11.1 Essential ideas

11.1.3b Osmoregulation in the kidney

Kidneys are responsible for producing urine for excretion as well as maintaining the balance of water and solutes in the blood (osmoregulation).

Ultrafiltration: the glomerulus and Bowman’s capsule

  • Ultrafiltration separates blood plasma contents based on particle size.
  • Smaller particles – urea, glucose, amino acids, ions and water – are pushed through filtration slits into the Bowman’s capsule by high pressure exerted by blood from the glomerulus.
  • Proteins and blood cells are prevented from moving into the Bowman’s capsule because they are too large.

Figure 11.1.3b/a – Ultrastructure of one capillary of the glomerulus (left) and transverse section of the filtration apparatus (right)Figure 11.1.3b/a – Ultrastructure of one capillary of the glomerulus (left) and transverse section of the filtration apparatus (right)

  • The ultrastructure of the glomerulus and Bowman’s capsule facilitate ultrafiltration.

Fenestrations

Holes in the membrane of the glomerular capillary

Basement membrane

Mesh of negatively charged glycoproteins between the capillary and the lumen of the Bowman’s capsule

Podocytes

Cells attached to the basement membrane that wrap around the capillary

Filtration slits

Small spaces between the ‘foot’ projections of each podocyte

Selective reabsorption: the proximal convoluted tubule

  • 80–90% of selective reabsorption occurs in the proximal convoluted tubule (PCT).
  • The cells in the lining of the PCT have microvilli that increase the surface area for absorption.
  • There are also a large number of mitochondria in these cells, providing energy for active transport of glucose and mineral ions.

Figure 11.1.3b/b – Selective reabsorption in the lining of the PCTFigure 11.1.3b/b – Selective reabsorption in the lining of the PCT

  • Protein pumps in the outer membrane of the tubule lining cells actively transport sodium from the tubule to the interstitial fluid of the cortex.
  • Sodium moves down the concentration gradient from the filtrate into the lining cell. This provides energy for glucose to be taken into the tubule cells. Sodium (moving down the concentration gradient) and glucose (moving up the concentration gradient) travel into the lining via a symport protein.
  • Chlorine and other ions move into and out of the lining cell because of charge and concentration gradients set up by pumping of sodium.
  • Water follows the movement of the ions passively (via osmosis).
  • Water, sodium, amino acids and ions then diffuse into the peritubular capillaries.

The loop of Henle maintains hypertonic conditions in the medulla

  • The function of the loop of Henle is to create a salt concentration gradient in the interstitial fluid. This forces water out of the tube and into the blood of the vasa recta.
  • The length of the loop of Henle is positively correlated with the need for water conservation in animals. A longer loop means there is a larger gradient, so the filtrate can become more highly concentrated. The animal will lose less water in the urine.
  • The loop of Henle starts in the cortex and extends into the medulla. The tube consists of a thin, water-permeable descending limb and a thick, water-impermeable ascending limb.
  • Membrane proteins in the ascending limb actively transport sodium into the interstitial fluid. This is what generates the solute gradient and causes water to diffuse out of the descending limb.
  • In the vasa recta, blood flows in the direction opposite to the filtrate in the loop. Water is exchanged between the surrounding fluid and the blood along the entire length of the capillary.
  • This is a counter-current multiplier system. If blood flowed in the same direction as the filtrate, the concentration gradient in the medulla would be weaker.

Figure 11.1.3b/c – Solute concentration gradient and mechanism of counter-current exchange along the loop of HenleFigure 11.1.3b/c – Solute concentration gradient and mechanism of counter-current exchange along the loop of Henle

ADH controls reabsorption in the collecting duct

  • The pituitary gland secretes ADH, or anti-diuretic hormone, when the hypothalamus detects a high solute concentration in circulating blood.
  • ADH is a hormone that acts at the end of the distal convoluted tubule and the entire length of the collecting duct to increase water permeability.
  • As a result of ADH, more water is reabsorbed into the bloodstream from the urine in the collecting duct. A smaller volume of more highly concentrated urine is excreted.

Figure 11.1.3b/d – Target cells for ADH are in the collecting duct as well as the distal convoluted tubule.Figure 11.1.3b/d – Target cells for ADH are in the collecting duct as well as the distal convoluted tubule.

Figure 11.1.3b/e – Electron micrograph of glomerulus with podocytes (yellow)Figure 11.1.3b/e – Electron micrograph of glomerulus with podocytes (yellow)

Key questions

  • Outline osmoregulation in the kidney.
  • Describe how structure of the glomerulus, Bowman’s capsule, loop of Henle and vasa recta are related to function.
  • Explain how active and passive transport work together to maintain solute concentration of the medulla.
  • Explain that the function of ADH is an example of negative feedback.

Concept help

  • As you go through this page, follow along with your annotated diagram of the nephron 11.1.3a Downloads.
  • Compare the size of fenestrations (80–100nm) and filtration slits (25nm) with those of various blood components: red blood cells (8mm), amino acids (1nm), antibodies (12nm), glucose (0.9nm). Remember: 1nm = 1000mm.
  • Counter-current exchange increases efficiency of water reabsorption. If blood and filtrate flowed in the same direction, the solute concentration of the filtrate would stop increasing at the equilibrium point.

Figure 11.1.3b/f – Concurrent exchangeFigure 11.1.3b/f – Concurrent exchange
Osmotic equilibrium limits water absorption in concurrent exchange.

Course link

Nature of Science

Curiosity: Investigations were carried out to determine how desert animals prevent water loss in their wastes.

Figure 11.1.3b/g – Kangaroo ratFigure 11.1.3b/g – Kangaroo rat
The loop of Henle of desert-dwelling kangaroo rats, genus Dipodomys, is much longer than that in other rodents of a similar size. Their urine is up to eight times more concentrated than a human’s.