The Kidney: Structure (Edexcel International A Level Biology)

• Humans have two kidneys
• The kidneys are responsible for carrying out two very important functions
  • As an osmoregulatory organ they regulate the water content of the blood 
    • This is essential for maintaining blood pressure and to prevent cell damage occurring due to osmosis
  • As an excretory organ they enable the excretion of the toxic waste products of metabolism, such as urea, and substances in excess of requirements, such as salts

The kidneys are located above the bladder. They are supplied with blood by the renal artery and connect to the bladder via the ureter.

The Function of the Kidneys & their Associated Structures Table

The gross structure of the kidney

• The kidney itself is surrounded by an outer layer known as the fibrous capsule
• Beneath the fibrous capsule, the kidney has three main regions
  • The cortex
  • The medulla
  • The renal pelvis

The kidney has three main regions; the cortex, the medulla, and the renal pelvis.

The microscopic structure of the kidney

• Each kidney contains thousands of tiny tubes, or tubules, known as nephrons
• Nephrons are the functional unit of the kidney and are responsible for the formation of urine
• Different parts of the nephron are found in different regions of the kidney
  • The cortex
    • Location of the glomerulus, Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule
  • The medulla
    • Location of the loop of Henle and collecting duct
  • The renal pelvis
    • All kidney nephrons drain into this structure, which connects to the ureter
• There are two types of nephrons in the kidney
  • Cortical nephrons
    • These occur mainly in the renal cortex and have a short loop of Henle that barely enters the medulla
    • They make up about 85% of the nephrons in a human kidney
  • Juxtamedullary nephrons
    • They have long loops of Henle that span across the entire medulla
    • Very efficient at conserving water in the body

The nephron spans the three regions of the kidney.

• There is also a network of blood vessels associated with each nephron
• Within the Bowman’s capsule of each nephron is a structure known as the glomerulus
  • Each glomerulus is supplied with blood by an afferent arteriole which carries blood from the renal artery
  • The afferent arteriole splits into a ball of capillaries that forms the glomerulus itself
  • The capillaries of the glomerulus rejoin to form the efferent arteriole
• Blood flows from the glomerulus into a network of capillaries that run closely alongside the rest of the nephron and eventually into the renal vein

The afferent arteriole supplies the capillaries of the glomerulus, which rejoin to form the efferent arteriole.