Excretion in Humans ICSE Class 10 Biology: Kidneys, Nephron, Urine Formation and Dialysis

Excretion is the process by which the body removes metabolic waste products that would otherwise become toxic. In humans, the kidneys are the main excretory organs, but the skin, lungs, and liver also play essential roles. This study note builds the concept from first principles and then adds the exam-relevant details you need for ICSE.

Why Excretion Matters: Waste Products and the Organs Involved

Every living cell produces waste while carrying out metabolism. Protein breakdown produces nitrogenous wastes such as urea and uric acid; carbohydrate and fat metabolism produce carbon dioxide and water. If these substances accumulate, they change the pH and osmotic balance of body fluids and can poison enzymes. Excretion is therefore not optional — it is the body's way of keeping its internal environment stable.

Excretion is often confused with egestion. Egestion is the removal of undigested food from the alimentary canal, and the material has never entered the body's cells. Excretion, in contrast, removes wastes that were actually produced inside cells by metabolic reactions. This distinction is a favourite ICSE question.

The main excretory organs and their wastes are:

• Kidneys: urea, uric acid, creatinine, excess water, and salts.
• Skin: water, salts, and a small amount of urea in sweat.
• Lungs: carbon dioxide and water vapour.
• Liver: not an excretory organ itself, but it converts toxic ammonia to urea and produces bile pigments that are eliminated in faeces.

The Kidney and Nephron: The Functional Unit

The human urinary system consists of a pair of kidneys, two ureters, a urinary bladder, and a urethra. The kidneys lie in the abdominal cavity, one on each side of the vertebral column. In a longitudinal section, each kidney shows an outer cortex, an inner medulla, and a central pelvis that narrows into the ureter. Urine is continuously formed in the kidneys, stored in the bladder, and released through the urethra.

The structural and functional unit of the kidney is the nephron. Each kidney contains about one million nephrons. A nephron begins as a cup-shaped Bowman's capsule that surrounds a tuft of capillaries called the glomerulus. From the capsule, the filtrate passes through the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT), before entering a collecting duct. Many collecting ducts converge to carry urine to the renal pelvis.

The blood supply of the nephron is specially arranged. Blood enters the glomerulus through a wider afferent arteriole and leaves through a narrower efferent arteriole. This difference in diameter creates high blood pressure inside the glomerular capillaries, which is essential for filtration. The efferent arteriole then forms a second capillary network around the tubules, supplying them with oxygen and picking up reabsorbed substances.

Urine Formation: Filtration, Reabsorption, and Secretion

Urine formation happens in three main steps. The first is glomerular filtration, also called ultrafiltration. The high pressure in the glomerulus forces water, glucose, amino acids, urea, and dissolved salts through the filtration membrane into Bowman's capsule. Blood cells and plasma proteins are too large to pass through, so the filtrate is almost protein-free. This filtrate is not urine yet; it is a fluid that still contains useful substances.

The second step is selective reabsorption, which occurs mainly in the PCT. Here, glucose and amino acids are reabsorbed by active transport into the blood. Water follows passively by osmosis, and needed salts are reabsorbed by active transport. The loop of Henle creates a concentration gradient in the medulla, which helps the collecting duct reabsorb water. The DCT and collecting duct adjust the final concentration of urine under the influence of hormones.

The third step is tubular secretion. The tubular cells actively secrete hydrogen ions, potassium ions, ammonia, and certain drugs into the filtrate. This process helps the body get rid of extra acids and toxins that were not filtered at the glomerulus. The final fluid that leaves the collecting duct is urine, whose concentration is regulated by ADH (antidiuretic hormone). When the body is dehydrated, ADH makes the collecting duct more permeable to water, so more water is reabsorbed and urine becomes concentrated.

Composition of Urine and the 180 L vs 1.5 L Numerical Idea

Normal urine is about 95% water. The remaining 5% contains urea, uric acid, creatinine, sodium, potassium, chloride, and other salts. The yellow colour comes from a pigment called urochrome, which is derived from bile pigments. The presence of glucose or albumin in urine is abnormal and suggests conditions such as diabetes mellitus or kidney damage.

A useful numerical idea is the difference between the volume of filtrate and the volume of urine. The kidneys receive about 1.2 litres of blood every minute. The glomerular filtration rate (GFR) is about 125 mL per minute. In one day, the filtrate formed is:

• 125 mL × 60 minutes = 7,500 mL per hour.
• 7,500 mL × 24 hours = 180,000 mL = 180 litres per day.
• Normal urine output is only about 1 to 1.5 litres per day.

This means about 99% of the filtrate is reabsorbed by the tubules. This is why the kidney can precisely control water and salt balance. If the GFR is too low, wastes accumulate; if it is too high, useful substances are lost. The body constantly adjusts the GFR through autoregulation and hormones.

Dialysis: The Artificial Kidney

When both kidneys fail, waste products such as urea and creatinine accumulate in the blood, a condition called uraemia. Dialysis is a life-saving procedure that performs the filtering function of the kidney outside the body. It is not a cure, but it replaces kidney function until a transplant is possible.

In haemodialysis, blood is taken from an artery, mixed with an anticoagulant, and passed through a machine called a dialyser. Inside the dialyser, blood flows on one side of a semipermeable membrane, while a specially prepared fluid called dialysate flows on the other side. Urea, creatinine, and excess salts diffuse from blood into the dialysate because their concentration is higher in blood. The cleaned blood is then returned to a vein.

The dialysate is carefully balanced to contain normal concentrations of glucose and essential salts. If these were absent, useful substances would diffuse out of the blood along with the wastes. By keeping their concentration equal to normal blood, the dialysate prevents their loss while still allowing waste products to be removed. This principle of diffusion across a membrane is the core exam idea for dialysis.

Beyond Kidneys: Skin, Lungs, Liver, and Osmoregulation

The skin helps excretion through sweat glands. Sweat contains water, sodium chloride, and small amounts of urea. Sweating also cools the body by evaporation, so the skin serves both excretion and thermoregulation. However, sweat is not a major route for nitrogenous waste removal; the kidneys handle that task far more efficiently.

The lungs excrete carbon dioxide and water vapour continuously. The liver plays a crucial role by deaminating excess amino acids. This process removes the amino group and produces ammonia, which is highly toxic. The liver immediately converts ammonia into the less toxic compound urea through the urea cycle. The urea is then released into the blood and removed by the kidneys. The liver also breaks down haemoglobin to produce bile pigments, which are excreted in faeces.

Osmoregulation is the regulation of water and solute balance in the body. The kidney achieves this by adjusting the amount of water reabsorbed. When water intake is low, the pituitary gland releases more ADH, making the collecting ducts more permeable to water and producing concentrated urine. When water intake is high, less ADH is released, and the urine becomes dilute. This negative-feedback system keeps blood volume and osmotic pressure within safe limits.

Key takeaways

• Excretion removes metabolic wastes from the body; egestion removes undigested food, so the two must not be confused.
• The nephron is the functional unit of the kidney; its blood supply creates high pressure for ultrafiltration.
• Urine formation involves glomerular filtration, selective reabsorption, and tubular secretion.
• The kidneys filter about 180 L of fluid per day but produce only 1–1.5 L of urine, meaning about 99% is reabsorbed.
• Haemodialysis removes urea and creatinine by diffusion across a semipermeable membrane against a balanced dialysate.
• The liver converts toxic ammonia to urea, while skin and lungs also excrete wastes; ADH controls water reabsorption and osmoregulation.

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