Excretion

Every living cell produces waste as it carries out its chemical reactions. Excretion is the removal of the toxic waste products of metabolism from the body. In CSEC Biology you must be able to distinguish excretion from two ideas it is often confused with: egestion (removing undigested food as faeces — that material was never part of a cell's metabolism) and secretion (releasing useful substances such as enzymes or hormones).

The main excretory products

Note that two of these — carbon dioxide and water — also link to respiration, while urea links to the diet, so excretion ties several topics together.

The organs of excretion

• Lungs excrete carbon dioxide and some water vapour.
• Skin excretes water, salts and a little urea in sweat (its main job, though, is temperature control).
• Liver breaks down excess amino acids by deamination, producing urea, and breaks down old red blood cells to form bile pigments.
• Kidneys are the main excretory organs, removing urea, excess water and excess salts as urine.

How the kidney works

Each kidney contains about a million tiny filtering units called nephrons. Blood arrives under high pressure through the renal artery. The process has two key stages.

1. Ultrafiltration. In the cup-shaped Bowman's capsule, the high blood pressure forces small molecules — water, glucose, salts, urea — out of the blood capillaries (the glomerulus) and into the nephron. Large molecules such as proteins and blood cells are too big to pass through and stay in the blood. The liquid formed is the filtrate.

2. Selective reabsorption. As the filtrate flows along the nephron, substances the body needs are reabsorbed back into the blood: all of the glucose, much of the water, and the right amount of salts. Urea and excess water and salts are left behind to form urine. Urine passes down the ureter to the bladder, where it is stored before being released through the urethra.

The clean, filtered blood leaves the kidney through the renal vein.

Osmoregulation — controlling water balance

The kidneys also keep the water content of the blood constant. This is part of homeostasis and is controlled by the hormone ADH (anti-diuretic hormone) from the pituitary gland.

• If the blood is too concentrated (e.g. on a hot day, or after sweating), more ADH is released. The nephron reabsorbs more water, so a small volume of concentrated urine is produced and the blood water rises back to normal.
• If the blood is too dilute (e.g. after drinking a lot), less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced.

This is a classic example of negative feedback: the response always opposes the change.

Kidney failure and its treatment

If the kidneys fail, urea and excess water build up to dangerous levels. Two treatments are studied:

• Dialysis uses a machine with a partially permeable membrane to filter the blood. The patient's blood flows on one side and dialysis fluid on the other; urea and excess salts diffuse out into the fluid, which contains the correct concentrations of glucose and salts so these are not lost. Dialysis must be repeated several times a week.
• Kidney transplant replaces the failed kidney with a healthy donor kidney. It is a long-term cure but the donor tissue must be a close match to reduce the risk of rejection, and the patient takes immunosuppressant drugs.

A good comparison answer notes that dialysis is repeated and time-consuming but avoids surgery and rejection, while a transplant is a one-off that restores normal life but depends on a matching donor and lifelong medication.

The liver's role in excretion

The liver deserves special attention because it prepares two of the body's main wastes. Proteins and amino acids cannot be stored, so the liver removes the amino group from any excess amino acids in a process called deamination. This produces ammonia, which is highly toxic, so the liver immediately converts it to the much less toxic urea. The urea then travels in the blood to the kidneys to be excreted. The liver also breaks down worn-out red blood cells, and the iron-free part of the haemoglobin becomes the bile pigments that colour the bile (and, later, the faeces). So although the kidney does the filtering, much of the waste it removes was actually made by the liver — a link worth stating in extended answers.

Comparing the excretory organs

A table that pulls the system together helps in revision:

Why excretion matters

Excretion is essential because metabolic wastes are toxic if allowed to build up. Carbon dioxide dissolves to form an acid that would lower the blood's pH and disrupt enzymes; urea and ammonia are poisonous to cells; excess salts and water would upset the osmotic balance of the blood and tissues. By removing these continuously, the excretory organs keep the internal environment of the body steady — which is exactly what homeostasis means. This is why excretion is studied alongside osmoregulation and temperature control rather than on its own: together they maintain the stable conditions the body's cells need to work.

Common exam mistakes

• Calling faeces an excretory product — it is egested, not excreted.
• Saying the kidney "makes" urine from nothing — urine is filtered from the blood.
• Forgetting that glucose is completely reabsorbed; finding glucose in urine is a sign of diabetes, not normal excretion.
• Mixing up ureter (kidney → bladder) with urethra (bladder → outside).

Key terms to remember

• Excretion — the removal of the toxic waste products of metabolism.
• Egestion — the removal of undigested food (faeces); not excretion.
• Deamination — the liver process that removes the amino group from excess amino acids, producing urea.
• Urea — the nitrogen-containing waste made by the liver and removed by the kidneys.
• Nephron — the microscopic filtering unit of the kidney.
• Ultrafiltration — filtering small molecules out of the blood under high pressure in the kidney.
• Selective reabsorption — taking useful substances (all glucose, needed water and salts) back into the blood.
• Osmoregulation — controlling the water content of the blood.
• ADH — the hormone that controls how much water the nephron reabsorbs.

Quick recap

• Excretion removes the toxic waste of metabolism: mainly CO₂ (lungs), urea (kidneys, skin) and excess water/salts.
• The kidney works by ultrafiltration then selective reabsorption; glucose and needed water/salts return to the blood, urea leaves as urine.
• ADH controls water balance by altering how much water the nephron reabsorbs.
• Kidney failure is treated by dialysis (repeated machine filtration) or a transplant (matched donor kidney).