Digestive System Functions and Anatomy

Digestive System Functions

• Ingestion—taking in food
• Digestion—breaking food down physically and chemically
• Absorption—movement of nutrients into the bloodstream
• Defecation—rids the body of indigestible waste

Organs of the Digestive System

• Two main groups of organs:
  • Alimentary canal (gastrointestinal or GI tract)—continuous, coiled hollow tube. These organs ingest, digest, absorb, and defecate.
  • Accessory digestive organs—include teeth, tongue, and other digestive organs.

Organs of the Alimentary Canal

• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

Mouth (Oral Cavity) Anatomy

• Lips (labia)—protect the anterior opening
• Cheeks—form the lateral walls
• Hard palate—forms the anterior roof
• Soft palate—forms the posterior roof
• Uvula—fleshy projection of the soft palate
• Vestibule—space between lips externally and teeth and gums internally
• Oral cavity proper—area contained by the teeth
• Tongue—attached at hyoid bone and styloid processes of skull; attached to floor of the mouth by lingual frenulum.
• Tonsils (palatine and lingual)—located at posterior end and base of tongue respectively.

Mouth Physiology

• Mastication (chewing) of food
• Mixing masticated food with saliva
• Initiation of swallowing by the tongue
• Allows for the sense of taste

Pharynx Anatomy

• Nasopharynx—not part of the digestive system
• Oropharynx—posterior to oral cavity
• Laryngopharynx—below the oropharynx and connected to the esophagus

Pharynx Physiology

• Serves as a passageway for air and food
• Food is propelled to the esophagus by two muscle layers:
  • Longitudinal inner layer
  • Circular outer layer
• Food movement is by alternating contractions of the muscle layers (peristalsis)

Esophagus Anatomy and Physiology

• Anatomy:
  • About 10 inches long
  • Runs from pharynx to stomach through the diaphragm
• Physiology:
  • Conducts food by peristalsis (slow rhythmic squeezing)
  • Passageway for food only (respiratory system branches off after the pharynx)

Layers of Tissue in the Alimentary Canal Organs

• Four layers from deep to superficial:
  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa

Mucosa Layer

• Innermost, moist membrane consisting of:
  • Surface epithelium
  • Small amount of connective tissue (lamina propria)
  • Small smooth muscle layer
• Lines the cavity (known as the lumen)

Submucosa Layer

• Just beneath the mucosa
• Soft connective tissue with blood vessels, nerve endings, mucosa-associated lymphoid tissue, and lymphatics

Muscularis Externa Layer

• Smooth muscle
  • Inner circular layer
  • Outer longitudinal layer
• Serosa—outermost layer of the wall contains fluid-producing cells
• Visceral peritoneum—outermost layer that is continuous with the innermost layer
• Parietal peritoneum—innermost layer that lines the abdominopelvic cavity

Alimentary Canal Nerve Plexuses

• Two important nerve plexuses that serve the alimentary canal.
  • Submucosal nerve plexus
  • Myenteric nerve plexus
• Function is to regulate mobility and secretory activity of GI tract organs.

Stomach Anatomy

• Location: Left side of abdominal cavity
• Food enters at the cardioesophageal sphincter
• Food empties into the small intestine at the pyloric sphincter (valve)
• Regions of the stomach:
  • Cardiac region—near the heart
  • Fundus—expanded portion lateral to the cardiac region
  • Body—midportion
  • Pylorus—funnel-shaped terminal end
• Rugae—internal folds of the mucosa
• Stomach can stretch and hold 4 L (1 gallon) of food when full
• External regions:
  • Lesser curvature—concave medial surface
  • Greater curvature—convex lateral surface

Stomach Layers

• Muscularis externa has a third layer (oblique)
• Layers of peritoneum attached to the stomach:
  • Lesser omentum—attaches the liver to the lesser curvature
  • Greater omentum—attaches the greater curvature to the posterior body wall; embedded fat insulates, cushions, and protects abdominal organs

Stomach Physiology

• Temporary storage tank for food
• Site of food breakdown
• Chemical breakdown of protein begins
• Delivers chyme (processed food) to the small intestine

Structure of Stomach Mucosa

• Mucosa is simple columnar epithelium
• Mucous neck cells—produce a sticky alkaline mucus
• Gastric glands—situated in gastric pits and secrete gastric juice
• Chief cells—produce protein-digesting enzymes (pepsinogens)
• Parietal cells—produce hydrochloric acid
• Enteroendocrine cells—produce gastrin

Small Intestine

• The body's major digestive organ
• Site of nutrient absorption into the blood
• Muscular tube extending from the pyloric sphincter to the ileocecal valve
• Suspended from the posterior abdominal wall by the mesentery

Subdivisions of the Small Intestine

• Duodenum —attached to the stomach; curves around the head of the pancreas
• Jejunum—attaches anteriorly to the duodenum
• Ileum—extends from jejunum to large intestine.

