Digestive System: Organs and Functions

Digestive System Overview

• Food travels from the mouth to the anus through the digestive system.
• Each digestive organ contributes to the digestion process.

Digestive Organs

• Mouth: Initiates mechanical digestion
• Pharynx: Acts as a passageway for food.
• Epiglottis: Prevents food from entering the trachea.
• Esophagus: Transports food to the stomach using peristalsis.
• Stomach: Stores food while performing mechanical and chemical digestion and produces chyme and contains intrinsic factor for B12 absorption
• Small Intestine:
  • Duodenum: Receives bile from the liver/gallbladder and pancreatic enzymes from the pancreas.
  • Jejunum & Ileum: Primary site for nutrient absorption and lined with villi and microvilli to increase the surface area.
• Large Intestine: Absorbs water, electrolytes, and vitamins (B, K), houses gut bacteria (microbiota), forms, and stores feces.
• Rectum: Stores the feces and initiates the defecation reflex.
• Anus: Releases feces. The internal anal sphincter is involuntary, and the external anal sphincter is voluntary.

Major and Accessory Organs

• Major organs directly involved in digestion include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.
• Accessory organs support digestion but food does not pass through them: teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
• Mouth is responsible for ingestion, mechanical digestion (chewing), and chemical digestion (salivary enzymes).
• Pharynx propels food to esophagus and helps in swallowing.
• Esophagus transports food to the stomach via peristalsis.
• Stomach mechanically mixes food and starts protein digestion via HCl and pepsin.
• Small Intestine is where most digestion and nutrient absorption occurs in the duodenum, jejunum, and ileum.
• Large Intestine absorbs water and vitamins and then forms and stores feces.
• Rectum stores feces before elimination.
• Anus releases feces via defecation.
• Teeth aid in mechanical digestion, specifically chewing.
• Tongue mixes food and helps form a bolus.
• Salivary Glands (parotid, submandibular, sublingual) produce saliva with enzymes like amylase and lipase.
• Liver produces bile to emulsify fats.
• Gallbladder stores and releases bile into the duodenum.
• Pancreas produces digestive enzymes (proteases, lipases, amylase, nucleases) and bicarbonate to neutralize stomach acid.

Digestive Processes

• Mouth is responsible for ingestion and both mechanical and some chemical digestion.
• Stomach is responsible for mechanical digestion in addition to beginning protein digestion.
• Small Intestine is a site for most chemical digestion, absorption, and segmentation.
• Large Intestine absorbs water while also forming feces.
• Rectum/Anus is responsible for defecation.
• Ingestion is the act of eating.
• Propulsion moves food along via peristalsis.
• Peristalsis squeezes material through the digestive tube using smooth muscle.
• Mechanical Digestion is chewing, churning, etc.
• Segmentation mixes food in the small intestine.
• Chemical Digestion occurs when enzymes digest food molecules.
• Absorption of nutrients occurs into the body.
• Defecation is when waste material is released as feces.

Layers of the Alimentary Canal

• Serosa:
  • Outermost layer with the function of protection and providing supply (blood, nerves, etc).
  • Tissue type is serosal epithelium.
• Muscularis externa:
  • Layer responsible for producing segmentation and peristalsis.
  • Tissue type is smooth muscle.
• Submucosa:
  • Layer that has elastic fibers to allow stretching.
  • Tissue type is areolar connective tissue.
• Mucosa:
  • No data listed in text

Major Digestive Enzymes

• Amylase digests starch.
• Lipase digests lipids, although it is produced in very little amounts.
• Lysozyme is an antibacterial enzyme.

Teeth Information

• 20 deciduous teeth (fall out) and 32 permanent teeth.
• Incisors are chisel-shaped for cutting.
• Canines are fang-like teeth that tear or pierce.
• Premolars (bicuspids) have broad crowns with rounded cusps to grind or crush.
• Molars have broad crowns with rounded cusps and function as the best grinders.

