Excretion & the Kidneys

Excretion: what it means

Excretion is the removal from the body of the toxic or waste products of metabolism — the chemical reactions happening inside cells. These substances would build up and poison you if they were not removed.

It is easy to confuse excretion with egestion, but they are not the same thing.

Key terms Excretion — removal of the waste products of metabolism (e.g. carbon dioxide, urea) made inside the body's cells.

Egestion — removal of undigested food (faeces) that has passed through the gut but was never absorbed into cells.

The simple test: was the substance ever part of a cell's chemistry? If yes, getting rid of it is excretion. Faeces is mostly food that never entered cells, so removing it is egestion.

The main excretory products and organs

Different waste products are removed by different organs.

Urea deserves a closer look. The body cannot store excess amino acids (the units that make proteins). In the liver they are broken down by a process called deamination: the nitrogen-containing part is removed and turned into urea, which is toxic in large amounts. The urea is carried in the blood to the kidneys, which filter it out.

Watch out A very common exam error is saying urea is made in the kidneys. Urea is made in the liver by deamination of excess amino acids. The kidneys only remove it from the blood.

The urinary system

The organs that make and remove urine form the urinary system.

Kidneys — two bean-shaped organs that filter the blood and make urine.
Ureters — two tubes carrying urine from the kidneys down to the bladder.
Bladder — a muscular bag that stores urine.
Urethra — the tube that carries urine out of the body.

Blood is delivered to each kidney by the renal artery and leaves, now cleaned, through the renal vein.

How the kidney makes urine

Each kidney contains about a million tiny filtering units called nephrons. Urine is made in two main stages: ultrafiltration followed by selective reabsorption.

#### Stage 1: Ultrafiltration

Blood enters a tight knot of capillaries called the glomerulus, sitting inside a cup-shaped Bowman's capsule. The blood here is under high pressure, which forces small molecules out of the blood and into the capsule. This is ultrafiltration.

Filtered out (small enough to pass): water, glucose, salts (ions) and urea.
Stay in the blood (too big to pass): blood cells and large plasma proteins.

The liquid now in the capsule is called the filtrate.

Key terms Ultrafiltration — filtering the blood at high pressure in the glomerulus, so small molecules pass into the Bowman's capsule while blood cells and proteins are held back because they are too large.

#### Stage 2: Selective reabsorption

The filtrate is still far too valuable to throw away — it contains useful substances. As it flows along the nephron tubule, the body takes back what it needs. This is selective reabsorption.

All the glucose is reabsorbed back into the blood — the body never wastes glucose.
Some of the water is reabsorbed (how much depends on how hydrated you are).
Some salts are reabsorbed, keeping the right balance in the blood.

Whatever is left in the tubule after reabsorption forms urine: water, salts and urea. There should be no glucose and no protein in healthy urine.

Exam tip Remember the difference between the two stages:

Ultrafiltration = non-selective (everything small is pushed out).

Reabsorption = selective (the body chooses what to take back, e.g. all glucose).

A question about glucose appearing in urine is usually testing whether you know reabsorption normally removes it all.

The nephron

The filtrate travels from the Bowman's capsule, along the tubule, and finally into the collecting duct, which carries the finished urine towards the ureter.

Osmoregulation

The kidneys do more than remove waste — they also control how much water the body keeps. This is called osmoregulation: keeping the water content of the blood steady.

On a hot day, or after little to drink, the body reabsorbs more water back into the blood. The urine produced is small in volume and concentrated (a darker colour).
After drinking a lot, less water is reabsorbed. The urine is large in volume and dilute (paler).

This adjustment happens at the nephron, by changing how much water is reabsorbed from the filtrate. It is an example of homeostasis — keeping internal conditions constant.

When kidneys fail: dialysis vs transplant

If the kidneys stop working, urea and excess water build up in the blood, which is fatal if untreated. There are two treatments.

In dialysis, the patient's blood is passed through a machine. It flows on one side of a partially permeable membrane, with dialysis fluid on the other. Urea and excess salts diffuse out of the blood into the fluid (which contains no urea), while glucose and useful ion levels are kept correct because the fluid is set to match healthy blood. Clean blood is returned to the body.

In a transplant, a healthy kidney from a donor replaces the failed ones.

Real world A transplant is generally the better long-term option because it frees the patient from the machine, but the body may reject the new kidney as "foreign", so patients take immunosuppressant drugs for life. With far more people needing kidneys than there are donors, many rely on dialysis while they wait.