The Kidney and Nephron Function

Overview of the Kidney

• Main Functions
  • Excretion of waste
  • Maintenance of the body's water level and electrolytes (e.g., salts)
  • Regulation of blood pressure
  • Production of certain hormones (not detailed here)
• Anatomical Position
  • Two kidneys, located near the back on either side of the spine, behind the liver.
• Zoomed-in View
  • Discussion of renal cortex (outer layer) and renal medulla (inner part), both play significant roles in filtration and excretion.

The Nephron

• Functional Unit of the Kidney
  • Responsible for waste excretion and water level maintenance.
• Structure
  • Each kidney contains approximately one million nephrons.
  • The nephron starts in the cortex, dips into the medulla, and ends in collecting ducts.

Blood Flow and Filtration Process

• Afferent Arteriole
  • Blood enters here and goes into the glomerulus.
• Glomerulus
  • A network of capillaries where filtration occurs; about 1/5 of the blood plasma gets filtered.
• Efferent Arteriole
  • Blood exits; maintains high pressure needed for filtration.
• Bowman’s Capsule and Space
  • Surrounds the glomerulus; initial collection site of filtrate (not yet urine).
• Filtrate
  • Contains ions (e.g., sodium), glucose, amino acids; large proteins and red blood cells do not get filtered.

Proximal Convoluted Tubule (PCT)

• Structure
  • The next segment after Bowman’s capsule; involves convoluted and straight parts.
• Function
  • Reabsorption of valuable substances (e.g. glucose, amino acids, and sodium) back into the bloodstream.
  • Active process using ATP to pump sodium and consequently reabsorb other molecules.

Loop of Henle

• Structure
  • Descends into the medulla and then ascends back up.
• Functions
  • Makes the renal medulla salty by actively pumping out salts (sodium, potassium, chloride) in the ascending limb, using ATP.
  • The descending limb is permeable only to water, which leaves due to the hypertonic (salty) environment of the medulla.
  • Major site for reabsorption of water; no ATP is required to reabsorb water.

Distal Convoluted Tubule (DCT)

• Structure
  • Located further along the nephron; near the Bowman’s capsule again.
• Function
  • Further reabsorption of calcium, sodium, and water.

Collecting Duct

• Function
  • Collects processed filtrate (now closer to urine) from multiple nephrons.
  • Passes through the hypertonic medulla again, and water reabsorption is regulated here by Antidiuretic Hormone (ADH).
  • Final urine is concentrated and transported to the urinary bladder via ureters.

Important Points

• Active Reabsorption and Filtration
  • ATP is used extensively in the reabsorption of ions and molecules.
• Water Reabsorption
  • Passive process driven by the osmotic gradient created by saltiness in the medulla.
  • ADH regulates water reabsorption in the collecting duct to concentrate urine.