The Kidney (Cambridge O Level Biology)

Organs of the urinary system table

Organ	Explanation
Kidney	Two bean-shaped organs that filter the blood
Ureter	The tube that connects the kidney to the bladder
Bladder	The organ that stores urine (excess water, salts and urea) after the kidney produces it
Urethra	The tube that connects the bladder to the exterior; urine is released this way

The human urinary system diagram

The urinary system in humans contains the kidneys and bladder, as well as the ureter and urethra

The Nephron 

• Each kidney contains around a million tiny structures called nephrons, also known as kidney tubules or renal tubules
• The nephrons 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 and the urine collects there before it flows into the ureter to be carried to the bladder for storage

Nephron diagram

The kidney contains many tiny structures known as nephrons

•  Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries  known as the glomerulus, which sits inside the cup-shaped Bowman’s capsule 
• The smaller molecules carried in the blood are forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate
  • The substances forced out of the capillaries are
    • Glucose
    • Water
    • Urea
    • Salts
• Some of these filtered molecules are useful and will be reabsorbed back into the blood further along the nephron 
• The urea, water and mineral salts that remain in the filtrate will form urine which is stored in the bladder

Filtration of blood diagram

The blood is filtered by the glomerulus

Components of filtrate table

Component	Location of reabsorption
Water	Loop of Henle and collecting duct
Salts	Loop of Henle
Glucose	Proximal convoluted tubule
Urea	Not reabsorbed

• After the Bowman’s Capsule, glucose is the first substance to be reabsorbed; this occurs in the proximal convoluted tubule
• As the filtrate passes through the Loop of Henle, necessary salts are also reabsorbed back into the blood
• When salts are reabsorbed back into the blood, water follows by osmosis
• • Water is also reabsorbed from the collecting duct in different volumes, depending on the requirements of the body

Reabsorption diagram

Reabsorption occurs primarily in the proximal convoluted tubule and loop of Henle