Control of Blood Water Potential

What is osmoregulation?

The control of water potential in the blood by negative feedback

What are the organs which control osmoregulation?

Kidneys- maintain the balance of water and salt ions in the body, and therefore the water potential of the blood

How do the kidneys work?

• Blood arrives to the kidney through the renal artery, which branches off the aorta. This brings oxygenated blood to the respiring tissues of the kidney, and the blood is also filtered as it passes through the kidney

• Filtered, deoxygenated blood leaves the kidney via the renal vein which leads back to the vena cava

Describe what a nephron is and its structure

• Basic structural and functional unit of the kidney (millions in the kidney)

• Associated with each nephron are a network of blood vessels

What is the role of the part of the nephron Bowman’s capsule?

Formulation of glomerular filtrate (ultrafiltration)

What is the role of the part of the nephron proximal convoluted tubule?

Reabsorption of water and glucose (selective reabsorption)

What is the role of the part of the nephron loop of Henlé?

Maintenance of gradient of sodium ions in the medulla

Describe the formation of glomerular filtrate (ultrafiltration)

• High hydrostatic pressure in glomerulus, as diameter of afferent arteriole (in) is wider than efferent arteriole (out)

• Small substances (water, glucose, urea, ions) are forced into glomerular filtrate

• These small substances are filtered by fenestrations between capillary endothelial cells, capillary basement membrane and podocytes into Bowmans capsule. As the filtrate contains some useful substances we don’t wish to lose, these must be reabsorbed back into the blood before the filtrate reaches the end of the tubule

• Large proteins/ blood cells remain in blood as too big to pass through basement membrane

What does a podocyte allow?

Free movement of substances into cell

Describe the reabsorption of glucose by the proximal convoluted tubule

• Sodium ion pump uses ATP from mitochondria to actively transport sodium ions out of proximal convoluted epithelial cells into blood

• Sodium ions facilitatedly diffuse down concentration gradient from the filtrate into the epithelial cells via a co-transport protein which brings glucose into the cell at the same time

• Glucose moves into blood from epithelial cell down its concentration gradient

Describe the reabsorption of water by the proximal convoluted tubule

• Glucose and the small substances are at a lower water potential

• Water moves by osmosis down a water potential gradient into the capillary cells of the proximal convoluted tubule

How are the features/ structure of the cells in the proximal convoluted tubule adapted to allow rapid reabsorption of glucose into the blood?

• Microvilli/ folded cell surface membrane: provides a large surface area so faster rate of diffusion

• Many channel/ carrier protein: faster rate of facilitated diffusion/ co-transport

• Many carrier protein: active transport

• Many mitochondria: produces ATP for active transport

• Many ribosomes: produces carrier/ channel proteins

Why could glucose be found in the urine of an untreated diabetic person?

• Blood glucose concentration is too high, so not all glucose is reabsorbed at PCT

• As glucose carrier/ co-transporter proteins are saturated/ working at maximum rate

Explain the importance of maintaining a gradient of sodium ions in the medulla (concentration increases as you get 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

In the ascending limb:

• Sodium ions actively transported out into medulla, so filtrate concentration decreases

• Water remains as ascending limb is impermeable to water

• This increases concentration of sodium ions down the medulla (area around limbs) as actively transported out, lowering water potential

In the descending limb:

• Permeable to water so water moves out by osmosis, so sodium ion concentration increases and ascending limb receives high conc of sodium ion filtrate

• High conc of salt in medulla as well as water molecules means water potential is always lower in medulla than filtrate

• ‘Recycled’ sodium ions diffused back into ascending limb (high conc at bottom, lower conc at top of medulla)

  
  
  

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

• More sodium ions will move out via active transport, so the sodium ion gradient is maintained for a longer amount of time in medulla and have a higher sodium ion conc

• So water potential gradient is maintained for longer

• So more water can be reabsorbed from collecting such by osmosis

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

• Water moves out via osmosis down a water potential gradient

• Controlled by ADH which increases their permeability

Describe the role of the hypothalamus in osmoregulation

• Contains osmoreceptors which detect increase or decrease in blood water potential

• Produces more ADH hormone 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 hormone into blood due to signals from hypothalamus

Describe the role of the the hormone ADH in osmoregulation when there’s a DECREASE in blood water potential

• More ADH is secreted by the posterior pituitary gland in response to signal from hypothalamus

• More ADH attaches to receptors on collecting duct and distal convoluted tubule

• This stimulates addition of channel proteins into cell-surface membranes

• So increases permeability of cells of collecting duct and DCT to water

• So increases water reabsorption from collecting duct/ DCT back into blood by osmosis

• So decreases volume and increases concentration of urine produced

Describe the role of the the hormone ADH in osmoregulation when there’s a INCREASE in blood water potential

• Less ADH is secreted by the posterior pituitary gland in response to signal from hypothalamus

• Less ADH attaches to receptors on collecting duct and distal convoluted tubule

• This means/ inhibits (less addition) of channel proteins into cell-surface membranes

• So decreases permeability of cells of collecting duct and DCT to water/ remains low

• So less water reabsorption from collecting duct/ DCT back into blood by osmosis

• So higher volume and increased concentration of urine produced