How do kidneys handle and excrete urea?

Kidneys manage and remove urea by filtering blood in the nephrons, reabsorbing useful substances, and excreting the leftover urea and extra water as urine through the renal tubule, ureters, and bladder. The liver first changes toxic ammonia into safer urea, which then enters the bloodstream to be filtered by the kidneys.

1. Urea Production in the Liver

• When proteins break down, they release extra amino acids that produce ammonia, which is harmful to the brain.
• The liver changes ammonia into a less harmful form called urea through the urea cycle.
• This urea then enters the blood and travels to the kidneys.

2. Glomerular Filtration

• Blood flows into the kidneys through the renal arteries and into millions of tiny units called nephrons.
• Inside each nephron, a part called the glomerulus (a cluster of small blood vessels) filters the blood.
• Water, urea, and waste pass into the renal tubule, while larger molecules like proteins and blood cells stay in the blood.

3. Tubular Reabsorption and Secretion

• As the fluid moves through the tubule, the body reabsorbs most of the water, nutrients, and minerals it needs back into the blood.
• The tubule also helps remove extra acids from the blood.
• Urea, extra water, and other wastes remain in the fluid and move forward.

4. Urine Formation and Excretion

• The remaining fluid, now urine, collects in the kidney’s renal pelvis.
• The ureters carry urine from the kidneys down to the bladder, where it is stored.
• Muscles in the ureters contract and relax to push urine into the bladder.
• When the bladder fills, urine leaves the body through the urethra.