Body Systems in Motion

The heart has four chambers. The right atrium and right ventricle take oxygen-poor blood returning from the body and send it to the lungs. The left atrium and left ventricle take freshly oxygenated blood from the lungs and push it out to the entire body. The left ventricle has the thickest wall because it has the longest trip.

A typical resting heart rate for an adult is 60–100 beats per minute. Well-trained endurance athletes often sit well below 60 — sometimes in the 40s — because a stronger heart moves more blood per beat and needs fewer beats to do the same work. This is called an increased stroke volume.

During exercise, heart rate and stroke volume both climb, and blood flow is actively redistributed: more to working muscle and skin, less to the digestive organs. That redistribution is why running hard right after a big meal feels terrible.

Air travels nose or mouth, then pharynx, larynx, trachea, bronchi, bronchioles, and finally to roughly 300 million alveoli — tiny air sacs wrapped in capillaries. Oxygen crosses into the blood and carbon dioxide crosses out, both by simple diffusion across a membrane thinner than a sheet of paper.

The diaphragm is the primary muscle of breathing. It contracts and flattens to pull air in; it relaxes and domes upward to push air out. Breathing rate rises during exercise not mainly because you are low on oxygen, but because rising carbon dioxide in the blood is the strongest trigger your brainstem watches.

The central nervous system is the brain and spinal cord. The peripheral nervous system is every nerve branching off it. Motor neurons carry commands out to muscle; sensory neurons carry information back in.

Proprioception is your sense of where your body is in space without looking — the reason you can touch your nose with your eyes closed, and the reason a well-trained athlete can correct a landing before their conscious brain notices. Proprioceptors sit in muscles, tendons, and joint capsules, and injury damages them along with the tissue. That is why balance work is a standard part of rehabilitation, not an afterthought.

Skin is the largest organ in the body. During exercise it becomes a cooling system: blood vessels near the surface dilate to dump heat, and sweat glands release fluid that carries heat away as it evaporates.

The key word is evaporates. Sweat that drips off does almost nothing for you. In high humidity, evaporation slows down, cooling fails, and core temperature climbs — which is the physiology behind every heat-illness policy in high school athletics.