Cambridge IGCSE Biology 13 Excretion in Humans Exam Questions

About Cambridge IGCSE Biology 13 Excretion in Humans

Excretion in humans covers how the body removes waste products of metabolism in Cambridge IGCSE Biology. Core candidates need the excretory products (carbon dioxide from the lungs, and urea, excess water and ions in urine) and must identify the kidneys, ureters, bladder and urethra. Extended candidates also learn the cortex and medulla, the structure and function of the nephron, ultrafiltration in the glomerulus, selective reabsorption of glucose, water and ions, and how the liver makes urea by deamination of excess amino acids. Examiners ask why glucose is not normally found in urine and why urea is toxic if it builds up.

What you need to know

• Excretory products Carbon dioxide is excreted by the lungs. The kidneys excrete urea, excess water and excess ions in urine. Urine volume and concentration depend on water intake, temperature and exercise.
• Urinary system Kidneys filter the blood; ureters carry urine to the bladder; the bladder stores urine; the urethra carries urine out of the body.
• Deamination (Extended) Excess amino acids are deaminated in the liver: the nitrogen-containing part is removed and converted to urea. The liver also assimilates amino acids by converting them to proteins, such as plasma proteins.
• Kidney structure (Extended) The kidney has an outer cortex and inner medulla. Each nephron has a glomerulus inside a Bowman's capsule, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule and a collecting duct.
• Ultrafiltration (Extended) High blood pressure in the glomerulus forces water, glucose, salts and urea into the Bowman's capsule. Blood cells and large proteins are too big to pass through.
• Reabsorption (Extended) All glucose is reabsorbed in the proximal convoluted tubule. Some salts and much of the water are reabsorbed along the nephron, including from the collecting duct. Urea is not reabsorbed, so it becomes concentrated in urine.

Common mistakes in 13 Excretion in Humans

• Saying the kidneys excrete faeces or undigested food. Faeces are egested.
• Saying urea is made in the kidneys. Urea is made in the liver by deamination and removed by the kidneys.
• Saying protein appears in normal filtrate. Proteins are too large to be filtered from the glomerulus.
• Confusing ureter and urethra. Ureters go from kidney to bladder; the urethra goes from bladder to outside.
• Saying glucose is excreted in urine normally. All glucose is reabsorbed in a healthy person.

Try three 13 Excretion in Humans exam-style questions