Human Anatomy and Physiology: Systems, Feedback, and Transport
Organization and control
Start with the body's levels of organization and homeostasis. Negative feedback reverses a change and is the dominant control mechanism (thermoregulation, blood glucose). Positive feedback amplifies a change and is rare (childbirth, blood clotting). The sympathetic division drives fight-or-flight responses; the parasympathetic division supports rest-and-digest functions.
Systems worth mastering
Cardiovascular: arteries carry blood away from the heart, veins carry it back, and the left ventricle has the thickest wall because it pumps to the whole body. Respiratory: gas exchange happens in alveoli by simple diffusion. Digestive: most nutrient absorption occurs in the small intestine via villi; the large intestine reclaims water. Urinary: filtration occurs at the glomerulus, most reabsorption in the proximal tubule, and ADH increases water reabsorption. Endocrine: insulin lowers blood glucose, glucagon raises it.
Traps
Arteries always carry blood away from the heart regardless of oxygen content - the pulmonary artery is deoxygenated. Oxygen travels mostly bound to hemoglobin, but most carbon dioxide travels as bicarbonate. Insulin lowers glucose; glucagon raises it. Breathing is driven more strongly by rising CO₂ than by falling O₂.
Worked example
Trace a red blood cell from the body: right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body tissues. Rule of thumb: the right side of the heart sends deoxygenated blood to the lungs; the left side sends oxygenated blood to the body.