Digestive, Excretory, Reproductive, and Immune Systems

Core Review

The Digestive System

Mouth: Mechanical digestion (chewing) and chemical digestion begin here. Salivary glands secrete salivary amylase (initiates starch breakdown to maltose) and mucus. The tongue forms a bolus.

Esophagus: Peristalsis (wave-like smooth muscle contractions) propels the bolus to the stomach. The lower esophageal (cardiac) sphincter prevents reflux.

Stomach: The stomach churns food into chyme. Gastric glands secrete:

• HCl (parietal cells): Denatures proteins, activates pepsinogen, kills microbes.
• Pepsinogen (chief cells): Converted to active pepsin by HCl. Pepsin begins protein digestion.
• Mucus (goblet cells): Protects the stomach lining from acid. Gastric ulcers result from H. pylori infection or NSAIDs disrupting this protection.
• Intrinsic factor (parietal cells): Essential for vitamin B12 absorption in the ileum.

Small Intestine: The PRIMARY site of digestion and absorption (~6 meters long). Three segments: duodenum, jejunum, ileum. The mucosa is folded into villi (finger-like projections) covered with microvilli (brush border), creating an enormous surface area (~200 m²).

Accessory organs:

• Pancreas: Secretes pancreatic juice (bicarbonate-rich to neutralize acidic chyme) containing enzymes: trypsin/chymotrypsin (proteins), pancreatic amylase (starch), pancreatic lipase (fats). These are released as inactive zymogens, activated in the duodenum.
• Liver: Produces bile (stored in the gallbladder, released into the duodenum). Bile emulsifies fats — physically breaking large fat globules into smaller droplets, increasing surface area for lipase action. Bile contains bile salts, not digestive enzymes.
• Gallbladder: Stores and concentrates bile. CCK (cholecystokinin) stimulates gallbladder contraction and bile release.

Brush Border Enzymes (small intestine epithelial cells): Complete digestion — disaccharidases (maltase, sucrase, lactase), peptidases. Absorption: monosaccharides and amino acids enter capillaries via facilitated diffusion/active transport → hepatic portal vein → liver. Fats are absorbed as monoglycerides and fatty acids, reassembled into triglycerides in the epithelial cells, packaged into chylomicrons, and enter lacteals (lymphatic vessels).

Large Intestine (Colon): Absorbs water and electrolytes. Houses gut microbiota (ferment undigested fiber, produce vitamins K and B). Forms and stores feces.

The Excretory System

The nephron is the functional unit of the kidney (~1 million per kidney).

1. Glomerular Filtration: Blood enters the glomerulus (a tuft of capillaries) via the afferent arteriole. High pressure forces water, ions, glucose, amino acids, and urea through the filtration membrane into Bowman's capsule. Blood cells and large proteins are retained. The resulting filtrate is essentially plasma without proteins. Glomerular filtration rate (GFR) ~125 mL/min.

2. Proximal Convoluted Tubule (PCT): Bulk reabsorption. ~65% of filtered Na⁺, Cl⁻, and water; ~100% of glucose and amino acids; ~90% of bicarbonate are reabsorbed. Glucose reabsorption is via Na⁺/glucose cotransporters — saturable (if exceeded → glucose in urine, glucosuria, a sign of diabetes).

3. Loop of Henle: Creates the medullary osmotic gradient for urine concentration.

• Descending limb: Permeable to water, impermeable to NaCl. Water moves out by osmosis → filtrate becomes more concentrated.
• Ascending limb: Impermeable to water. Na⁺ and Cl⁻ are actively transported out (thick ascending limb) → filtrate becomes dilute. This countercurrent multiplier system establishes a high osmolarity in the medullary interstitium.

4. Distal Convoluted Tubule (DCT): Na⁺ reabsorption, K⁺ and H⁺ secretion. Aldosterone acts here to increase Na⁺ reabsorption and K⁺ secretion. PTH increases Ca²⁺ reabsorption here.

5. Collecting Duct: Final water reabsorption. ADH increases water permeability by inserting aquaporin-2 channels. When ADH is high, concentrated urine is produced. When ADH is absent (or inhibited — e.g., by alcohol), dilute urine results. The collecting duct also handles urea recycling and acid-base balance.

