2iExcretion

Excretion is the removal from the body of the waste products of metabolism (the chemical reactions going on inside cells).

Do not confuse it with egestion — faeces are undigested food that never entered the cells, so egestion is not excretion.

Waste gases in a leaf

Two gases are produced as waste products of metabolism in a plant:

Both gases leave the leaf by diffusion through the stomata (singular: stoma), the pores mainly on the lower epidermis, opened and closed by guard cells.

What actually comes out, and when

• In bright light: photosynthesis is faster than respiration, so there is a net loss of oxygen and a net uptake of carbon dioxide.
• In darkness: only respiration happens, so there is a net loss of carbon dioxide and net uptake of oxygen.
• At the point where they balance: the two rates are equal and there is no net movement of either gas.

Water vapour is also lost through the stomata, but that is transpiration, not excretion.

Human organs of excretion

Urea is made in the liver from excess amino acids (which cannot be stored). The urea is carried dissolved in the blood plasma to the kidneys.

Common mistake: examiners repeatedly see students writing that carbon dioxide is excreted by the kidneys. It is not. Carbon dioxide is excreted by the lungs only. Kidneys excrete urea, water and ions.

The skin's main job is temperature control — but because sweat contains urea and ions, the skin still counts as an organ of excretion.

Examiner tip. When a question asks about carbon dioxide removal by a plant, say it is used up / removed by photosynthesis, not just that it 'goes into the leaf'. Vague answers that repeat the wording of the question earn nothing. Equally, never list carbon dioxide as a kidney product — that error costs a mark almost every time it appears.

(Separate Biology only — not in Science (Double Award))

Structure of the urinary system

Learn the parts in order and be able to label them:

Cut a kidney open and you see three regions: the outer cortex, the inner medulla, and the renal pelvis where urine collects before entering the ureter.

The two roles of the kidney

1. Excretion

The kidney removes urea from the blood. Urea is the toxic waste made in the liver when the liver breaks down excess amino acids. Excess ions (such as sodium and chloride) are also excreted.

2. Osmoregulation

The kidney controls the water content of the blood, keeping the blood plasma at a constant concentration. This is part of homeostasis.

• If the blood is too dilute (too much water), the kidney produces a large volume of dilute urine.
• If the blood is too concentrated (too little water — after sweating, or drinking little), the kidney produces a small volume of concentrated urine.

Both jobs happen at the same time, in the same structures: blood is filtered, then the useful substances are taken back.

Summary in one line: filter everything small out of the blood, then reabsorb what the body needs; what is left becomes urine.

Note that ureter and urethra are different tubes and are easy to muddle in a hurry. Ureter = kidney → bladder. Urethra = bladder → outside.

Examiner tip. Watch the command word. 'Describe the structure of the urinary system' wants named parts and what they look like/connect to — not an explanation of filtration. 'Explain' questions need reasons ('because…'). Examiners report that confusing describe and explain is one of the most frequent reasons for losing marks.

(Separate Biology only — not in Science (Double Award))

Each kidney contains about a million microscopic tubes called nephrons. This is where the blood is actually filtered.

Structure of a nephron — learn the order

1. Bowman's capsule — a cup-shaped structure in the cortex, containing…
2. Glomerulus — a knot of blood capillaries inside the Bowman's capsule
3. Proximal convoluted tubule — the first coiled tubule, in the cortex
4. Loop of Henle — a long hairpin loop that dips down into the medulla
5. Distal convoluted tubule — the second coiled tubule, back in the cortex
6. Collecting duct — runs down through the medulla to the renal pelvis

Blood arrives at the glomerulus through a wide vessel and leaves through a narrower one. This builds up high blood pressure inside the glomerulus.

Ultrafiltration

Ultrafiltration happens in the Bowman's capsule.

• The high pressure in the glomerulus forces liquid out of the capillaries and into the Bowman's capsule.
• The capillary walls and the capsule wall act as a filter with tiny pores — only small molecules get through.

The liquid that collects in the Bowman's capsule is called the glomerular filtrate. It is essentially blood plasma minus the proteins and cells.

