Sec 3 Biology Excretion in Humans notes

Excretion is the removal of waste products of metabolism — substances made by chemical reactions inside cells that the body does not need and cannot store. The big three routes: carbon dioxide from respiration leaves via the lungs; urea, made in the liver from the breakdown of excess amino acids, leaves via the kidneys in urine; and water and salts leave via the kidneys and the skin.

Why the urgency? Urea is poisonous: it raises the concentration of nitrogenous waste in the blood and, at high levels, damages cells — the body must keep it dilute and flowing out. Note the boundary the syllabus draws: egestion (passing out undigested food as faeces) is not excretion, because faeces never entered the cells and were never part of metabolism.

Blood arrives at each kidney through the renal artery — about a fifth of each heartbeat’s output — and leaves by the renal vein. The kidneys filter the blood and produce urine, which trickles down the two ureters into the bladder, a muscular storage bag. When the bladder is full, its sphincter relaxes and urine leaves through the urethra. Bladder and urethra move and store; the kidneys alone do the chemistry.

Inside each kidney sits a dense forest of roughly a million nephrons — microscopic filtration units that begin in the outer cortex, loop deep into the medulla, and drain their finished urine into the central pelvis of the kidney, which feeds the ureter.

Each nephron begins with a cup called the Bowman’s capsule, wrapped around a knot of capillaries called the glomerulus. Blood enters the glomerulus under high pressure (the vessel carrying blood in is narrower than the one carrying it out, keeping pressure up). That pressure squeezes the liquid part of the blood through the capillary walls into the capsule — a process called ultrafiltration.

The filter is size-selective, not need-selective: water, glucose, amino acids, salts and urea — all small molecules — pass into the filtrate. Blood cells and large plasma proteins are too big to cross, so they stay in the blood, which leaves the glomerulus through the vessel leading to the tubule.

As the filtrate flows along the coiled tubule, useful substances are selectively taken back into the surrounding capillaries: all of the glucose and amino acids, most of the water (and some salts) are reabsorbed. The word "selective" is doing real work here — the tubule decides by need, re-absorbing exactly what the body must keep. Urea is not reabsorbed: it stays in the tubule and becomes the main solute of urine.

The finished urine therefore contains water, urea and salts — and, in a healthy person, no glucose and no protein. Glucose appears only if blood glucose is so high the tubule cannot reabsorb it all (the signature of uncontrolled diabetes); protein appears only if the glomerular filter has been damaged and leaks.

When kidneys fail, urea and other wastes climb to toxic levels. A dialysis machine takes blood from a vein-side line, runs it along a partially permeable membrane with dialysis fluid on the other side, and returns the cleaned blood. Everything moves by diffusion down its own concentration gradient — no pumping of solutes, no active transport.

The fluid’s recipe is engineered to make those gradients perfect: it contains no urea, so urea diffuses out of the blood; it contains glucose and salts at the same concentrations as healthy blood plasma, so none of them leave the patient’s blood (no gradient, no loss). Sessions last hours and repeat several times a week, because the machine cleans only what is in the blood passing through at that moment.