Physiology — GI, Blood & Homeostasis — USMLE Step 1 Notes
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GI secretion & its control
- Parietal cells secrete gastric acid (H+/K+ ATPase) and intrinsic factor; chief cells secrete pepsinogen.
- Acid secretion is stimulated by gastrin, histamine and acetylcholine (vagal) and inhibited by somatostatin and low luminal pH.
- Gut hormones: gastrin (acid), secretin (bicarbonate from pancreas/bile), cholecystokinin (enzyme secretion, gallbladder contraction), GIP (insulin release).
Digestion & absorption
- Pancreatic enzymes digest all three macronutrients; bile emulsifies fat for absorption of fatty acids and the fat-soluble vitamins (ADEK).
- Iron is absorbed in the duodenum; vitamin B12 needs intrinsic factor and is absorbed in the terminal ileum.
- The liver processes absorbed nutrients (first pass), makes bile and plasma proteins, and detoxifies via cytochrome P450.
Blood & haematopoiesis
- All blood cells arise from bone-marrow stem cells; erythropoietin (from the kidney, in response to hypoxia) drives red-cell production.
- Haemoglobin (4 haem + globin) carries oxygen; each gram carries ~1.34 mL O2. Fetal Hb binds oxygen more tightly than adult Hb.
Haemostasis in brief
- Primary haemostasis: platelet plug (von Willebrand factor bridges platelets to collagen).
- Secondary haemostasis: the coagulation cascade generates thrombin → fibrin. PT tests the extrinsic pathway; aPTT the intrinsic.
Homeostasis & thermoregulation
- Homeostasis keeps the internal environment stable by negative feedback (sensor → control centre → effector); positive feedback is rarer (e.g. the LH surge, clotting, labour).
- The hypothalamus is the thermostat: heat loss via sweating and cutaneous vasodilation; heat conservation via vasoconstriction and shivering.
Written by StepSherpa in our own words, grounded in Human Physiology (Wikibooks), used under CC BY-SA 3.0.
GI hormones & blood buffering
- Gastrin (acid), CCK (enzymes/gallbladder), secretin (bicarbonate), somatostatin (inhibits all), GIP/GLP-1 (incretins → insulin). Migrating motor complex clears the gut between meals (motilin).
- Blood buffering: bicarbonate/CO2 is the main extracellular buffer; haemoglobin and phosphate contribute. The Bohr and Haldane effects couple O2 and CO2 transport.
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