The Kidneys (OCR Gateway GCSE Biology)

• The kidneys are located in the back of the abdomen and have two important functions in the body:
  • They regulate the water content of the blood (vital for maintaining blood pressure)
    • Kidneys alter blood water content by controlling the volume of water lost in the urine
  • They excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirement (such as salts)

The structure of the kidney

• There are three regions of the kidney
  • Cortex - the outermost region
  • Medulla - the inner section of the kidney
  • Renal pelvis - the tube linking the kidney to the ureter
• Each kidney contains around a million tiny structures called kidney tubules or renal tubules
  • The tubules start in the cortex of the kidney, loop down into the medulla and back up to the cortex
• The contents of the nephrons drain into the innermost part of the kidney (the renal pelvis) and the urine collects there before it flows into the ureter to be carried to the bladder for storage

• A kidney tubule has several sections:
  • Bowman's capsule
  • Proximal convoluted tubule
  • Loop of Henlé
  • Distal convoluted tubule
  • Collecting duct
• Surrounding the tubule is a network of capillaries with a knotted section, called the glomerulus, which sits inside the Bowman's capsule

Kidney tubule structure

Urine Formation

• The role of the tubules in the kidney is to filter out the waste products from the blood for excretion in the urine
• Another fundamental role of the kidney is to ensure that all the required substances are reabsorbed back into the blood 
• This process occurs in different stages throughout the structure of the tubule

1. Ultrafiltration

• Ultrafiltration occurs when all the small molecules are squeezed from the blood in the glomerulus into the Bowman's capsule of the kidney tubule
• Small molecules include: glucose, water, urea, mineral ions
• Proteins and blood cells are too large and so remain in the capillaries

Small molecules are filtered out of the blood in the glomerulus into the Bowman's capsule of the kidney tubule

2. Reabsorption

•  Reabsorption of the required substances occurs throughout the rest of the tubule structure which includes:
  • The first convoluted tubule
  • The loop of Henle
  • The second convoluted tubule
  • The collecting duct 
• Substances that are reabsorbed include:
  • Glucose: All of the glucose actively transported back into the blood to be used in respiration
  • Water: Water is reabsorbed by osmosis, the quantity depends on the requirements of the body
  • Mineral salts: Minerals are reabsorbed by diffusion, like water, the quantity depends on the requirements of the body

Glucose, salts and water are reabsorbed from the tubule into the blood

Water content of the body

• There are two sources of water in the body:
  • Water produced as a result of aerobic respiration
  • Water in the diet
• Water is lost from the body in the following ways:
  • Via the lungs during exhalation (breathing out)
  • Lost from the skin as sweat (alongside mineral ions and urea)
  • Excreted in urine
• Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys

Urine production

• Urine produced by the kidneys contains a mixture of urea, excess mineral ions and excess water in varying proportions
• The quantity of water in the urine varies depending on reabsorption in the nephron
• This allows the kidney to regulate blood water content and excrete any excess water in the urine
• When blood water concentrations are high
  • Less water is reabsorbed and more is excreted in the urine
  • Large volumes of more dilute urine are produced
  • Urine is pale in colour as the urea is less concentrated
  • This may happen when water intake is high and sweating is minimal
• When blood water concentrations are low
  • More water is reabsorbed and less is excreted in the urine
  • Small volumes of more concentrated urine are produced
  • Urine is darker yellow or orange in colour as the urea is less diluted
  • This may happen when water intake is too low or sweating has increased due to high temperatures or exercise