D4.2 - Formation of Urine

Formation of Urine

• Urine formation involves three processes: glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular Filtration

• Blood enters the glomerulus, acting as a high-pressure filter.
• Dissolved solutes pass through the glomerulus walls into Bowman's capsule, creating a filtrate similar to blood plasma.
• Plasma proteins, platelets, and red blood cells don't pass through the membrane, as they are too large.

Tubular Reabsorption

• Approximately 65% of the filtrate passes through the proximal tubule (including the loop of Henle).
• This process returns useful substances to the blood.
• Involves both active and passive transport mechanisms.
• Loop of Henle's function is to reabsorb water and ions from the glomerular filtrate.
• Water diffuses from filtrate into capillaries by osmosis.
• Cells of the descending limb are permeable to water.
• Near the bottom of the loop, the permeability to water and ions changes, now impermeable to water and slightly permeable to ions.
• Na+ ions diffuse from the filtrate into surrounding blood vessels.
• At the thick-walled portion of the ascending limb, carrier molecules actively transport Na+ ions, followed by Cl⁻ and bicarbonate ions passively.
• This replenishes the salty environment in the medulla, aiding in water absorption from the filtrate in the descending limb.
• Nutrients (e.g., glucose, amino acids, Na+, K+) are actively reabsorbed.
• Negatively charged ions (e.g., Cl⁻) are passively reabsorbed due to electrical attraction.
• Water is reabsorbed by osmosis.

Tubular Secretion

• Potassium (K⁺) and hydrogen (H⁺) ions are actively secreted from capillaries into the distal tubule to maintain blood pH.
• Drugs and substances not normally part of the body are secreted into the distal tubule.
• Filtrate is collected in the collecting duct, still containing a large amount of water.
• Passive reabsorption of water from filtrate through osmosis occurs
• If dehydrated, the permeability to water in the distal and collecting ducts increases, causing more water to be reabsorbed.
• The filtrate, now 4x more concentrated is called urine, representing only 1% of the original filtrate volume.

This quiz covers the formation of urine, focusing on the three essential processes: glomerular filtration, tubular reabsorption, and tubular secretion. Understand how blood filtration works and the mechanisms involved in reabsorbing vital substances back into the bloodstream.