How the body’s organ systems keep internal conditions steady
In digestion, enzymes break food into absorbable units: salivary amylase starts on starch, pepsin acts on proteins in the stomach, and pancreatic enzymes and bile complete digestion in the small intestine, where villi absorb nutrients. In breathing, the diaphragm and intercostal muscles change chest volume, and gas exchange occurs in alveoli by diffusion. Oxygen is carried mostly by haemoglobin, and the oxygen dissociation curve is sigmoid; carbon dioxide travels mainly as bicarbonate.
The human heart has four chambers and double circulation. Its beat starts at the sinoatrial node, the pacemaker, and spreads through the atrioventricular node and bundle of His, recorded on an ECG. Blood groups follow the ABO and Rh systems. The kidneys contain nephrons that filter blood in the glomerulus, reabsorb useful substances in the tubules and concentrate urine through the counter-current mechanism, regulated by ADH and the renin–angiotensin system. Muscle contraction is explained by the sliding filament theory, with actin and myosin, while neural and hormonal control coordinate all systems.
Why it matters
Human physiology is the foundation of medicine and nursing. It explains conditions like asthma, heart attacks, kidney failure and dialysis, and why blood must be matched before transfusion. Understanding how organs work together helps people make healthy choices about exercise, diet and smoking.
Key terms
- Alveoli
- Tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged.
- Sinoatrial node
- The heart’s natural pacemaker that starts each heartbeat.
- Nephron
- The structural and functional unit of the kidney that filters blood and forms urine.
- Sliding filament theory
- The explanation that muscle contracts as actin filaments slide over myosin filaments.