D3.3.8 Role of the glomerulus, Bowman’s capsule and proximal convoluted tubule in excretion (HL) Notes

The Role of the Glomerulus, Bowman's Capsule, and Proximal Convoluted Tubule in Excretion

1. Your kidneys are essentially a state-of-the-art filtration system, working tirelessly to remove waste while retaining essential nutrients.
2. This process begins in the nephron, the kidney's functional unit, specifically in the glomerulus, Bowman's capsule, and proximal convoluted tubule.

High Pressure Drives Filtration in the Glomerulus and Bowman's Capsule

1. Blood enters the glomerulus, a dense ball-shaped network of capillaries, through the afferent arteriole and exits via the narrower efferent arteriole.
2. The difference in diameters creates high pressure within the glomerulus.
3. This pressure forces water, ions, glucose, and other small molecules from the blood into Bowman’s capsule, forming the initial filtrate.

Glomerulus and Bowman's Capsule Is A Sieve for the Blood

1. The filtration barrier allows only small and medium-sized molecules to pass, retaining larger molecules like proteins and blood cells.
2. It consists of three layers:
   1. Fenestrated Capillary Endothelium:
      Contains pores (~100 nm) that allow fluid and solutes to exit the blood while blocking larger particles.
   2. Basement Membrane:
      A gel-like layer made of negatively charged glycoproteins. It repels plasma proteins due to their size and charge, preventing their passage into the filtrate.
   3. Podocytes in Bowman’s Capsule:
      Specialized cells with interlocking foot processes that create narrow filtration slits, allowing only small molecules to pass.

Composition of Glomerular Filtrate

1. The filtrate in Bowman's capsule contains:
   1. Water
   2. Glucose
   3. Amino acids
   4. Ions (e.g., $Na^{+}$, $Cl^{-}$)
   5. Urea
2. However, it lacks large molecules like proteins or blood cells, ensuring only waste and necessary substances for reabsorption proceed.

Selective Reabsorption in the Proximal Convoluted Tubule

1. After ultrafiltration, the filtrate enters the proximal convoluted tubule (PCT), where essential substances are reabsorbed into the blood.
2. This process is highly selective and efficient.

Key Reabsorption Processes

1. Sodium Ions (Na⁺):
   1. Actively transported out of the tubule cells into the interstitial fluid.
   2. This creates an electrochemical gradient that drives the reabsorption of other substances.
2. Chloride Ions (Cl⁻):
   1. Follow $Na^{+}$ passively due to the charge gradient.
3. Glucose and Amino Acids:
   1. Reabsorbed via co-transporter proteins that couple their movement with $Na^{+}$.
4. Water:
   1. Follows solutes by osmosis, driven by the solute concentration gradient.

Structural Adaptations of the PCT

1. Microvilli: Increase surface area for absorption.
2. Mitochondria: Provide ATP for active transport.
3. Tight Junctions: Prevent leakage of reabsorbed substances back into the tubule.

Why Is This Process Important?

1. Efficient Waste Removal: Ultrafiltration ensures that toxins and excess solutes are removed from the blood.
2. Conservation of Valuable Substances: Selective reabsorption in the PCT prevents the loss of essential nutrients and water.
3. Homeostasis: The kidney maintains the body's internal environment by regulating solute concentrations and fluid balance.