Stomach Sphincters and Gastric Secretion

• Cardiac sphincter allows food to enter the stomach.
• Pyloric sphincter keeps food in the stomach.
• Phases of gastric secretion:
  • Cephalic phase: Triggered by smell, sight, or thought of food, leading to gastric secretions.
  • Gastric phase: Dramatically increases secretions and occurs when food reaches the stomach.
  • Intestinal phase: Occurs as food enters the duodenum.

Gland Cells in Digestion

• Mucous neck cells secrete acidic mucus to buffer gastric acid and prevent damage to the epithelium.
• Parietal cells secrete HCl and intrinsic factor.
  • HCl kills microbes.
  • Intrinsic factor aids in B12 absorption.
• Chief cells secrete pepsinogen and lipases.
  • Pepsinogen converts to pepsin for acid-protein digestion.
  • Gastric lipases digest fats.

Small Intestine Regions

• Duodenum: 25 cm long, where most secretions are produced and deposited.
• Jejunum: 2.5m long.
• Ileum: 3.6 m long, ending at the ileocecal sphincter.
• Villi and microvilli increase the surface area of the small intestine.
• Intestinal crypts produce secretions.

Intestinal Cells

• Enterocytes are basic epithelium cells.
• Goblet cells produce mucus.
• Enteroendocrine cells aid digestive activity.
• Paneth cells fight infection with enzymes.
• Stem cells perform mitosis to form all cell types.

Large Intestine Regions

• Composed of the cecum (with appendix), colon, rectum, anal canal, and anus.
• Cecum receives food from the ileum.
• Appendix houses beneficial bacteria and lymphoid tissue.
• Rectum allows gases to pass by feces.
• Internal anal sphincter is involuntary and relaxes and is made of smooth muscle.
• External anal sphincter is voluntary and is the last resort and is made of skeletal muscle.

Nutrition and Metabolism

• Macronutrients include carbohydrates, lipids, proteins, and complete/incomplete proteins.
• Complete proteins must contain all of the body’s amino acid requirements for tissue maintenance and growth.
• Micronutrients include vitamins and minerals.
• Vitamins are organic molecules containing carbon that mostly function as coenzymes.

Essential vs Non-Essential Nutrients

• An essential nutrient must be obtained from the diet, since the body cannot synthesize it.
• Non-essential nutrients are made naturally by the body.
• Complete proteins have 9 essential amino acids.
• Incomplete proteins do not have all 9 essential amino acids, and may lack one or more.
• Infants need 10 essential amino acids, and adults only need 8.

Protein Synthesis

• All amino acids for a given protein must be present when synthesizing.
• Sufficient calories must be available for synthesis because, otherwise, proteins will be used for energy instead.
• Anabolic hormones, which include growth hormones, estrogens, and glucocorticoids, encourage protein synthesis and growth.

Fat-Soluble Vitamins

• Fat-soluble vitamins: A, D, E, K;
• They are absorbed with lipids and are easily stored.
• Excess doses can lead to toxicity.

Water-Soluble Vitamins

• B complex and C, are water-soluble.
• Excess is urinated out and not stored.

Major Minerals

• Needed in large amounts, specifically, more than 200 mg/day
• Includes Calcium (Ca), Phosphorus (P), Sulfur, Potassium, Chlorine (Cl), Sodium (Na), and Magnesium (Mg).

Trace Minerals

• Needed in small amounts (10 or more).

Metabolic Reactions

• Anabolism: Reactions that build larger molecules and require energy (endergonic).
• Catabolism: Reactions that break down large molecules and release energy (exergonic).
• Phosphorylation: Powers up a molecule via covalent bond formation.
• Reduction: Gain of electrons (RIG).
• Oxidation: Loss of electrons (OIL).