Chemical Digestion in the Small Intestine

• Chemical digestion begins in the small intestine
• Enzymes are produced by:
  • Intestinal cells
  • Pancreas
• Pancreatic ducts carry enzymes to the small intestine
• Bile, formed by the liver, enters via the bile duct.

Small Intestine Anatomy

• Three structural modifications that increase surface area:
  • Microvilli—tiny projections of the plasma membrane
  • Villi—fingerlike structures formed by the mucosa
  • Circular folds (plicae circulares)—deep folds of mucosa and submucosa

Large Intestine

• Larger in diameter, but shorter in length, than the small intestine
• Extends from the ileocecal valve to the anus
• Subdivisions:
  • Cecum
  • Appendix
  • Colon
    • Ascending
    • Transverse
    • Descending
    • Sigmoid
  • Rectum
  • Anal canal

Large Intestine Anatomy

• Cecum—saclike first part of the large intestine; appendix hangs from the cecum
• Appendix—accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis)
• Colon (ascending, transverse, descending, sigmoid)—travels up right side of abdomen, across abdominal cavity, down left side, and into the pelvis.
• Rectum—and anus also located in the pelvis
• Anus—opening of the large intestine; has external and internal sphincters (normally closed except during defecation)

Large Intestine Anatomy

• No villi present
• Goblet cells produce alkaline mucus which lubricates the passage of feces
• Muscularis externa layer is reduced to three bands of muscle called teniae coli
• These bands cause the wall to pucker into haustra (pocketlike sacs)

Accessory Digestive Organs

• Teeth
• Salivary glands
• Pancreas
• Liver
• Gallbladder

Teeth

• Function is to masticate (chew) food
• Humans have two sets of teeth:
  • Deciduous ("milk") teeth—20 teeth by age two
  • Permanent teeth—32 teeth (some people don't have wisdom teeth); replace deciduous teeth between ages 6 and 12
• Classification of Teeth:
  • Incisors-cutting
  • Canines (eyeteeth)—tearing or piercing
  • Premolars (bicuspids)—grinding
  • Molars—grinding

Regions of a Tooth

• Crown—exposed part
• Enamel—hardest substance in the body
• Dentin—found deep to the enamel and forms the bulk of the tooth
• Pulp cavity—contains connective tissue, blood vessels, and nerve fibers.
• Root canal—where the pulp cavity extends into the root.
• Neck—region in contact with the gum
• Cementum—covers outer surface of root; attaches tooth to periodontal membrane

Salivary Glands

• Three pairs of salivary glands empty secretions into the mouth:
  • Parotid glands—found anterior to the ears
    • Submandibular glands
    • Sublingual glands —both empty saliva into the floor of the mouth through small ducts

Saliva

• Mixture of mucus and serous fluids
• Helps to form a food bolus
• Contains salivary amylase to begin starch digestion
• Dissolves chemicals so they can be tasted.

Pancreas

• Found posterior to the parietal peritoneum (retroperitoneal)
• Extends across the abdomen from spleen to duodenum
• Produces digestive enzymes that break down all categories of food.
• Enzymes enter the duodenum
• Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from the stomach; hormones produced by the pancreas (insulin, glucagon).

Liver

• Largest gland in the body; located on the right side of the body under the diaphragm.
• Consists of four lobes
• Suspended from the diaphragm and abdominal wall by the falciform ligament
• Connected to the gallbladder via the common hepatic duct

Bile

• Produced by cells in the liver
• Leaves the liver through the common hepatic duct
• Composition:
  • Bile salts
  • Bile pigments (mostly bilirubin from the breakdown of hemoglobin)
  • Cholesterol
  • Phospholipids
  • Electrolytes
• Function: Emulsify fats; physically break large fat globules into smaller ones.

Gallbladder

• Sac found in a hollow fossa of the liver
• When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder
• When digestion of fatty food is occurring, bile is introduced into the duodenum from the gallbladder.
• Gallstones are crystallized cholesterol which can cause blockages.

Functions of the Digestive System

• Ingestion—placing food into the mouth
• Propulsion—moving foods from one region of the digestive system to another
• Peristalsis—alternating waves of contraction and relaxation that squeezes food along the GI tract
• Segmentation—moving materials back and forth to aid with mixing in the small intestine.

Digestion in the Small Intestine

• Enzymes from the brush border function to:
  • Break double sugars into simple sugars
  • Complete some protein digestion
• Pancreatic enzymes play the major digestive function:
  • Help complete digestion of starch (pancreatic amylase)
  • Carry out about half of all digestion of protein
  • Digest fats using lipases from the pancreas
  • Digest nucleic acids using nucleases
  • Alkaline content neutralizes acidic chyme.

