodies to move around normally. A humongous and gargantuan thanks to our hormones, the infinitesimal beings that matter the most!