Cellular Respiration

• Glycolysis:
  • Pyruvate makes acetyl coa used for citric acid cycle
  • What Goes In: glucose, 2 ATP
  • Products: 4 ATP (2 Net), 2 NADH, 2 Pyruvate
• Transition reaction:
  • What Goes In: 2 pyruvate, coA
  • Products: 2 NADH, 2 acetylcoA, CO2
• Citric acid cycle:
  • Produces NADH (electron carriers) and co2
  • What goes in: 2 acetyl coA
  • Products: 2 ATP, 6 NADH, 2 FADH2, CO2
• Electron transport chain:
  • What Goes In: O2, NADH, FADH2
  • Products: 34 ATP, metabolic water

Metabolic Reactions

• Emulsification: Breaks down fat globules into small pieces from bile salts.
• Glycogenesis: Makes glycogen from glucose when ATP/sugar levels are high.
• Glycogenolysis: Breaks glycogen down into glucose when they are low.
• Gluconeogenesis: New glucose made from non-carbohydrates.
• Lipogenesis: Converts glycerol, fatty acids, and glucose to triglycerides.
• Lipolysis: The opposite of lipogenesis.
• Protein Metabolism:
  • AAs can be used for protein synthesis or energy.
  • Proteins are regularly broken down and recycled.
  • Extra proteins are not stored and are oxidized for energy or converted into fats.

Absorptive vs Postabsorptive States

• Absorptive state: Lasts for about 4 hours after eating. Anabolic reactions dominate inside cells, especially in the liver, muscles, and fat. Insulin stimulates glycogenesis, lipogenesis, and protein synthesis.
• Postabsorptive state is after the gut is empty, and catabolic reactions dominate.
  • Glycogenolysis, gluconeogenesis, lipolysis, and protein breakdown all occur.
  • Glucagon stimulates the first three.
  • Glucose sparing can occur if the postabsorptive state goes on too long.
  • Glucose is saved for the brain and other molecules are used for ATP synthesis.

Energy Balance

• Energy intake equals energy output over the long term.
• Intake is from oxidation of food molecules.
• 60% of output is immediate heat loss during reactions.
• The remainder is used for work or stored as fat or glycogen.
• Imbalance:
  • Obesity: BMI is over 30.
  • Overweight: BMI is between 25 and 30.
  • Weight loss/starvation/underweight occurs if output exceeds intake.

Basal and Total Metabolic Rates

• Basal Metabolic Rate: kcal/m2 (of body surface area)/ hour.
  • Energy the body needs to perform its most essential activities (involuntary).
  • Measured at rest, in a postabsorptive state, and at a comfortable air temperature.
  • Higher for those who are thinner, younger, male, stressed, have a high temperature, or have high thyroxine levels.
• Total Metabolic Rate: Kcal consumption including all activities.
  • Energy the body needs to perform all voluntary and involuntary functions.
  • Exercise dramatically raises TMR.
  • Higher in absorptive state.
  • Higher for athletes (fit people) as muscle consumes calories. Factors in Heat Production and Heat Loss
• Metabolism: All biochemical reactions in the body.
• Heat production results from muscular activity, exergonic, catabolic reactions, increases in total metabolic rate, shivering, and increased thyroxine.
• Heat loss occurs through anabolic, endergonic, cutaneous vasodilation, sweating, and relaxing.

Urinary System Functions

• Regulates total water volume and solute concentration.
• Regulates ion concentration in extracellular fluid.
• Ensures long-term acid-base balance.
• Excretes metabolic wastes, toxins, and drugs.
• Produces EPO to regulate blood pressure and renin to regulate RBC production.

