Digestive, Nutrition, Urinary, and Acid-Base Balance: Comprehensive Study Guide

Digestive System

Digestive Organs and Their Functions

The digestive system consists of specialized organs that process food, absorb nutrients, and eliminate waste. Each organ has distinct layers and cell types that contribute to its function.

• Mouth: Initiates mechanical and chemical digestion; contains salivary glands.

• Esophagus: Transports food via peristalsis.

• Stomach: Secretes acid and enzymes; churns food.

• Small Intestine: Major site of nutrient absorption; contains villi and microvilli to increase surface area.

• Large Intestine: Absorbs water and electrolytes; forms feces.

Layers of the Intestines:

• Mucosa: Innermost layer; contains epithelial cells, goblet cells (mucus secretion), and enteroendocrine cells (hormone secretion).

• Submucosa: Connective tissue with blood vessels and nerves.

• Muscularis: Smooth muscle responsible for peristalsis and segmentation.

• Serosa: Outermost layer; protective.

Cell Types: Epithelial cells (absorption), goblet cells (mucus), chief cells (pepsinogen), parietal cells (HCl), enteroendocrine cells (hormones).

Constituents of Saliva

Saliva is produced by salivary glands and aids in digestion and oral health.

• Water: Moistens food.

• Electrolytes: Na+, K+, Cl-, HCO3-.

• Enzymes: Salivary amylase (digests starch), lingual lipase (digests fats).

• Mucus: Lubricates food.

• Antibacterial compounds: Lysozyme, IgA.

Peristalsis vs. Segmentation

• Peristalsis: Wave-like contractions that propel food through the digestive tract.

• Segmentation: Localized contractions that mix food and increase contact with absorptive surfaces.

Physical vs. Chemical Digestion

• Physical Digestion: Mechanical breakdown (chewing, churning) increases surface area for enzymes.

• Chemical Digestion: Enzymatic breakdown of macromolecules into absorbable units (e.g., proteins to amino acids).

• Outcome: Physical digestion prepares food; chemical digestion produces nutrients for absorption.

Enzymes, Hormones, and Chemicals in Digestion

• Enzymes: Amylase (salivary, pancreatic), pepsin (stomach), lipase (pancreatic).

• Hormones: Gastrin (stimulates acid), secretin (stimulates bicarbonate), cholecystokinin (stimulates bile and pancreatic enzymes).

• Chemicals: HCl (stomach acid), bile (emulsifies fats).

• Origin: Enzymes from salivary glands, stomach, pancreas; hormones from enteroendocrine cells.

Qualities of the GI System

• Surface Area: Villi and microvilli in small intestine maximize absorption.

• Mucosal Barrier: Protects stomach lining from acid; mucus and tight junctions.

• Predominant Processes: Stomach (protein digestion), small intestine (absorption), large intestine (water/electrolyte absorption).

GI System Disorders and Effects

• Ulcers: Damage mucosal barrier; can lead to bleeding and impaired digestion.

• Malabsorption: Reduced surface area or enzyme deficiency impairs nutrient uptake.

• Inflammatory conditions: Affect motility and absorption.

Additional info: Disorders can alter GI surface area, enzyme secretion, and absorption efficiency.

Urinary System

Organ Structure and Function: The Nephron

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine.

• Renal Corpuscle: Glomerulus (capillary network) and Bowman's capsule (filtration site).

• Proximal Convoluted Tubule (PCT): Reabsorbs water, ions, nutrients.

• Nephron Loop (Loop of Henle): Descending limb (water reabsorption), ascending limb (ion reabsorption).

• Distal Convoluted Tubule (DCT): Fine-tunes ion and water balance.

• Collecting Duct: Final adjustments; regulated by hormones.

Composition and Flow of Urine

• Normal Constituents: Water, urea, creatinine, ions (Na+, K+, Cl-).

• Abnormalities: Glucose (diabetes), proteins (kidney damage), blood (trauma/infection).

• Flow: Nephron → collecting duct → renal pelvis → ureter → bladder → urethra.

Diuretics and Diuresis

• Diuretics: Substances that increase urine output (e.g., caffeine, certain medications).

• Diuresis: Increased production of urine.

