The Kidney and Excretion: KS3 Biology Guide

Excretion is the removal of metabolic waste products — substances produced by the body's chemical reactions — from the blood. The kidneys are the main excretory organs, filtering roughly 180 litres of blood per day to produce about 1–2 litres of urine, removing urea, excess salts, and water.

What is excretion and why does the body need it?

Start with the big picture: every living cell in the body carries out chemical reactions (metabolism) — breaking down glucose for energy, building proteins, and so on. These reactions produce waste products that, if allowed to accumulate, would poison the very cells that made them. Excretion is the body's solution: the continuous removal of these metabolic waste products via the blood and excretory organs.

Excretion is specifically the removal of waste products of metabolism — it is not the same as egestion (which is the removal of undigested food as faeces; faeces was never actually inside the body's cells).

The main waste products requiring excretion are:

Carbon dioxide (CO₂): produced by aerobic respiration in every cell (glucose + oxygen → carbon dioxide + water). CO₂ dissolves in blood plasma; if it accumulates, it forms carbonic acid, making blood too acidic. Excreted via the lungs.

Urea: produced in the liver when excess amino acids are broken down (deamination). Urea is toxic at high concentrations. Excreted by the kidneys in urine.

Water and mineral salts: produced or consumed by metabolic reactions; their concentrations in blood must be kept within narrow limits. Regulated primarily by the kidneys.

Without efficient excretion, urea builds up in the blood — a condition called uraemia — causing fatigue, nausea, and ultimately life-threatening complications. The kidneys prevent this, working continuously every moment of a person's life.

Where are the kidneys and what do they do?

Zoom out to the organ level. The two kidneys are bean-shaped organs, each roughly 12 cm long, situated in the lower back on either side of the spine. The blood supply to each kidney arrives via the renal artery, which branches directly from the aorta (the body's main artery). Filtered blood leaves via the renal vein, which drains into the vena cava and returns to the heart.

Urine produced in each kidney drains down a muscular tube called the ureter into the bladder, where it is stored until it is released through the urethra.

Each kidney contains approximately one million nephrons — the microscopic filtration units that do the actual work. At the level of the whole organ, the kidneys have two primary jobs:

Excretion of metabolic waste: removing urea, excess water, and excess salts from the blood

Osmoregulation: maintaining the correct water and ion balance in the body's fluids

How do the kidneys filter blood — what is ultrafiltration?

Now zoom in from the kidney to the nephron. Each nephron begins with a tight knot of capillaries called the glomerulus, enclosed in a cup-shaped structure called the Bowman's capsule.

Blood enters the glomerulus at high pressure (the vessel leading in is wider than the vessel leading out, which builds pressure). This high pressure forces small molecules out of the capillary blood and into the Bowman's capsule — a process called ultrafiltration. The substances that pass through include: glucose, urea, water, amino acids, and mineral ions (salts).

Large molecules — plasma proteins and red blood cells — are too large to squeeze through the filtration membrane. They remain in the blood. This is why healthy urine contains no protein and no blood cells: their presence in urine indicates a problem with the filtration membrane.

The liquid that collects in the Bowman's capsule is called the filtrate. It contains many useful substances that the body cannot afford to lose alongside the waste products.

How does selective reabsorption work?

The filtrate flows along the tubule of the nephron. As it travels, useful molecules are actively transported back into the surrounding capillaries — a process called selective reabsorption:

All glucose is reabsorbed (by active transport against the concentration gradient — this requires energy from ATP)

All amino acids are reabsorbed

Useful mineral ions (sodium, potassium) are reabsorbed in regulated amounts

Some water is reabsorbed (how much varies with the body's hydration state)

The cells lining the tubule are packed with mitochondria — the organelles that generate ATP — because active transport demands a large and continuous energy supply.

What is not reabsorbed stays in the tubule and becomes urine: urea (the toxic waste — it must go); excess water (any water the body does not currently need); and excess salts. This two-stage process — filter everything small, then take back what is needed — is far more efficient than trying to filter selectively in the first place.

What is urine made of and how does it form?

Urine is the end product of ultrafiltration followed by selective reabsorption. Its composition reflects what has been filtered out but not reabsorbed.

