Your blood stays at about 37 °C, with a steady water content, salt level and pH, no matter how hot the day or how salty the meal. That constancy is no accident — it is the active work of homeostasis, the body keeping its internal environment stable so every cell can function.
Homeostasis is the maintenance of a constant internal environment — keeping the conditions inside the body within narrow, steady limits despite changes outside. The "internal environment" means the tissue fluid (and blood plasma) that bathes every cell.
Cells are extremely fussy. Their enzymes only work over a narrow range of temperature and pH; they swell or shrink if the surrounding water and salt balance is wrong; and waste such as urea is toxic if it builds up. Homeostasis holds all of these — temperature, water, salts (osmotic balance), blood glucose, pH and waste removal — at their correct set points.
Almost all homeostasis works by negative feedback: when a variable moves away from its set point, the body's response acts in the opposite direction to cancel the change and restore the norm.
The loop has three parts: a receptor that senses the change, a coordinator (the brain — especially the hypothalamus — or an endocrine gland) that processes it, and an effector that acts. Because the response opposes and switches off the original stimulus, the value never runs away — it oscillates gently around the set point.
Excretion is the removal of the toxic waste products of metabolism from the body. The chief nitrogenous waste in mammals is urea, made in the liver when excess amino acids are broken down (deamination) — the toxic ammonia produced is at once converted to the far less toxic urea, carried in the blood to the kidneys, and excreted in urine.
Osmoregulation is the control of the water and salt (solute) concentration of the body fluids — keeping the blood's osmotic pressure steady. In mammals both jobs — excretion of urea and osmoregulation — are carried out together by the kidneys.
| Organ | Excretes |
|---|---|
| Kidneys | Urea, excess water, excess salts (as urine) |
| Lungs | Carbon dioxide and some water vapour |
| Skin | Water, salts and a little urea (in sweat) |
| Liver | Bile pigments (from broken-down haemoglobin) |
The two kidneys lie against the back wall of the abdomen. Blood enters each kidney under high pressure through the renal artery and leaves, cleaned, by the renal vein. Urine drains down the ureter to the bladder.
Cut a kidney lengthways and you see three regions:
Each kidney is built from about a million microscopic tubules called nephrons — the actual functional units that make urine.
A nephron is a long, fine tubule with a cup at one end and a rich blood supply along its length. Trace it from start to finish:
Urine is made in three steps, in this order:
Blood in the glomerulus is under high pressure (the efferent arteriole is narrower than the afferent). This hydrostatic pressure forces small molecules — water, glucose, amino acids, salts and urea — out of the blood, through the filter, and into the capsule. The filter holds back large molecules: blood cells and plasma proteins are too big to pass and stay in the blood. The fluid that enters the capsule is the glomerular filtrate.
The filtrate still contains valuable substances the body cannot afford to lose. As it flows along the tubule these are taken back into the surrounding capillaries: all the glucose, all the amino acids, most of the salts and most of the water are reabsorbed. Glucose and amino acids are reabsorbed by active transport (needing energy), and water follows by osmosis. Healthy urine therefore contains no glucose.
Finally, some unwanted substances are actively added from the blood into the tubule — extra hydrogen ions (H⁺), potassium ions (K⁺), ammonia and certain drugs. This fine-tunes the blood's pH and ion balance. What remains in the tubule — water, urea and excess salts — is urine.
The body must keep the water content of the blood constant. This is controlled by the hormone ADH (antidiuretic hormone), made in the hypothalamus and released by the pituitary gland, acting by negative feedback:
Thermoregulation is keeping the core body temperature near 37 °C — vital because our enzymes have an optimum there and denature if it climbs too high. The control centre is the thermoregulatory centre in the hypothalamus, which monitors blood temperature and skin receptors and runs a negative-feedback loop, mostly through the skin.
The skin is the body's main temperature regulator — its blood vessels, sweat glands and hair work together under the hypothalamus, while a layer of fat insulates. The liver is a homeostatic powerhouse: it makes urea (deamination), regulates blood glucose (storing it as glycogen or releasing it), breaks down toxins and worn-out red cells, and — being so chemically active — generates much of the body's heat.
Homeostasis explains why a high fever is dangerous (enzymes denature), why people on hot days drink and pass little dark urine, why diabetes (loss of glucose control) and kidney failure are so serious, and how dialysis machines stand in for failed kidneys. The single principle — detect a change, respond to oppose it — runs through the kidney, the skin, blood glucose and far beyond.