Key hormones regulating kidney function:

• ADH: ↑ water reabsorption (collecting duct); released in response to ↑ plasma osmolarity.
• Aldosterone: ↑ Na⁺ reabsorption, ↑ K⁺ secretion (DCT and collecting duct); released via RAAS.
• ANP (atrial natriuretic peptide): ↓ Na⁺ reabsorption (opposes aldosterone); released when atria are stretched by high blood volume.

The Reproductive System

Male: Testes produce sperm (seminiferous tubules, stimulated by FSH) and testosterone (Leydig/interstitial cells, stimulated by LH). Sperm mature in the epididymis, travel through the vas deferens, and exit via the urethra. Accessory glands (seminal vesicles, prostate, bulbourethral glands) contribute to seminal fluid.

Female: Ovaries produce ova (follicles, stimulated by FSH) and estrogen/progesterone (stimulated by LH). The ovarian cycle:

• Follicular phase (days 1–13): FSH stimulates follicle development. Follicles secrete estrogen. Estrogen builds up the endometrial lining (proliferative phase).
• Ovulation (day 14): LH surge triggers release of the secondary oocyte.
• Luteal phase (days 15–28): Corpus luteum forms from the ruptured follicle, secretes progesterone (and some estrogen). Progesterone maintains the endometrium (secretory phase). If no fertilization, corpus luteum degenerates → progesterone drops → menstruation. If fertilization occurs, hCG (human chorionic gonadotropin) from the embryo maintains the corpus luteum.

The Immune System

Innate Immunity (non-specific, present from birth):

• Physical barriers: Skin, mucous membranes, cilia.
• Chemical defenses: Stomach acid, lysozyme (tears/saliva), antimicrobial peptides.
• Cellular defenses: Phagocytes — neutrophils (first responders, most abundant WBC), macrophages (derived from monocytes, also present antigens). Natural killer (NK) cells destroy virus-infected and tumor cells.
• Inflammatory response: Histamine from mast cells → vasodilation, increased capillary permeability → redness, heat, swelling. Complement system: a cascade of plasma proteins that lyse pathogens and enhance phagocytosis (opsonization).

Adaptive Immunity (specific, develops over time):

• Antigen: A molecule (usually foreign) that elicits an immune response.
• Humoral immunity (B cells → antibodies): B cells recognize antigens via membrane-bound antibodies. Activation (with helper T cell assistance) → plasma cells secrete antibodies. Antibodies neutralize pathogens, opsonize (mark for phagocytosis), and activate complement. Memory B cells persist for rapid future response.
• Cell-mediated immunity (T cells):
  • Helper T cells (CD4+): Recognize antigens presented on MHC II (on antigen-presenting cells: dendritic cells, macrophages, B cells). Secrete cytokines to activate B cells, cytotoxic T cells, and macrophages. HIV targets CD4+ cells.
  • Cytotoxic T cells (CD8+): Recognize antigens presented on MHC I (on nearly all nucleated cells). Kill infected or cancerous cells via perforin/granzyme or Fas-FasL.
• Active vs. Passive immunity:
  • Active: Your body produces its own antibodies and memory cells. Natural (infection) or artificial (vaccination). Long-lasting.
  • Passive: Receiving pre-formed antibodies from another source. Natural (mother's IgG crossing placenta, or IgA in breast milk/colostrum) or artificial (antivenom, monoclonal antibodies). Immediate but temporary (no memory cells formed).

Common Traps

• "The stomach digests everything": The stomach primarily digests proteins. Most digestion and absorption occurs in the small intestine. Carbohydrate digestion begins in the mouth.
• "The liver produces enzymes for digestion": The liver produces BILE (emulsifies fat, not enzymatic). The PANCREAS produces the major digestive enzymes.
• "ADH and aldosterone do the same thing": ADH = water reabsorption (aquaporins, collecting duct). Aldosterone = Na⁺ reabsorption (DCT, collecting duct), and water follows osmotically.
• "All T cells kill infected cells": Only CD8+ cytotoxic T cells kill. CD4+ helper T cells coordinate the immune response but do NOT directly kill.
• "Antibiotics treat viruses": Antibiotics target bacteria. Viruses require antiviral drugs or rely on the immune system. This is more of a clinical trap but appears in context.