Organismal Structure and Function (Anatomy and Physiology)
Physiology
Items test how organ systems maintain internal stability, how exchange surfaces are built, and how structure at each level serves function.
Homeostasis and control
Negative feedback reverses a deviation: a sensor detects change, an integrating center compares it with a set point, and an effector counters it (insulin lowers glucose; sweating lowers temperature). Positive feedback amplifies a change and must terminate (oxytocin in labor, clotting cascade). Levels of organization run from cells to tissues (epithelial, connective, muscle, nervous) to organs to systems.
Systems
- Circulation: mammals have a four-chambered heart and double circulation; arteries carry blood away from the heart, and pulmonary arteries carry deoxygenated blood.
- Gas exchange: alveoli and gill lamellae maximize surface area and minimize diffusion distance; countercurrent flow in fish gills maintains a gradient along the whole lamella.
- Digestion: enzymes act at specific pH (pepsin acid; pancreatic amylase and lipase alkaline); villi and microvilli increase absorptive area.
- Excretion: the nephron filters at the glomerulus, reabsorbs glucose, water, and ions, and secretes wastes; the loop of Henle builds a concentration gradient.
- Nervous and endocrine: fast neuronal action potentials involve sodium and potassium channel currents; the sodium-potassium pump maintains gradients over time rather than directly causing rapid repolarization; hormones act on target cells bearing receptors.
- Plants: xylem moves water and minerals upward under transpiration pull (cohesion–tension); phloem translocates sugars by pressure flow; guard cells open stomata when turgid.
Worked example
After a carbohydrate-rich meal, blood glucose rises. Pancreatic beta cells detect it and secrete insulin; insulin promotes glucose uptake in responsive muscle and adipose tissue, promotes glycogen storage and suppresses hepatic glucose production, so glucose falls toward the set point. When glucose drops below the set point, alpha cells release glucagon, triggering glycogen breakdown and gluconeogenesis in the liver. Both arms are negative feedback: the response opposes the original deviation. In diabetes mellitus, insulin secretion and/or action is impaired and hyperglycemia can result; this model is not a clinical diagnosis.
Traps
Labeling a loop positive because the outcome is "good"; forgetting that pulmonary arteries carry deoxygenated blood; confusing xylem and phloem direction; assuming diffusion suffices in large animals; mixing the roles of the loop of Henle and the collecting duct; assuming all hormones are lipid-soluble.