Homeostasis: Question 2

Worked solution

Part (a): Ultrafiltration in the glomerulus

The glomerulus is a network of capillaries supplied by an afferent arteriole and drained by an efferent arteriole. The afferent arteriole has a wider lumen than the efferent arteriole, so blood is delivered to the glomerulus faster than it can drain away. This creates a high hydrostatic pressure within the glomerular capillaries, much higher than in ordinary capillaries elsewhere in the body.

This pressure pushes fluid out of the blood:

• Fenestrations (pores) in the capillary endothelium allow water and small solutes through.
• The basement membrane acts as the main filter, its mesh of protein fibres physically excluding large molecules.
• Podocytes, specialised cells lining the Bowman’s capsule, wrap around the capillaries with foot processes that leave narrow filtration slits, providing a further physical barrier.

Water, glucose, amino acids, urea and mineral ions are small enough to pass through all three layers and collect in the space of the Bowman’s capsule as the glomerular filtrate.

Part (b): Why blood cells and plasma proteins are excluded from the filtrate

Blood plasma is the liquid, cell-free portion of the blood: it consists of water together with dissolved solutes such as glucose, amino acids, urea, mineral ions and plasma proteins. Blood cells (red blood cells, white blood cells and platelets) are not part of the plasma at all, they are a separate, cellular component of blood, simply suspended within the plasma as it flows.

Both are nonetheless absent from the glomerular filtrate, for related but distinct reasons:

• Blood cells are far too large to pass through the fenestrations (pores) of the glomerular capillary endothelium in the first place, so they never leave the blood.
• Plasma proteins (e.g. albumin), despite being genuinely dissolved within the plasma, are still large soluble molecules that cannot pass through the basement membrane or the narrow filtration slits between podocyte foot processes.

Because both are excluded, the fluid that enters the Bowman’s capsule is essentially cell-free, protein-free plasma, while the smaller plasma solutes (water, glucose, amino acids, urea and mineral ions) pass through freely and appear in the filtrate.

Part (c): Adaptations of the proximal convoluted tubule for selective reabsorption

The proximal convoluted tubule (PCT) reabsorbs almost all of the useful solutes from the filtrate (all of the glucose and amino acids, plus much of the water and mineral ions) back into the blood. Its epithelial cells are adapted for this in several ways:

• Microvilli on the apical (lumen-facing) membrane greatly increase the surface area available for reabsorption.
• Large numbers of mitochondria supply the ATP needed for active transport.
• Na+/K+ pumps on the basal membrane (facing the blood) actively pump sodium ions out of the cell, keeping the intracellular sodium concentration low.
• This low intracellular sodium concentration means sodium ions diffuse back into the cell down their concentration gradient through co-transporter proteins in the apical membrane, and this sodium movement is coupled to the simultaneous uptake of glucose and amino acids, which are carried into the cell even against their own concentration gradient (a form of active transport, since it ultimately depends on the ATP used by the Na+/K+ pump).
• Glucose and amino acids then leave the cell across the basal membrane (by facilitated diffusion, since their concentration is now higher inside the cell than in the blood) and enter the surrounding blood capillaries.

Final answers

• (a) A wider afferent than efferent arteriole raises hydrostatic pressure in the glomerulus, forcing water and small solutes through the fenestrated capillary wall, basement membrane and podocyte filtration slits to form the glomerular filtrate.
• (b) Blood cells never leave the blood because they cannot cross the capillary fenestrations at all; plasma proteins, though genuinely part of the plasma, are still too large to cross the basement membrane/filtration slits, so both are absent from the filtrate.
• (c) Microvilli (surface area), many mitochondria (ATP), and sodium-linked co-transport (driven by basal Na+/K+ pumps) actively reabsorb glucose and amino acids from the filtrate into the blood.