At this stage the filtrate contains plenty of useful substances — glucose, most of the water and many of the ions. The rest of the nephron's job is to take those back.

Common mistake: if a diagram labels structures 'Area X' and 'Area Y', you must still write the proper names — glomerulus, Bowman's capsule, proximal convoluted tubule. Examiners report answers that only say 'in area X' and score nothing.

Examiner tip. Use the full name proximal convoluted tubule and keep it distinct from the distal convoluted tubule and the loop of Henle — examiners note that these three are regularly swapped over. Remember the running order: capsule → proximal → loop of Henle → distal → collecting duct.

(Separate Biology only — not in Science (Double Award))

Ultrafiltration is not fussy — it forces out useful substances as well as waste. Everything the body still needs must now be taken back into the blood.

Selective reabsorption of glucose (proximal convoluted tubule)

All of the glucose in the filtrate is reabsorbed at the proximal convoluted tubule, into the blood capillary wrapped around it.

Why it must happen:

• Glucose is a valuable substance — it is the respiratory substrate used by cells to release energy.
• Losing glucose in the urine would waste it, so none should normally appear in urine.

How it happens:

• Glucose is at a lower concentration in the filtrate than in the blood by the time reabsorption is nearly complete, so it must move against the concentration gradient.
• This requires active transport, which uses energy from respiration — which is why the cells lining the proximal convoluted tubule contain many mitochondria.

It is called selective reabsorption because only useful substances (glucose, some ions) are taken back; urea is left behind.

Water reabsorption from the collecting duct

Water is reabsorbed all along the nephron, but the final adjustment happens in the collecting duct.

• As the collecting duct passes down through the medulla, the surrounding tissue fluid is very concentrated (has a low water concentration).
• So water moves out of the filtrate, through the partially permeable walls of the collecting duct, into the blood, by osmosis — down a water concentration gradient.
• How much water leaves depends on how permeable the collecting duct walls are, and that is controlled by the hormone ADH.

Composition of urine

The liquid left at the end of the collecting duct is urine. It contains:

• water
• urea
• ions (mineral salts)

Normally it contains no glucose (all reabsorbed by active transport) and no protein (too large to be filtered out in the first place).

Examiner tip. When explaining glucose reabsorption, name the process — active transport — and say it needs energy from respiration because glucose moves against a concentration gradient. Saying only 'the glucose goes back into the blood' is a description, not an explanation, and examiners flag exactly this lack of depth.

(Separate Biology only — not in Science (Double Award))

ADH (anti-diuretic hormone) is made by the pituitary gland and released into the blood. It controls the water content of the blood — osmoregulation.

Its target is the collecting duct (and distal convoluted tubule) of the nephron.

The key rule: ADH makes the walls of the collecting duct MORE permeable to water.

More permeable → more water reabsorbed by osmosis into the blood → less water lost in urine.

When water content of the blood is too LOW

(e.g. after sweating heavily on a hot day, exercising, eating salty food, or drinking little)

1. The brain detects that the blood is too concentrated.
2. The pituitary gland releases more ADH into the blood.
3. The collecting duct becomes more permeable to water.
4. More water is reabsorbed into the blood by osmosis.
5. A small volume of concentrated urine is produced.
6. The water content of the blood rises back to normal.

When water content of the blood is too HIGH

(e.g. after drinking a lot of water)

1. The brain detects that the blood is too dilute.
2. The pituitary releases less ADH.
3. The collecting duct becomes less permeable to water.
4. Less water is reabsorbed.
5. A large volume of dilute urine is produced.
6. The water content of the blood falls back to normal.

Summary table

This is negative feedback: a change away from the normal level triggers a response that reverses it. It is part of homeostasis — keeping a constant internal environment.

Alcohol reduces ADH release, which is why it causes a large volume of dilute urine and dehydration. (Only mention this if a question gives you the context.)

Examiner tip. Examiners specifically report candidates writing that ADH reduces the permeability of the collecting duct. It does the opposite — more ADH = more permeable = more water reabsorbed = less urine. If you can only remember one line about ADH, remember that one, and always finish the answer by stating the volume and concentration of the urine produced.