Kidney Anatomy

• Cortex: Outer region, and the site of urine filtration.
• Medulla: Deep to the cortex, containing lots of tubules and capillaries with renal pyramids and columns.
• Pelvis: Funnel that connects to ureters and calyces that are extensions of the pelvis. Minor calyces merge into major calyces, then empty into the ureter.
• Nephron: Is the kidney's functional unit, producing urine via filtration.
• Renal corpuscle: Includes Glomerulus and Bowman’s capsule.
  • Glomerulus:
    • Ball of fenestrated capillaries (porous).
    • Produces filtrate, plasma-derived fluid that will form urine, filtrate forced into glomerular capsule.
• Glomerular (Bowman’s capsule)
  • Parietal layer: Simple squamous.
  • Visceral layer: Branching epithelial cells and podocytes.
• Podocytes:
  • Capsular cells that are stuck to capillaries.
  • Filtrate enters through silts between cells.
• Renal tubule:
  • Proximal convoluted tubule:
    • Proximal to corpuscle, is a simple cuboidal, and is responsible for reabsorption and secretion.
  • Nephron loop:
    • Involved in reabsorption and secretion, with descending and ascending limbs, composed of simple squamous.
  • Distal convoluted tubule:
    • Responsible for reabsorption and secretion and is simple cuboidal.
• Collecting duct: Receives filtrate from many nephrons, found in medullary pyramids, and is used for reabsorption/secretion. It takes urine to ureters through minor calyces.
• Nephron capillary beds:: Glomerulus for initial filtration, has:
  • Afferent arteriole that feeds the glomerulus
  • Efferent arteriole that drains glomerulus
• High BP:
  • Afferent arterioles have a larger diameter than efferent arterioles.
  • High resistance vessels.
• Peritubular capillaries: Surround tubules, have low pressure and are porous, allowing for absorption of water and solutes.
• Vasa recta: Surrounds nephron loops of juxtamedullary nephrons and is a type of peritubular capillaries that concentrates urine.

Nephron Classes

• Cortical nephrons:
  • Most numerous (85%) and is mostly found in cortex
  • Has a little loop into the medulla, produces regular, everyday urine, and peritubular capillaries surround tubules that allow for absorption of water and solutes are low pressure.
• Juxtamedullary nephrons:
  • Less common whose corpuscles are in the cortex
  • Has a long loop into the medulla, produces concentrated urine, and the vasa recta surrounds nephron loops of juxtamedullary nephrons primarily in juxtamedullary nephrons, which serve as a type of peritubular capillaries that concentrates urine.

Glomerular Filtration

• A passive process driven by hydrostatic blood pressure that occurs in the glomerulus.
• Glomerular filtration rate: Volume of filtrate formed per minute; 120-125 mL/min.
• Afferent arterioles feed the glomerulus, and efferent arterioles drain the glomerulus.

Renal Autoregulation

• Intrinsic controls regulates by the kidney .
  • Myogenic Mechanism:
    • If BP increases, afferent arterioles constrict, and vice versa.
    • Started by muscles of afferent arteriole.
    • Maintains normal Net Filtration Pressure and Glomerular Filtration Rate.
  • Tubuloglomerular Feedback Mechanism:
    • Juxtaglomerular complex responds to NaCl concentration and GFR (ascending limb).
    • If GFR and Nacl concentration increases - ATP is released —> vasoconstriction of afferent arteriole and decreased GFR and vice versa.

Extrinsic Controls

• Regulation from outside the kidney (to stabilize BP).
• Sympathetic nervous system controls:
  • Baroreceptor reflex that baroreceptors respond to low systemic BP, causing norepinephrine and epinephrine (sympathetic NS hormones) release, which causes afferent arteriole vasoconstriction (and system-wide), and reduces GFR causing less urine/filtrate produced, less plasma lost to help maintain BP
• Renin-angiotensin mechanism:
  • Low BP causes JGC to release renin.
  • Renin converts angiotensin I to angiotensin II.
  • Angiotensin II causes systemic vasoconstriction (raises BP).
  • Angiotensin II also stimulates aldosterone release.
  • Aldosterone causes Na and water absorption (raises BP).
  • High blood pressure signals the reverse process for this mechanism

