Human Biology and Anatomy: Systems and Homeostasis
What you must understand
Work system by system, but always connect each to homeostasis. Circulation: the heart has four chambers; the left ventricle drives systemic flow; arteries carry blood away from the heart (not necessarily oxygenated - the pulmonary artery carries deoxygenated blood). Gas exchange occurs in alveoli, with diffusion driven by concentration gradients. Digestion: mechanical and chemical breakdown, enzymes by substrate (amylase for starch, pepsin for protein in acid, lipase for fat), absorption in the small intestine via villi. Excretion: the nephron filters, reabsorbs and secretes; the loop of Henle builds the medullary gradient. Nervous and endocrine systems coordinate rapid electrical and slower hormonal signalling; negative feedback restores set points.
Traps
Blood in arteries is not always oxygenated. Insulin lowers blood glucose; glucagon raises it - do not swap them. The liver is not part of the digestive tract proper but secretes bile and performs metabolic regulation. Breathing and cellular respiration are different processes; ventilation supplies oxygen, respiration makes ATP in cells. In the nephron, glucose should be fully reabsorbed in a healthy person; glucose in urine signals a problem.
Worked example
A person hyperventilates. CO₂ is washed out, blood pH rises (respiratory alkalosis). Rule: ventilation controls CO₂, which sets acid–base balance; the kidney adjusts bicarbonate more slowly. So a fast respiratory change is compensated renally over hours to days - a favourite reasoning question.