Cell Types in Urinary Physiology

• Podocytes: Specialized cells in Bowman's capsule; filtration.

• Principal cells: Collecting duct; respond to hormones (ADH, aldosterone).

• Intercalated cells: Acid-base balance.

Qualities of the Urinary System (Nephron Loop)

• Surface Area: Extensive tubules for reabsorption.

• Osmotic Pressure: Drives water movement; countercurrent mechanism in loop of Henle.

• Hydrostatic Pressure: Drives filtration in glomerulus.

Hormones Affecting Urine Output

• Antidiuretic Hormone (ADH): Increases water reabsorption; decreases urine output.

• Aldosterone: Increases Na+ reabsorption; increases water retention.

• Atrial Natriuretic Peptide (ANP): Increases Na+ excretion; decreases blood volume/pressure.

Additional info: Hormonal regulation is crucial for maintaining blood volume and pressure.

Acid-Base Balance

Fluid Volume and Ion Regulation

The body maintains fluid volume and acid-base balance through regulation of ions and water.

• Major Ions: Na+ (extracellular), K+ (intracellular), Ca2+, Cl-, HCO3-.

• Disorders: Dehydration (fluid loss), hypotonic hydration (excess water), edema (fluid accumulation).

• Retention of Ions: Affects osmotic balance and fluid distribution.

Location and Role of Major Ions

• Na+: Most abundant in extracellular fluid; regulates fluid volume and nerve function.

• K+: Most abundant in intracellular fluid; important for cell function.

• Ca2+: Bone structure, muscle contraction.

• Cl-: Follows Na+; maintains osmotic balance.

• HCO3-: Buffer for blood pH.

Hormonal Regulation of Ions

• Aldosterone: Increases Na+ reabsorption; released in response to low Na+ or high K+.

• ANP: Decreases Na+ reabsorption; released when blood volume/pressure is high.

• PTH: Increases Ca2+ reabsorption; released when Ca2+ is low.

• ADH: Increases water reabsorption; released when blood osmolarity is high.

Hypo- vs. Hypertonic Solutions

• Hypotonic: Lower solute concentration; water moves into cells (swelling).

• Hypertonic: Higher solute concentration; water moves out of cells (shrinking).

Bicarbonate and Blood pH Regulation

• Bicarbonate Buffer System: Maintains blood pH.

• Equation:

$\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

• Importance: Neutralizes excess acids or bases.

Hydrogen Ion Regulation

• Chemical Buffer Systems: Immediate response; includes bicarbonate, phosphate, and protein buffers.

• Respiratory Centers: Adjust CO2 exhalation; slower response.

• Renal Mechanisms: Slowest; kidneys excrete or conserve H+ and HCO3-.

Additional info: Renal compensation is essential for long-term acid-base balance.

Diseases and Disorders

Respiratory and Metabolic Acidosis/Alkalosis

• Respiratory Acidosis: Caused by hypoventilation; increased PCO2.

• Respiratory Alkalosis: Caused by hyperventilation; decreased PCO2.

• Metabolic Acidosis: Decreased blood pH and HCO3-.

• Metabolic Alkalosis: Increased blood pH and HCO3-.

Indicators:

• Respiratory: PCO2 levels.

• Metabolic: Blood pH and HCO3- levels.

Nephron Events and Urine Formation

• Three Major Steps:

  1. Filtration: Blood filtered in glomerulus.

  2. Reabsorption: Useful substances returned to blood.

  3. Secretion: Waste and excess ions added to filtrate.

• Events in Each Part: PCT (bulk reabsorption), Loop of Henle (concentration), DCT (fine-tuning), Collecting Duct (final adjustments).

Summary Table: Hormones and Their Effects

Hormone	Target	Effect	Trigger
Aldosterone	Kidney (DCT, Collecting Duct)	Increases Na+ reabsorption, water retention	Low Na+, high K+, low BP
ADH	Kidney (Collecting Duct)	Increases water reabsorption	High blood osmolarity
ANP	Kidney	Increases Na+ excretion, decreases BP	High blood volume/pressure
PTH	Kidney, Bone	Increases Ca2+ reabsorption	Low Ca2+ levels

Additional info: Hormonal pathways are essential for homeostasis of fluid, electrolytes, and acid-base balance.