Tubular Reabsorption

• Reabsorption of most filtrate from the tubules.
• Active reabsorption requires ATP and goes against the concentration gradient.
• Passive reabsorption does not require ATP and follows the concentration gradient.
  • Na reabsorption (active): - Occurs in the entire tubule except for the descending limb of the loop. - Na gradient powers reabsorption of most other molecules.
  • Water, glucose, amino acids, and ions (Cl, Ca) urea are absorbed in varying amounts.
    • "Water (and stuff) follows salt”.
    • 65% of filtrate volume is reabsorbed by the PCT.
    • Water can be reabsorbed everywhere but the ascending limb of the loop.
    • Solutes can be reabsorbed everywhere but the descending limb of the loop.
• Hormones regulating reabsorption:
  • Aldosterone stimulates Na (and thus water) reabsorption.
  • Antidiuretic hormone (ADH) stimulates water reabsorption by collecting ducts.
  • Atrial natriuretic peptide (ANP) inhibits Na reabsorption, increasing water loss and decreasing BP. Parathyroid hormone (PTH) stimulates Ca reabsorption.
• Tubular secretion is the secretion back into the tubule.
  • Lose drugs, urea, uric acid, and some ions in urine, as well as some actively/ passively secreted.

Components of Urine

• 95% water and 5% solutes under normal conditions.
• Include urea, Na, K, PO4, SO4, creatinine, and uric acid, in that order of concentration.
  • Creatinine is the breakdown product of nucleic acids.
  • Uric acid is the product of proteins.
  • Urea is a product of carbs.
• Abnormal substances in the urine indicate bad conditions and glucose indicates glycosuria.
• Proteins indicate proteinuria and albuminuria.
• Ketone bodies indicate ketonuria.
• Hemoglobin indicates hemoglobinuria.
• Bile pigments indicate bilirubinuria.
• Erythrocytes indicate hematuria.
• Leukocytes indicate pyuria.

Elimination of Urine

• Ureters: One exits each kidney containing smooth muscle in their walls to move urine.
• Urinary Bladder: Urine storage that can hold 500ml or more and whose detrusor muscle squeezes out urine and is made of smooth muscle. Contains transitional epithelium for stretching and internal (involuntary) and external sphincters where the internal involuntary sphincter used when you do not need to use the restroom. However, if you have an urge to use the restroom badly, the internal sphincter opens and external voluntary sphincter may be used.
• Urethra:
• Exits the bladder, has urogenital function in males(used for urination and reproduction, approximately 20 cm long, and infection is less likely. Consequently, it is harder for bacteria to get through the longer urethra and into the bladder
• Urinary only in females, is 3 or 4 cm long, making it easier for bacteria to enter into the bladder and more prone to infection
• Micturition: Urination process

Fluids and Electrolytes

• Body fluid compartments and their proportions are:
  • Intracellular fluid (ICF): 2/3
  • Extracellular fluid (ECF): 1/3
  • Plasma: 20% of ECF
  • Interstitial fluid (IF): 80% of ECF
• Electrolytes dissociate into charged ions in water (Ex- NaCl).
• Nonelectrolytes do not associate in water (Ex- Glucose, urea, lipids).
• Fluid movement between compartments is based on osmotic and hydrostatic pressures.
  • Power movement with leaks across capillary walls between plasma and interstitial fluid.
  • Fluids also move across plasma membranes.
• Na+ and Cl- are concentrated in plasma/IF.
• K+ and HPO4-concentrated in ICF.
• Water intake:
  • Metabolism- 10%
  • Foods- 30%
  • Beverages- 60%
• Water output:
  • Feces- 4%
  • Sweat- 8%
  • Insensible loss via skin and lungs- 28%
  • Urine- 60%
• Hormones that regulate water: ADH, aldosterone (sodium reabsorption), and ANP(atrial natriuretic peptide).
  • ADH and aldosterone decrease water loss and increase water reabsorption to increase blood pressure.
  • ANP does the opposite, increasing water loss and decreasing blood pressure.
• Dehydration results in loss of water by ECF, leading to loss by ICF.
• Hypotonic hydration occurs due to renal insufficiency or overconsumption of water.
• Insensible loss is evaporation via diffusion through skin and lungs (not the same as sweat).
• Sensible loss is measurable and includes urine, sweat, and feces.

