What you'll learn
Every living cell produces waste as it carries out its chemical reactions, and those wastes are toxic if allowed to accumulate. This topic covers what the body's excretory products are and where each comes from, the organs that remove them, the structure and working of the kidney and nephron, osmoregulation and the role of ADH, the treatment of kidney failure, and the liver's part in preparing wastes for removal. Examiners test the distinction between excretion and two processes it is easily confused with, and they test the kidney as a sequence of stages rather than as a list of parts, so learn it as a process.
Excretion is the removal of the toxic waste products of metabolism from the body. Distinguish it from egestion (removing undigested food as faeces — that material was never part of a cell's metabolism) and from secretion (releasing useful substances such as enzymes or hormones).
Key terms and definitions
Excretion — the removal of the toxic waste products of metabolism.
Egestion — the removal of undigested food as faeces; not excretion.
Secretion — the release of a useful substance made by a cell, such as an enzyme or hormone.
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 — the filtering of small molecules out of the blood under high pressure in the kidney.
Selective reabsorption — the return of useful substances (all glucose, needed water and salts) to the blood.
Osmoregulation — the control of the water content of the blood.
ADH — anti-diuretic hormone, which controls how much water the nephron reabsorbs.
Homeostasis — the maintenance of a constant internal environment.
Core concepts
The main excretory products
| Waste product | Where it comes from | Excreted by |
|---|---|---|
| Carbon dioxide | aerobic respiration | lungs |
| Urea | breakdown of excess amino acids in the liver | kidneys (in urine), skin (in sweat) |
| Water and salts | diet and metabolism | kidneys, skin, lungs |
| Bile pigments | breakdown of old red blood cells | liver → gut |
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.
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.
Structure of the kidney and nephron
Each kidney has a dark outer cortex, a paler inner medulla, and a central space, the pelvis, which collects urine and leads into the ureter. Blood enters through the renal artery and leaves through the renal vein.
Each kidney contains about a million nephrons. A nephron begins with a cup-shaped Bowman's capsule in the cortex, enclosing a knot of capillaries called the glomerulus. From the capsule the tubule runs as the proximal convoluted tubule, dips down into the medulla as the loop of Henlé, returns as the distal convoluted tubule, and joins a collecting duct that carries urine to the pelvis. Knowing that filtration happens in the cortex and that the loop dips into the medulla is enough detail for CSEC, but knowing the order of the regions makes the two-stage process easier to describe.
How the kidney works
1. Ultrafiltration. In the Bowman's capsule, high blood pressure forces small molecules — water, glucose, salts and urea — out of 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 into the blood: all of the glucose, much of the water, and the right amount of salts. Urea and the excess water and salts are left behind to form urine. Urine passes down the ureter to the bladder, where it is stored before release through the urethra.
The word selective carries the meaning: the nephron does not simply take back some of everything, it takes back particular substances in particular amounts. Glucose is reabsorbed completely because the body cannot afford to lose it, and reabsorbing it against a concentration gradient requires active transport — which is why the cells lining the tubule contain many mitochondria.
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 from the pituitary gland.
- If the blood is too concentrated — 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 content rises back to normal.
- If the blood is too dilute — after drinking a large quantity — 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 that triggered it.
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 that these are not lost. Dialysis must be repeated several times a week.
A 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 for life.
A good comparison 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.
Why excretion matters
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 steady — which is what homeostasis means. This is why excretion is studied alongside osmoregulation and temperature control rather than on its own.
Worked examples
Example 1. A urine sample from a patient is found to contain glucose. Explain what this suggests and why glucose is not normally present.
Glucose is normally reabsorbed completely from the filtrate during selective reabsorption, so none should appear in urine. Its presence suggests that the concentration of glucose in the blood was so high that the carrier proteins reabsorbing it were working at their maximum rate and could not return it all — an indication of diabetes. Note that the glucose was filtered normally; the fault lies in reabsorption being unable to keep pace.
Example 2. Explain why protein is found in the blood leaving the kidney but not in the filtrate in the Bowman's capsule.
Ultrafiltration separates molecules by size. Protein molecules are too large to pass through the capillary wall and the capsule wall, so they remain in the blood and are absent from the filtrate. Finding protein in urine would therefore suggest damage to the filtering membrane rather than a failure of reabsorption.
Example 3. A student drinks a litre of water. Describe and explain the change in the urine produced over the next hour.
The blood becomes more dilute, so the pituitary gland releases less ADH. With less ADH, the nephron reabsorbs less water from the filtrate, so a large volume of dilute urine is produced. This returns the water content of the blood towards normal, which is negative feedback.
Example 4. Explain why a person who eats a diet very high in protein excretes more urea than one who does not.
Amino acids cannot be stored, so any excess beyond what is needed for growth and repair is deaminated in the liver. Deamination removes the amino group and produces ammonia, which the liver converts to urea. A high-protein diet therefore supplies more excess amino acids, more deamination occurs, more urea is made, and the kidneys excrete more of it.
Common mistakes and how to avoid them
Calling faeces an excretory product. Faeces are egested. The material was never part of a cell's metabolism, so removing it is not excretion.
Saying the kidney "makes" urine from nothing. Urine is filtered from the blood. Describe it as formed from the filtrate, not manufactured.
Forgetting that glucose is completely reabsorbed. Glucose in urine indicates diabetes, not normal function.
Mixing up ureter and urethra. The ureter runs from kidney to bladder; the urethra runs from bladder to the outside. One letter changes the meaning, so write carefully.
Saying ADH removes water. ADH controls how much water is reabsorbed. More ADH means more water returned to the blood and less urine, which is the opposite of what candidates often write.
Treating deamination as a kidney process. Deamination happens in the liver. The kidney excretes the urea the liver has already made.
Exam technique for Excretion
Describe the kidney as a two-stage process and name both stages. An answer that mentions filtering but never uses the words ultrafiltration and selective reabsorption rarely gains full marks.
When a question gives a substance and asks where it ends up, reason from molecule size and from need. Large molecules stay in the blood; small useful ones are filtered then reabsorbed; small waste ones are filtered and stay in the urine.
For osmoregulation questions, always complete the feedback loop: state the change in blood concentration, the change in ADH, the change in water reabsorbed, the resulting urine, and the return to normal. Marks are allocated along that chain.
Where dialysis and transplant are compared, give a balanced answer covering an advantage and a disadvantage of each rather than arguing for one.
Watch the command word. Name wants one word; describe wants what happens; explain wants why it happens. Many marks in this topic are lost by describing when the question asked for an explanation.
Quick revision summary
- Excretion removes toxic metabolic waste: mainly CO₂ (lungs), urea (kidneys, skin) and excess water and salts.
- Egestion is not excretion; secretion releases useful substances.
- The liver deaminates excess amino acids, making ammonia then urea, and forms bile pigments from old red blood cells.
- The kidney works by ultrafiltration (size-based, under high pressure) then selective reabsorption (all glucose, needed water and salts).
- Nephron order: Bowman's capsule → proximal tubule → loop of Henlé → distal tubule → collecting duct.
- Glucose reabsorption uses active transport, so tubule cells have many mitochondria.
- ADH raises water reabsorption: concentrated blood → more ADH → small volume of concentrated urine. This is negative feedback.
- Dialysis is repeated and avoids surgery; a transplant is a one-off but needs a matched donor and immunosuppressants.
- Glucose in urine suggests diabetes; protein in urine suggests damage to the filter.
- Ureter = kidney to bladder; urethra = bladder to outside.