The kidney is a very important organ of the urinary system that produces urine. The two important functions of the kidney are to:

1. Maintain water balance in the body
2. Excrete toxic waste products from metabolism out of the body. The main component of urine is urea, which is a waste product generated when protein is broken down.

A General Idea of How the Urinary System Removes Waste:

• Blood is transported to the kidney through the renal artery, which branches out from the aorta.
• Urine is produced in the kidney through these processes:
  • Ultrafiltration: Water, salt ions, urea, glucose and other small molecules are filtered out from the blood.
  • Selective reabsorption: Useful substances, such as glucose, water and salts are reabsorbed into the capillaries.
• The urine, which contains water, salt ions and urea, is transported from the kidney to the bladder through ureter. Urine is stored in the bladder until it is excreted out of the body through the urethra.
• The blood, after waste has been removed, returns to the circulatory system through the renal vein and to the heart through vena cava.

Let's take a closer look at how urine is produced in the kidney:

Each kidney has about 1 million nephrons, which is the functional unit that produces urine.

👇 The structure of nephron 👇 

• Blood enters the nephron through glomerulus, which is a ball of capillary network.
• A high pressure is created at the glomerulus because the diameter of the arteriole leaving (efferent arteriole) is smaller than the diameter of the arteriole entering (afferent arteriole).
• Ultrafiltration
  • Due to the high pressure at the glomerulus, small molecules are filtered out of the capillary into the Bowman's capsule. 
  • Proteins are too large to leave the capillary so stay in the blood.
  • The filtrate contains glucose, urea, water and ions, and it continues to travel through the tubule.
• Selective reabsorption
  • Some essential molecules, such as glucose, were filtered out from the blood, so they need to be reabsorbed into the blood.
  • All of the glucose is reabsorbed at the proximal convoluted tube through active transport. There are a lot of mitochondria at the proximal convoluted tube to provide energy for the active transport of glucose.
  • Some water and ions are also reabsorbed along the tubule.
  • Water reabsorption at the collecting duct is controlled by ADH (anti-diuretic hormone).
• The remaining filtrate (water, ions, urea) in the collecting ducts form the urine, and urine is transported to the bladder through ureter and then excreted through urethra.

ADH (Anti-diuretic hormone)

The water level in the blood plasma is balanced by a hormone called ADH (anti-diuretic hormone).

ADH increases the reabsorption of water to the bloodstream in the collecting duct.

When the brain detects that there is too less water in the blood:

• Pituitary gland releases more ADH
• More water is reabsorbed in the kidney
• Less water is excreted through urine
• The water level in blood increases

• Pituitary gland releases less ADH
• Less water is reabsorbed in the kidney
• More water is excreted through urine
• The water level in blood decreases

Wanna Boost Up Your IGCSE Biology?