7.11 - Kidneys - The Nephron

Gross structure of the kidney and the nephron

• Each kidney is part of the urinary system.
• Blood is brought to the kidney by the renal artery and filtered blood leaves by the renal vein.
• Urine produced by the kidney drains out through the ureter, which carries it to the bladder, where it's stored before being released through the urethra.

Connection to the central blood vessels

• The renal artery branches from the aorta - the body's main artery, carrying blood away from the heart.
• The renal vein empties into the vena cava - the body's main vein, returning blood to the heart.
• So both kidneys connect to these large central vessels via their renal arteries and renal veins.

Regions inside the kidney

• Cortex - The outer part of the kidney.
• Medulla - The inner part of the kidney.
• Pelvis - A wide, funnel-shaped region at the top of the ureter (where it meets the kidney) that collects the urine and channels it into the ureter.

The nephron

• Filtering happens in tiny tube-like structures called nephrons (also called kidney tubules). A single kidney contains around a million of them.
• The nephrons run across both the cortex and the medulla.
• A network of capillaries wraps around the tubule alongside the nephron.

Each nephron is made up of the following, in order:

1. Glomerulus (also called the capillary knot) - A ball of capillaries where the nephron begins.
2. Bowman's capsule - A cup-shaped structure that the glomerulus sits inside.
3. Proximal convoluted tubule - The first section of the long, looping tubule leading from the Bowman's capsule.
4. Loop of Henle - The middle looping section of the tubule.
5. Distal convoluted tubule - The final section of the tubule.
6. Collecting duct - Carries the finished urine away towards the ureter.

High pressure in the glomerulus

• The arteriole carrying blood into the glomerulus is wider than the one carrying blood away from it.
• This difference builds up a high pressure inside the glomerulus.

Ultrafiltration and selective reabsorption

Inside each nephron, urine is made in two stages: ultrafiltration, then selective reabsorption.

Stage 1 - Ultrafiltration

• Blood arriving at the glomerulus is under high pressure, because the arteriole leaving the glomerulus is narrower than the one entering it.
• This high pressure squeezes the liquid part of the blood - water, plus the small dissolved molecules glucose, urea, and ions - out of the glomerulus and into the Bowman's capsule.
• The membranes between the glomerulus and the capsule act like filters with very small holes:
  • Larger molecules such as proteins, and the blood cells, are too big to be squeezed out and stay behind in the blood.
• The high-pressure filtration is called ultrafiltration, and the filtered liquid now in the Bowman's capsule is the glomerular filtrate.

Stage 2 - Selective reabsorption

• As the filtrate flows along the tubule, substances the body needs are taken back into the blood, while waste is left behind.
• It's called selective reabsorption because only some substances are reabsorbed:

From filtrate to urine

1. The substances not reabsorbed - urea, excess water, and excess ions - stay in the tubule and form urine.
2. Urine passes through the collecting duct, into the ureter, and down to the bladder, where it's stored before being released through the urethra.