Physiology — GI, Blood & Homeostasis — USMLE Step 1 Notes

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.