IGCSE Biology / Homeostasis and excretion
Kidneys, ureters and bladder revision notes
Edexcel International GCSE Biology (4BI1), chapter 8: Homeostasis and excretion, specification point 2.72B, 2.73B. Free lesson.
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The kidneys remove urea from the blood and produce urine from the filtered material that is not reabsorbed. Knowing the parts of the urinary system and keeping the paths of blood and urine separate helps you explain how excretion works.
What each part of the urinary system does
The kidneys sit at the back of the abdomen, one on each side. Blood arrives through the renal artery and leaves through the renal vein. The kidneys remove urea from the blood, so the blood leaving contains much less urea. The ureters carry urine from the kidneys to the bladder, one tube from each kidney. The bladder is a muscular bag that stores urine. The urethra carries urine from the bladder out of the body. There is one urethra.
How a kidney is arranged
A kidney cut in half has three main regions. The cortex is the outer region, and contains the glomeruli and Bowman's capsules. The medulla is the inner region, where the loops of Henle and collecting ducts run. Urine collects in the renal pelvis, then leaves through the ureter. A nephron is a filtering unit in the kidney. Each nephron spans the cortex and medulla. There are about a million nephrons in each kidney.
How blood travels through the kidney
Blood enters through the renal artery carrying urea, a waste substance made in the liver. In the nephrons, blood is filtered and useful substances are reabsorbed. The blood then leaves through the renal vein with much less urea. The glomeruli, where filtration begins, are in the cortex. Urea is made in the liver, not the kidney. The kidney removes urea from the blood.
How urine leaves the body
The filtered material that is not reabsorbed becomes urine. It collects in the renal pelvis before travelling to the bladder and then out of the body. Urine follows the route nephron → renal pelvis → ureter → bladder → urethra. Blood follows a separate route through the kidney: renal artery → kidney → renal vein.