Regulation of Pancreatic Juice Secretion

• Release of pancreatic juice into the duodenum is stimulated by:
  • Vagus nerve
  • Local hormones
    • Secretin
    • Cholecystokinin (CCK)
• Hormones travel the blood to stimulate the pancreas to release enzyme- and bicarbonate-rich product.
• Secretin causes the liver to increase bile output.
• CCK causes the gallbladder to release stored bile.
• Bile is necessary for fat absorption and absorption of fat-soluble vitamins (K, D, A).

Absorption in the Small Intestine

• Water is absorbed along the length of the small intestine
• End products of digestion:
  • Most substances are absorbed by active transport through cell membranes
  • Lipids are absorbed by diffusion
• Substances are transported to the liver by the hepatic portal vein or lymph

Propulsion in the Small Intestine

• Peristalsis is the major means of moving food
• Segmental movements
  • Mix chyme with digestive juices
  • Aid in propelling food

Food Breakdown and Absorption in the Large Intestine

• No digestive enzymes are produced
• Resident bacteria digest remaining nutrients
• Produce some vitamin K and B
• Release gases
• Water and vitamins K and B are absorbed
• Remaining materials are eliminated via feces
• Feces contains:
  • Undigested food residues
  • Mucus
  • Bacteria
  • Water

Propulsion in the Large Intestine

• Sluggish peristalsis
• Mass movements
  • Slow, powerful movements
  • Occur three to four times per day
• Presence of feces in the rectum causes a defecation reflex
• Internal anal sphincter is relaxed
• Defecation occurs with relaxation of the voluntary (external) anal sphincter

Nutrition

• Nutrient-substance used by the body for growth, maintenance, and repair
• Major nutrients:
  • Carbohydrates
  • Lipids
  • Proteins
  • Water
• Minor nutrients:
  • Vitamins
  • Minerals

Dietary Sources of Major Nutrients

• Carbohydrates (mostly from plants; exceptions: lactose from milk and small amounts of glycogen from meats)
• Lipids (saturated fats from animal products; unsaturated fats from nuts, seeds, and vegetable oils; cholesterol from egg yolk, meats, and milk)
• Proteins (complete proteins from animal products, essential amino acids that our bodies cannot make must be consumed via our diet. Legumes and beans are incomplete proteins)
• Vitamins (used as coenzymes, found in all major food groups)
• Minerals (play many roles in the body, found in vegetables, legumes, milk, and some meats).

Metabolism

• Chemical reactions necessary to maintain life
• Catabolism—substances are broken down; energy released
• Anabolism—larger molecules are built from smaller ones.

Carbohydrate Metabolism

• Carbohydrates are the body's preferred source to produce cellular energy (ATP)
• Glucose (blood sugar)
  • Major breakdown product of carbohydrate digestion
  • Fuel used to make ATP

Cellular Respiration

• Oxygen-using events within the cells to create ATP from ADP.
• Carbon leaves cells as carbon dioxide (CO2)
• Hydrogen atoms combine with oxygen to form water
• Energy produced adds a phosphorus to ADP to produce ATP
• ATP can be broken down to release energy for cellular use

Metabolic Pathways Involved in Cellular Respiration

• Glycolysis—energizes a glucose molecule so it can be split into two pyruvic acid molecules and yield ATP.
• Krebs cycle—produces virtually all the carbon dioxide and water resulting from cell respiration, yields small amounts of ATP
• Electron transport chain—hydrogen atoms removed during glycolysis and the Krebs cycle are delivered to protein carriers; hydrogen is split into hydrogen ions and electrons in the mitochondria; electrons give off energy in steps to enable the production of ATP

Metabolism of Carbohydrates

• Hyperglycemia—excessively high levels of glucose in the blood; Excess glucose stored in body cells as glycogen; if blood glucose levels are too high, excesses are converted to fat.
• Hypoglycemia—low levels of glucose in the blood; Liver breaks down stored glycogen and releases glucose into the blood

Fat Metabolism

• Handled mostly by the liver
• Use some fats to make ATP
• Synthesizes lipoproteins, thromboplastin, and cholesterol
• Releases breakdown products into the blood
• Body cells remove fat (and cholesterol) to build membranes and steroid hormones.
• Fats must first be broken down to acetic acid, in mitochondria, acetic acid is completely oxidized, producing water, carbon dioxide, and ATP
• Acidosis (ketoacidosis) results from incomplete fat oxidation in which acetoacetic acid and acetone accumulate in the blood; breath has a fruity odor. common with "no carbohydrate" diets, uncontrolled diabetes mellitus, starvation.