Substance | In blood arriving at kidney | In filtrate | In urine (excreted) | Notes
Urea | Present | Yes | Yes | Not reabsorbed — toxic waste
Glucose | Present | Yes | No (normally) | Completely reabsorbed by active transport
Water | Present | Yes | Some | Amount varies with hydration status
Proteins | Present | No | No | Too large to pass through filtration membrane
Red blood cells | Present | No | No | Too large to pass through filtration membrane
Salts (ions) | Present | Yes | Some | Amount regulated according to body's needs

Normal urine is approximately 95% water, with around 2% urea and small amounts of creatinine, uric acid, and dissolved salts making up the remainder. Its characteristic pale yellow colour comes from urochrome — a pigment formed when the liver breaks down old red blood cells.

How do the kidneys regulate water balance — what is osmoregulation?

The kidneys do not produce a fixed volume of urine each day; they adjust output continuously in response to the body's hydration state. This regulation of water and ion concentration in body fluids is called osmoregulation.

The control mechanism works via a hormone called ADH (anti-diuretic hormone), secreted by the pituitary gland at the base of the brain:

When you are dehydrated: blood becomes more concentrated (higher solute concentration). Receptors in the hypothalamus detect this and signal the pituitary to release more ADH into the blood. ADH travels to the kidney tubules and collecting ducts, making them more permeable to water — so more water is reabsorbed back into the blood. The result: a small volume of dark, concentrated urine.

When you are well hydrated: blood is more dilute. Less ADH is released. The tubules become less permeable to water — less water is reabsorbed. The result: a large volume of pale, dilute urine.

This is a classic negative feedback loop: the response (more or less water reabsorption) opposes the stimulus (blood being too concentrated or too dilute) and returns the system to the set point.

What happens when kidneys fail?

When the kidneys can no longer filter blood adequately, waste products and excess fluid accumulate in the blood — a dangerous condition. Causes of kidney failure include uncontrolled diabetes, chronic high blood pressure, certain infections, and inherited conditions such as polycystic kidney disease.

There are two main treatment options:

Dialysis mimics kidney filtration artificially. In haemodialysis (the most common form), the patient's blood is pumped through a machine containing a semi-permeable dialysis membrane. On the other side flows a carefully prepared dialysate fluid containing the correct concentrations of glucose and salts but no urea. Urea, excess water, and excess salts pass from the blood across the membrane into the dialysate by diffusion and osmosis; the cleaned blood is returned to the patient. Sessions typically last around four hours and are needed three times per week.

Kidney transplant involves surgically placing a healthy donor kidney — from either a deceased donor or a living donor (usually a close relative) — into the patient. The transplanted kidney is connected to the patient's blood supply and ureter. Immunosuppressant drugs must be taken lifelong to prevent the immune system rejecting the donor organ. A successful transplant restores normal kidney function and is generally the preferred long-term outcome for eligible patients.

Frequently asked questions

What is the difference between excretion and egestion?

Excretion is the removal of metabolic waste products generated by the body's own chemical reactions — carbon dioxide (from respiration), urea (from amino acid breakdown), and excess water. Egestion is the removal of undigested food material as faeces. Faeces passed through the gut but was never inside the body's cells or part of its metabolism — so egestion is emphatically not excretion. This distinction is a common source of lost marks in biology exams.

What is urea and where does it come from?

Urea (chemical formula CO(NH₂)₂) is produced in the liver during deamination — the breakdown of excess amino acids. The amino group (–NH₂) is removed, forming ammonia (NH₃), which is highly toxic. The liver immediately converts it to the far less toxic urea, which is released into the bloodstream for the kidneys to filter and excrete in urine. The remaining carbon skeletons are used for energy or converted to glucose or fat.

Why does urine change colour?

Urine's pale yellow colour comes from urochrome, a pigment produced when the liver breaks down haemoglobin from old red blood cells. When well hydrated, a large water volume dilutes the urochrome, producing pale or nearly colourless urine. When dehydrated, the kidneys reabsorb more water (under ADH), concentrating the urine — the same urochrome dissolved in far less water gives a dark yellow or amber colour. Very dark urine with thirst signals you should drink more.

How does dialysis mimic kidney function?

In haemodialysis, blood is pumped through a machine containing a dialysis membrane — semi-permeable, allowing small molecules through but not large ones. On the other side flows dialysate containing the correct concentrations of glucose and salts but no urea. Urea diffuses from the high-concentration blood into the zero-urea dialysate; excess water and salts also move across. The cleaned blood is returned to the patient. Dialysis is needed typically three times per week because the machine replaces only the kidneys' filtering function, not their continuous operation.

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