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