Protein Metabolism

• Proteins are conserved by body cells (they are used for most cellular structures)
• Ingested proteins are broken down to amino acids
• Cells remove amino acids to build proteins
• Synthesized proteins are actively transported across cell membranes
• Amino acids are used to make ATP only when proteins are overabundant or there is a shortage of other sources.
• Amine groups are removed from proteins as ammonia, the rest of the protein molecule enters the Krebs cycle in mitochondria; the liver converts harmful ammonia to urea.

Role of the Liver in Metabolism

• Several roles in digestion, manufactures bile, detoxifies drugs and alcohol, degrades hormones, produces cholesterol, blood proteins (albumin and clotting proteins), plays a central role in metabolism, can regenerate.

Metabolic Functions of the Liver

• Glycogenesis—"glycogen formation"; glucose molecules are converted to glycogen, stored in the liver.
• Glycogenolysis—"glucose splitting"; glucose is released from the liver after conversion from glycogen.
• Gluconeogenesis—"formation of new sugar"; glucose is produced from fats and proteins.
• Fats and fatty acids absorbed by the liver; some are oxidized for liver energy production. Rest are broken down into simpler compounds and released into bloodstream.

Cholesterol Metabolism and Transport

• Cholesterol is not used to make ATP; Functions of cholesterol:
  • Serves as a structural basis of steroid hormones and vitamin D
  • Is a major building block of plasma membranes
• Most cholesterol is produced in the liver (85 percent)
• Cholesterol and fatty acids cannot freely circulate in the bloodstream; transported by lipoproteins (lipid-protein complexes). -Low-density lipoproteins (LDLs) transport to body cells; rated "bad lipoproteins" since they can lead to artherosclerosis
  • High-density lipoproteins (HDLs) transport from body cells to the liver

Body Energy Balance

• Energy intake = total energy output (heat + work + energy storage)
• Energy intake is the energy liberated during food oxidation
• Energy produced during glycolysis, Krebs cycle, and the electron transport chain
• Energy output: energy lost as heat (60 percent) and energy stored as fat or glycogen.

Regulation of Food Intake

• Body weight is usually relatively stable
• Energy intake and output remain about equal
• Mechanisms that may regulate food intake: levels of nutrients in the blood, hormones, body temperature, psychological factors.

Metabolic Rate and Body Heat Production

• Basic metabolic rate (BMR)—amount of heat produced by the body per unit of time at rest.
• Average BMR is about 60 to 72 kcal/hour
• Kilocalorie (kcal) is the unit of measure for the energy value of foods and the amount of energy used by the body
• Factors that influence BMR: surface area, gender, age, amount of thyroxine produced. More thyroxine means a higher metabolic rate.
• Total metabolic rate (TMR) = Total amount of kilocalories the body must consume to fuel ongoing activities; TMR increases with an increase in body activity; TMR must equal consumed calories to maintain weight homeostasis and a constant weight.

Body Temperature Regulation

• Most energy is released as foods are oxidized, most energy escapes as heat.
• The body has a narrow range of homeostatic temperature, must remain between 35.6°C and 37.8°C (96°F to 100°F)
• The body's thermostat is in the hypothalamus
• Initiates heat-loss or heat-promoting mechanisms
• Heat-promoting mechanisms:
  • Vasoconstriction of blood vessels
  • Blood rerouted to deeper, more vital body organs
  • Shivering (contraction of muscles produces heat)
• Heat-loss mechanisms:
  • Heat loss from the skin via radiation and evaporation
  • Skin blood vessels and capillaries are flushed with warm blood
  • Evaporation of perspiration cools the skin
• Fever—controlled hyperthermia (results from infection, cancer, allergic reactions, CNS injuries); if the body thermostat is set too high, body proteins may be denatured and permanent brain damage may occur.

Developmental Aspects of the Digestive System

• The alimentary canal is a continuous tube by the fifth week of development
• Digestive glands bud from the mucosa of the alimentary tube
• The developing fetus receives all nutrients through the placenta
• In newborns, feeding must be frequent, peristalsis is inefficient, and vomiting is common
• Newborn reflexes:
  • Rooting reflex (helps infant find the nipple)
  • Sucking reflex (helps infant hold on to the nipple and swallow)
• Teething begins around age six months
• Problems of the digestive system:
  • Gastroenteritis (inflammation of the gastrointestinal tract)
  • Appendicitis (inflammation of the appendix)
  • Metabolism decreases with old age
  • Middle-age digestive problems
  • Ulcers
  • Gallbladder problems
• Activity of digestive tract in old age: fewer digestive juices, peristalsis slows, diverticulosis and cancer are more common.