10 min readPathway sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B very short answer (2 marks) or short answer (3 marks), such as how dialysis helps

Excretion in humans: kidneys, nephron, dialysis and transplant

Excretion removes harmful wastes made in the body. Each kidney has about a million nephrons that filter blood and make urine. Dialysis cleans blood when kidneys fail.

Excretory systemTwo kidneys, two ureters, one urinary bladder, one urethra

NephronStructural and functional unit of the kidney: Bowman's capsule with the glomerulus, then the PCT, loop of Henle and DCT

Two main processesFiltration in Bowman's capsule, then reabsorption of useful substances along the tubule

OsmoregulationKeeping the right amounts of water and mineral ions in the blood

Parts of the human excretory system

Part	Job
Kidneys (two)	Bean-shaped organs below the diaphragm, towards the back of the abdomen; they filter blood and make urine
Ureters (two)	Tubes that carry urine from each kidney to the bladder
Urinary bladder	A bag that stores urine for a time
Urethra	A muscular tube that carries urine out through the urinary opening

How a nephron makes urine

1
An artery brings blood with wastes and extra water to the kidney. In each nephron it forms a knot of capillaries, the glomerulus, inside the cup-shaped Bowman's capsule.
2★
Filtration: blood is filtered into the capsule. This fluid is the nephric filtrate. Red blood cells and proteins do not pass; they stay in the blood.
3
The filtrate flows along the renal tubule: the proximal convoluted tubule (PCT), the loop of Henle and the distal convoluted tubule (DCT).
4★
Reabsorption: capillaries around the tubule take useful substances, extra water and salts such as sodium and chloride back into the blood.
5
What is left is mainly urea in water: this is urine. It flows into the collecting tubules.
6
Urine goes down the ureters to the bladder, and is passed out from time to time through the urethra.

★ key step

Jobs of the kidneys, and other excretory organs

Kidneys remove nitrogenous wastes such as urea.
They control the water content of the body (osmoregulation) and keep the mineral balance of the blood. If this balance is upset, a person falls sick.
Each kidney has about one million nephrons, so a person can live with one healthy kidney.
Lungs remove carbon dioxide.
Skin: sweat glands remove extra salts and water when we perspire.
Liver also helps remove wastes; for example, it sends the pigment bilirubin out in the bile.

Dialysis vs kidney transplant

Dialysis (artificial kidney)A tube takes blood from an artery in the patient's arm or leg to a kidney machine. The blood flows in an inner tube, surrounded by dialysis fluid and separated from it by a thin membrane. Wastes pass out of the blood into the fluid. The cleaned blood returns through a vein, and the used fluid is thrown away.

Kidney transplantA surgeon puts in a healthy kidney donated by another person; the kidney that does not work may be removed. Care is taken that the patient's body accepts the new kidney.

These are needed when disease or an accident damages both kidneys. Without treatment, wastes build up in the blood and the person cannot survive.

How to draw a nephron and the excretory system

Excretory system: draw two bean-shaped kidneys, one on each side of the backbone, below the diaphragm.
Show the renal artery and renal vein joining each kidney to the aorta and the vena cava in the middle.
From each kidney, draw a thin tube, the ureter, running down into a bag, the urinary bladder. Draw the urethra leaving the bladder.
Nephron: draw a cup, the Bowman's capsule, holding a knot of capillaries, the glomerulus.
From the capsule, draw a coiled PCT, a hairpin-shaped loop of Henle, a coiled DCT and a collecting tubule.
Draw blood capillaries wrapped around the tubule. Mark filtration at the capsule and reabsorption along the tubule, and label every part.

In real life

Drinking enough water, especially in a hot Indian summer, helps the kidneys flush out urea. Dialysis units in hospitals keep kidney patients alive, and a kidney donated by a relative can give a patient a new life.

Exam angle

2 marks: “What is a nephron? Name its two main parts.” The structural and functional unit of the kidney. Bowman's capsule with the glomerulus, and the renal tubule (PCT, loop of Henle and DCT).

3 marks: “Explain how urine is formed in a nephron.” Filtration: blood in the glomerulus is filtered into Bowman's capsule; RBCs and proteins stay behind. Reabsorption: useful substances, extra water and salts return to the blood along the tubule. The rest, mainly urea in water, is urine, which goes to the collecting tubules.

3 marks: “Describe how an artificial kidney machine removes urea from the blood when both kidneys have failed.” Blood from an artery flows through an inner tube in the kidney machine, with dialysis fluid outside it across a thin membrane. Wastes pass from the blood into the fluid. The cleaned blood returns through a vein. Without this, wastes build up and the patient cannot survive.

Remember it

A nephron in three words: Filter, Take back, Throw out (filtration, reabsorption, urine).