Digestive Physiology

Digestion

Digestion is the process by which food is broken down into smaller components that can be absorbed by the body.

• Chemical Digestion: Involves enzymatic breakdown of food molecules (e.g., proteins into amino acids).

• Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning in the stomach).

• Peristaltic Waves: Rhythmic contractions of smooth muscle that propel food through the digestive tract.

• Chyme: The semi-fluid mass of partly digested food expelled by the stomach into the duodenum.

Example: Chewing bread (mechanical) and salivary amylase breaking down starch (chemical).

Role of Dietary Fats

Dietary fats are essential for energy storage, cell membrane structure, and absorption of fat-soluble vitamins (A, D, E, K).

• Why needed: Provide essential fatty acids, support hormone production, and aid in nutrient absorption.

Metabolism

Metabolism, Anabolism, and Catabolism

Metabolism encompasses all chemical reactions in the body, divided into anabolism and catabolism.

• Anabolism: Synthesis of complex molecules from simpler ones (e.g., protein synthesis).

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy (e.g., glycolysis).

Cellular Respiration

Cellular respiration is the process by which cells generate ATP from nutrients.

• End goal: Production of ATP for cellular energy.

• Major processes (in order):

  1. Glycolysis

  2. Krebs Cycle (Citric Acid Cycle)

  3. Electron Transport Chain

Cholesterol

Cholesterol is a lipid molecule essential for cell membrane structure and hormone synthesis.

• Is it always harmful? No. It is necessary for normal body function, but high levels of LDL cholesterol are associated with cardiovascular disease.

Hypoproteinemia

Hypoproteinemia is a condition characterized by abnormally low levels of plasma proteins, especially albumin.

• Key sign: Edema (swelling due to fluid accumulation in tissues).

Acid-Base Balance and Diabetes

Metabolic and Respiratory Acidosis

• Metabolic Acidosis: Decrease in blood pH due to accumulation of acids or loss of bicarbonate.

• Respiratory Acidosis: Decrease in blood pH due to hypoventilation and CO2 retention.

Diabetes Mellitus and Diabetes Insipidus

• Diabetes Mellitus: Disorder of glucose metabolism due to insulin deficiency or resistance.

• Diabetes Insipidus: Disorder of water balance due to ADH deficiency or insensitivity, leading to excessive urination.

Hepatocytes

Hepatocytes are the main functional cells of the liver, involved in metabolism, detoxification, and bile production.

Mechanisms of Heat Production

• Shivering thermogenesis

• Non-shivering thermogenesis (e.g., brown fat metabolism)

• Increased metabolic rate (e.g., via thyroid hormones)

• Vasoconstriction (reduces heat loss)

Digestive System Structure & Function

Key Organs and Structures

• Mesenteries: Folds of peritoneum that support and stabilize abdominal organs.

• Liver: Produces bile, processes nutrients, detoxifies substances.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Secretes digestive enzymes and hormones (insulin, glucagon).

• Chief cells: Stomach cells that secrete pepsinogen.

• Esophagus: Muscular tube transporting food from mouth to stomach.

Renal Physiology

Nephron Structure and Function

• Nephron: Functional unit of the kidney, responsible for urine formation.

• Juxtaglomerular Apparatus: Regulates blood pressure and filtration rate.

• Loop of Henle: Concentrates urine by creating a gradient in the medulla.

• Proximal Convoluted Tubule: Reabsorbs nutrients, ions, and water.

• Peritubular Capillaries: Surround tubules, involved in reabsorption and secretion.

• Collecting Duct: Final site for water reabsorption and urine concentration.

Macula Densa and Granular Cells

• Macula Densa: Specialized cells in the distal tubule that sense sodium concentration.

• Granular Cells: Secrete renin in response to low blood pressure.

Micturition

Micturition is the process of urination, triggered by stretch receptors in the bladder wall.

Hormonal Regulation of Renal Function

Key Hormones and Their Effects

Digestive Enzymes and Secretions

Major Enzymes/Agents and Their Sources

Processes and Pathways

Sequence of Urine Formation and Elimination

1. Glomerulus

2. Bowman's capsule

3. Proximal convoluted tubule

4. Loop of Henle

5. Distal convoluted tubule

6. Collecting duct

7. Renal pelvis

8. Ureter

9. Bladder

10. Urethra

Comparisons and Clinical Applications

Heat Stroke vs. Heat Exhaustion

ACE Inhibitors and Blood Pressure

ACE inhibitors lower blood pressure by blocking the conversion of angiotensin I to angiotensin II, leading to vasodilation, decreased aldosterone secretion, increased urine output, and reduced blood volume and peripheral resistance.

Nutrition and Health

Omega-3 Fatty Acids and HDL

• Benefits: Reduce inflammation, lower triglycerides, increase HDL ("good" cholesterol), and support cardiovascular health.

• Food sources: Fatty fish (salmon, mackerel), flaxseeds, walnuts, chia seeds.

Key Equations

• General formula for cellular respiration:

• Glomerular Filtration Rate (GFR):

Where: = filtration coefficient, = glomerular hydrostatic pressure, = Bowman's capsule pressure, = glomerular oncotic pressure, = Bowman's capsule oncotic pressure.