Excretion is the removal from the body of unwanted and harmful products of metabolism (urea, uric acid, excess water and salts, CO₂). If they were not removed, they would build up in the blood and poison the cells. Excretory organs do more than “throw out waste”: they keep the internal environment constant — homeostasis — by regulating the water and salt content of the blood, the acid–base balance (pH) and blood pressure. Exams often ask about the structure of the kidney and nephron, the stages of urine formation, the difference between primary and final urine, and the functions of the skin.
§1. Excretory organs
Several organs take part in excretion in humans:
Kidneys — the main excretory organ: they remove urea, uric acid, excess water and mineral salts, and drug residues.
Lungs — remove carbon dioxide (CO₂) and water vapour.
Skin (sweat glands) — removes water, salts and a little urea.
Intestine and liver — the liver sends bilirubin (a breakdown product of haemoglobin) and other substances into the gut with bile; the intestine removes heavy-metal salts and undigested remains.
Another important job of the liver: toxic ammonia, formed when proteins are broken down, is converted in the liver into less toxic urea, which the kidneys then excrete.
The urinary system: kidneys → ureters → bladder → urethra. The bladder stores urine; when the smooth muscle in its wall contracts, urine is released. This is a reflex under the control of the spinal cord and brain.
The main excretory organ is the kidney; its structural and functional unit is the nephron.
§2. Structure of the kidney
The kidneys are a pair of bean-shaped organs on the back wall of the abdominal cavity, in the lower back, on either side of the spine. Each has a mass of about 150 g. Through the hilum on the concave side the renal artery enters and the renal vein and ureter leave.
A cut kidney shows three parts:
the outer cortex — dark, containing the glomeruli of the nephrons;
the inner medulla — made of renal pyramids containing tubules and collecting ducts;
the renal pelvis — collects urine draining from the pyramids and passes it into the ureter.
About 1500–1700 l of blood pass through the kidneys every day — the whole blood volume is filtered through them hundreds of times a day.
§3. The nephron — the working unit of the kidney
Each kidney has about 1 million nephrons. Parts of a nephron:
The glomerulus (Malpighian corpuscle) — a knot of capillaries. The afferent arteriole enters it and the efferent arteriole leaves. The afferent arteriole is wider than the efferent one, so blood pressure in the glomerulus is high — this drives filtration.
Bowman’s capsule (capsule of Shumlyansky–Bowman) — a double-walled cup around the glomerulus; filtered fluid collects in the space between its walls.
Tubules: the first (proximal) convoluted tubule, the loop of Henle, the second (distal) convoluted tubule. The tubules are wrapped in a network of capillaries — reabsorbed substances pass into them.
The tubules of several nephrons drain into a collecting duct, and the ducts open through the pyramids into the pelvis.
§4. How urine is formed
Urine forms in three stages.
1. Filtration. Under high pressure, water from blood plasma with small dissolved molecules passes from the glomerular capillaries into the capsule. The fluid formed — primary urine (filtrate) — amounts to about 150–170 l a day. Its composition is like plasma (water, glucose, amino acids, salts, urea), but it contains no proteins and no blood cells — they are too large to pass the filter.
2. Reabsorption. As the primary urine flows along the tubules, substances the body needs return to the blood: most of the water, glucose (all of it in a healthy person), amino acids, vitamins, needed salts. Urea is hardly reabsorbed, so its concentration rises.
3. Secretion. Tubule cells actively move some substances from the blood into the tubule (certain drugs, excess ions).
The result is final (secondary) urine, about 1.5 l a day. It contains water (about 95–96%), urea (about 2%), uric acid, creatinine and salts. The urine of a healthy person contains no glucose and no protein.
Glucose in urine is a sign of diabetes mellitus; protein in urine is a sign of damage to the kidney filter (glomeruli).
Control.Antidiuretic hormone (vasopressin) from the posterior pituitary increases water reabsorption in the tubules and collecting ducts — urine becomes scanty and concentrated. After drinking a lot, less hormone is released and more urine is made. Adrenal hormones control sodium and potassium balance.
§5. The skin: structure and functions
The skin is the body’s largest organ. It has three layers:
The epidermis (outer layer) — stratified squamous epithelium; its top cells become keratinised and are constantly shed, while the bottom layer divides to replace them. It contains the pigment melanin, which protects the skin from ultraviolet light (a suntan is the result).
The dermis (true skin) — connective tissue containing sweat and sebaceous glands, hair follicles, blood and lymph vessels, receptors and nerve endings.
The subcutaneous fat layer — keeps heat in, cushions blows, stores energy.
Functions of the skin:
protection — against microbes, mechanical injury, water loss, ultraviolet light;
thermoregulation — in heat the skin vessels widen, more blood flows in and heat is lost, and sweat evaporates and cools the body; in cold the vessels narrow and heat is kept;
excretion — water, salts and a little urea leave in sweat;
sensation — receptors for pain, temperature, pressure and touch;
vitamin D synthesis — under sunlight;
a little gas exchange.
§6. Hygiene and disease
Kidney diseases: nephritis (inflammation), kidney stones (deposits of salts, often linked to drinking water and diet). Colds, complications of tonsillitis and chilling can damage the kidneys.
Skin hygiene: regular washing, clean clothes, moderate time in the sun.
First aid: for a burn — cool the area under cold water for 10–15 minutes, do not break blisters, apply a clean dressing; for frostbite — warm gradually (do not rub with snow), give a warm drink.
In severe kidney failure an “artificial kidney” machine (haemodialysis) is used to clean the blood.
Worked example
In a day 170 l of primary urine and 1.7 l of final urine were formed. What percentage of the fluid was reabsorbed?
Volume reabsorbed: 170 − 1.7 = 168.3 l.
Fraction: 168.3 / 170 × 100% = 99%.
Answer: 99% of the fluid returned to the blood.
Worked example
The glucose concentration in primary urine equals that in blood (about 0.1%). How much glucose is in 150 l of a healthy person’s primary urine, and where does it go?
150 l of primary urine is about 150 000 g (its density is close to water).
Glucose: 150 000 × 0.001 = 150 g.
In a healthy person all this glucose is reabsorbed in the tubules.
Answer: about 150 g; all of it returns to the blood, so final urine has no glucose.
Worked example
A patient’s urine contains protein but no glucose. Which part of the nephron is damaged?
Protein is a large molecule that normally cannot cross the glomerular filter.
Protein in urine means the filter — the wall of the glomerular capillaries — is damaged.
There is no glucose, so reabsorption is working.
Answer: the glomerulus (the filtration stage) is damaged.
Check yourself
Where in the nephron does filtration take place?
What does primary urine lack compared with blood plasma?
At which stage does glucose return to the blood?
Glucose in urine is a sign of which disease?
If 160 l of primary urine give 1.6 l of final urine a day, how much fluid is reabsorbed?
Which hormone increases water reabsorption?
Which pigment protects the skin from ultraviolet light?
In which organ is ammonia converted into urea?
Common mistakes
Thinking primary urine contains protein — protein does not pass the filter.
Writing that filtration happens in the tubules — it happens in the glomerulus; the tubules reabsorb.
Not counting the lungs as excretory organs — they remove CO₂ and water vapour.
Saying glucose in final urine is normal.
Writing that urea is made in the kidney — it is made in the liver; the kidney only excretes it.
Mixing up the afferent and efferent arterioles — the afferent is wider.