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Excretion

Excretion is the removal of waste products of metabolism (cell activity) and excess substances from the body.

The body naturally produces a number of toxins. Excretion prevents the build-up of these substances in the body. It is an important part of homeostasis (maintaining stable internal conditions).

Excretion keeps the composition of bodily fluids like blood and tissue fluid stable.

Important! Do not confuse excretion with egestion. Egestion is the removal of waste (undigested material) from the gut, not from body tissue.

There are three important excretory substances:

  • Carbon dioxide: removed from the blood by the lungs.
  • Urea: is removed from the blood by the kidneys.
  • Water is removed from the body by the lungs, kidneys and skin.

If there is too much water in the blood, the kidneys increase the water content of urine. This is why urine appears paler after drinking a lot.

Air we breathe out contains excreted carbon dioxide and water vapour.
Air we breathe out contains excreted carbon dioxide and water vapour.

The urinary system produces and excretes urine.

Urine contains urea and other waste substances (e.g. excess water) that must be removed from the body.

  1. Two kidneys produce urine from the waste products in the blood.
  2. The ureter transports urine from the kidneys to the bladder.
  3. The bladder stores urine prior to excretion.
  4. The urethra transports urine from the bladder out of the body.

The kidneys are supplied with blood by the renal artery. Filtered blood exits the kidneys and returns towards the heart via the renal vein.

Renal refers to anything to do with the kidneys.

The renal artery and vein are branches of the main blood vessels to and from the heart (aorta and vena cava).

The urinary system of humans produces roughly 1.5 litres of urine every day.
The urinary system of humans produces roughly 1.5 litres of urine every day.

The kidneys are a pair of organs that filter the blood and produce urine.

The kidneys interact with the blood by:

  • Regulating blood water levels.
  • Regulating blood salt and ion levels.
  • Reabsorbing useful substances into the blood.
  • Excreting urea and other waste products.
Each kidney is about 12 cm long and 3 cm thick.
Each kidney is about 12 cm long and 3 cm thick.

Your kidneys filter roughly 180 litres of blood every day! However, only 1.5 litres is converted into urine. The rest stays as blood.

The nephron (also called the kidney tubule) is a repeating structure inside the kidney that filters the blood.

Each kidney contains roughly a million nephrons. Each nephron has its own blood supply that originates from the renal artery.

The cortex contains a knot of capillaries called the glomerulus . The glomerulus is enclosed by the Bowman's capsule .

The medulla contains the loop of Henle and the collecting duct .

Water is passed between the nephron tubule (orange line) and the blood capillary (red line) during blood filtration.
Water is passed between the nephron tubule (orange line) and the blood capillary (red line) during blood filtration.

The nephron's structure is vital for the kidney to filter blood and reabsorb nutrients.

There are two processes that occur simultaneously in the nephron:

  • Ultrafiltration : Blood in the glomerulus is under high pressure. Urea, glucose, ions and water are forced through the capillary walls of the glomerulus into the Bowman's capsule.

    The solution that is formed in the Bowman's capsule is called filtrate.

    The filtrate contains waste urea as well as glucose and useful ions!

  • Selective reabsorption : Useful molecules and some water in the filtrate are reabsorbed into the blood when the filtrate moves through the loop of Henle. Waste substances such as urea are not reabsorbed.

    The level of reabsorption can be altered by the body. The loop of Henle has a high salt concentration to increase the efficiency of selective reabsorption.

    The filtrate that remains forms urine. Urine is removed from the nephron via the collecting duct.

The loop of Henle has a high salt concentration due to the selective reabsorption of water.

Damage or stress to both kidneys can lead to kidney failure.

Diabetes, high blood pressure and some infections can damage the kidneys.

Kidney failure means that neither kidney is able to properly filter the blood or control the composition of urine. It is often irreversible.

Patients with kidney failure can be kept alive by dialysis. This uses a dialysis machine to filter the blood externally.

You can survive with only one kidney, but if both fail, dialysis or transplant are the only options.
You can survive with only one kidney, but if both fail, dialysis or transplant are the only options.
  • The coiled tube acts as the capillary network of the kidneys. The tube walls of the coiled tube are semi-permeable to allow diffusion of substances from the blood.
  • Dialysis fluid replaces the kidney filtrate. It has the same concentration of useful substances (such as glucose and amino acids) as the blood. This prevents the diffusion of these substances from the blood.
  • Dialysis fluid does not contain any urea. This causes urea to diffuse into the dialysis fluid from the blood.

Within the dialysis machine, blood and dialysis fluid flow in opposite directions. This maximises the concentration gradient of urea, increasing waste removal.

Osmoregulation is the process of regulating the water potential (water content) of fluids such as blood.

Osmoregulation is needed to maintain a stable internal environment (homeostasis). It protects cells against water damage (i.e. cell shrinkage or bursting).

There are two main ways the body regulates blood water levels:

  • Controlling water content of the blood:
  • The kidneys control how much water is reabsorbed back into the blood from the loop of Henle.

  • Controlling ion concentrations:
  • Tissues can absorb ions from the blood. This decreases the concentration of ions in the blood and increases the water potential of the blood.

There is a balance between different factors affecting osmotic concentration.

On a hot day, water is lost from the blood through sweating. The body compensates for this by reducing the water content of urine.

Antidiuretic hormone (ADH) controls the amount of water in the blood.

A 'diuretic' substance increases urine production. ADH has the opposite effect.

ADH increases reabsorption of water into the blood.

ADH is produced and released into the blood by the pituitary gland in the brain The amount of ADH released by the pituitary gland depends on signals from a small brain area called the hypothalamus.

This cycle is an example of a negative feedback system maintaining homeostasis.

Remember: the more water is retained by the kidney tubules, the less it is reabsorbed back into the blood!