6.4.3 Control of blood water potential

Describe the structure of a nephron

• nephron = basic structure and functional unit of the kidney millions in the kidney)

• associated with each nephron are a network of blood vessels

Summarise the role of different parts of the nephron

Bowman’s / renal capsule:

• formation of glomerular filtrate (ultrafiltration)

Proximal convoluted tubule:

• reabsorption of water and glucose (selective reabsorption)

Loop of Henle:

• maintenance of a gradient of sodium ions in the medulla

Distil convoluted tubule + Collecting duct:

• reabsorption of water (permeability controlled by ADH)

Describe the formation of glomerular filtrate

1. high hydrostatic pressure in glomerulus

• as diameter of afferent arteriole (in) is wider than the efferent arteriole (out)

2. small substances e.g. water, glucose, ions, urea, forced into glomerular filtrate, filtered by:

a) pores / fenestrations between capillary endothelial cells

b) capillary basement membrane

c) podocytes

3. large proteins / blood cells remain in blood

Describe the reabsorption of glucose by the proximal convoluted tubule

1. Na⁺ actively transported out of epithelial cells to capillary

2. Na⁺ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient

3. glucose moves into capillary by facilitated diffusion down its concentration gradient

Describe the reabsorption of water by the proximal convoluted tubule

• glucose etc in capillaries lower water potential

• water moves by osmosis down a water potential gradient

Describe and explain how features of the cells in the PCT allow the rapid reabsorption of glucose into the blood

• microvilli / folded cell-surface membrane → provides a large surface area

• many channel / carrier proteins → for facilitated diffusion / co-transport

• many carrier proteins → for active transport

• many mitochondria → produce ATP for active transport

• many ribosomes → produce carrier / channel proteins

Suggest why glucose is found in the urine of an untreated diabetic person

• blood glucose concentration is high so not all glucose is reabsorbed at the PCT

• as glucose carrier / cotransporter proteins are saturated / working at maximum rate

Explain the importance of maintaining a gradient of sodium ions in the medulla (concentration increases further down)

• so water potential decreases down the medulla (compared to filtrate in collecting duct)

• so a water potential gradient is maintained between the collecting duct and the medulla

• to maximise reabsorption of water by osmosis from filtrate

Describe the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla

1. In the ascending limb:

• Na⁺ actively transported out (so filtrate concentration decreases)

• water remains as ascending limb is impermeable to water

• this increases the concentration of Na⁺ in the medulla, lowering water potential

2. In the descending limb:

• water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases)

• Na⁺ ‘recycled’ → diffuses back in

Suggest why animals needing to conserve water have long loops of Henle (thick medulla)

• more Na⁺ moved out → Na⁺ gradient is maintained for longer in medulla / higher Na⁺ concentration

• so water potential gradient is maintained for longer

• so more water can be reabsorbed from collecting duct by osmosis

Describe the reabsorption of water by the distal convoluted tubule and collecting ducts

• water moves out of distal convoluted tubule and collecting duct by osmosis down a water potential gradient

• controlled by ADH which increases their permeability

What is osmoregulation?

control of water potential of the blood (by negative feedback)

Describe the role of the hypothalamus in osmoregulation

1. contains osmoreceptors which detect increase OR decrease in blood water potential

2. produces more ADH when water potential is low OR less ADH when water potential is high

Describe the role of the posterior pituitary gland in osmoregulation

secretes (more/less) ADH into blood due to signals from the hypothalamus

Describe the role of the antidiuretic hormone (ADH) in osmoregulation

1. attaches to receptors on collecting duct (and distal convoluted tubule)

2. stimulating addition of channel proteins (aquaporins) into cell-surface membrane

3. so increases permeability of cells of collecting duct and DCT to water

4. so increases water reabsorption from collecting duct / DCT (back into blood) by osmosis

5. so decreases volume and increases concentration of urine produced

The above applies to a decrease in water potential of the blood (e.g. increased sweating, reduced water intake, increased salt intake). The body responds to an increase in water potential in the opposite way (less ADH secreted, less attaches to receptors, less addition of channel proteins, permeability remains low, less water reabsorption, higher volumes and increased concentration of urine etc).

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