Urinary System & Nephrons

• The urinary system can hold up to 25% of the body's blood at any given time.
• Renal arteries carry blood from the aorta to the kidneys.
• Wastes are filtered by the kidneys as blood travels and then moved to the urinary bladder by the ureters.
• Ureters are tubes that move urine from the kidneys to the bladder.
• A valve/sphincter muscle at the base of the bladder will allow urine to be stored.
• At 200mL, stretch receptors on the bladder send a signal to the brain.
• At 400mL, more Stretch receptors are activated, sending a more urgent message.
• At 600mL, voluntary control is lost.

Nephrons

• Kidneys are made up of nephrons, which are responsible for kidney function.
• Each nephron is supplied with blood by the afferent arterioles.
• The afferent arterioles branch apart and form a high-pressure capillary bed, called the glomerulus.
• Efferent arterioles carry blood away from the glomerulus.
• The glomerulus is surrounded by the Bowman's capsule, which collects waste products to be released into urine.
• Bowman's capsule gradually forms the proximal tubule, which carries urine to the loop of Henle (found in the medulla of the kidney).
• Urine then travels from the loop of Henle toward the collecting duct via the distal tubule.
• The collecting duct collects urine from all of the nephrons in the kidney.

Urine Formation

• Urine formation depends on three steps/functions:
  • Filtration: movement of fluids from blood into Bowman's Capsule
  • Reabsorption: important solutes are returned to the blood via capillaries.
    • About 600ml of fluid goes through the kidney per minute.
    • 20% of this fluid is filtered by the nephrons.
  • Secretion: movement of materials from blood back into the nephrons/distal tubule.

Filtration

• Blood moves through the afferent arteriole into the glomerulus, which acts as a high-pressure filter.
• Dissolved solutes such as sodium chloride, water, amino acids, and H+ will move into Bowman's capsule.
• Plasma protein, blood cells, and platelets are too large to move through to the Bowman's Capsule.

Reabsorption

• Wastes go back into the blood against the concentration gradient.
• Sodium (Na+) ions are carried across using active transport
• Negative ions (Cl-) ions follow the positive ions across using passive transport.
• This causes a gradient to form, which will draw water out of the nephron back into the blood.
• Glucose may also be absorbed back to the blood stream, but if blood glucose levels are high, some of the excess glucose will be excreted.

Secretion

• Any other wastes are moved from the blood and into the nephron.
• This process will balance H+, nitrogen-containing wastes, and other minerals (K+).
• This occurs via active transport, so cells around the distal tubule are packed with mitochondria.