Thirst Mechanism

• Osmoreceptors in the hypothalamus that detect solute concentration in blood and, when solutes are too high, trigger the brain to drink.
• Dry mouth stimulates drinking and water quickly quenches, which prevents overconsumption.
• Baroreceptors are present in pressure receptors in blood vessels. Low pressure indicates the need to increase blood volume and drink.

Hormonal Regulators of Water Loss

• ADH, aldosterone (sodium reabsorption), and ANP(atrial natriuretic peptide).
• ADH and aldosterone decrease water loss and increase water reabsorption thus increasing blood pressure.
• ANP does the opposite, increasing water loss and decreasing blood pressure.

Effects of Dehydration

• If more water than solutes is lost, cells shrink.

Effects of Hypotonic Hydration

• If more water than solutes is gained, cells swell.

Sodium Regulation

• Aldosterone: Causes Na+ reabsorption from the distal convoluted tubule and collecting ducts. Water follows Na+, raising BP, and is stimulated by angiotensin II from renin- angiotensin mechanism for release.
• Atrial Natriuretic peptide: Has the opposite effect, which inhibits Na+ and water reabsorption and vasoconstriction, while also reducing BP.
• Estrogens: Stimulate Na+ and water reabsorption while also causing "water retention” during parts of the menstrual cycle.
• Progesterone: A mild diuretic, makes us urinate
• Glucocorticoids: Stimulate Na+ (and water) reabsorption

Potassium & Calcium Regulation

• Potassium: Affects resting membrane potential in neurons and muscle cells (especially cardiac muscle),Increases in ECF (K+) (hyperkalemia) causes decreased RMP, causing depolarization, followed by reduced excitability or decreases in ECF (K+) (hypokalemia) causes hyperpolarization and unresponsiveness.Disruption in (K+) (hyper or hypokalemia) in the heart can interfere with electrical conduction, leading to sudden death. Aldosterone also stimulates K+ secretion into the filtrate.
• Calcium: Ca2+ important for clotting, membrane permeability, neuromuscular excitability.Parathyroid hormone increases Ca+ reabsorption increasing osteoclast activity in bones as well and Ca+ absorption in intestines.Hypocalcemia: Increases neuromuscular excitability and can lead to muscle tetany Hypercalcemia: Inhibits neurons and muscle cells and may cause heart arrhythmias.

Acid-Base Balance

• Bicarbonate buffer system: H+ increase, combines with HC03- and shifts equation left. As a result, base binds H+, removing it from the solution and shift equation right.
  • HCO3- bicarbonate
  • H2CO3- carbonic acid
• Respiratory regulation- Eliminates CO2 - During CO2 unloading, reactions shifts left (H+ into H2O. Alkalosis- high pH decreases respiratory rate, increases CO2 and reaction shift right to maintain pH. If PCO2 in blood rises (hypercapnia), activates medullary chemoreceptors, causing increased respiratory rate and depth.Rising plasma H+ (acidosis) activates peripheral chemoreceptors increasing respiratory rate and depth which Both cause more CO2 to be removed from the blood, pushing reaction to left, which reduces H+ concentration. Alkalosis depresses respiratory center, and as a result, the respiratory rate and depth decrease, causing H+ concentration to increase.
• Renal regulation- kidneys regulate the amount of bicarbonate in the blood by conserving, making new HCO3, and secreting these and kidneys secrete or retain H+. H+ concentration controlled by chemical buffers, respiratory centers, and renal mechanisms Bicarbonate, phosphate, and protein buffers Also, pick up or release cations (+) and/or anions (-), altering blood pH and deeper, rapid breathing increases blood pH (by dumping CO2).
  • Kidney activity: Vice versa for shallow, slow breathing, retain and generate HCO3- to increase pH, they also secrete H+ to increase pH, They excrete HCO3- to decrease pH.