6.4.3 Control of blood water potential

Question 1

How do we ensure fairly constant water potential of blood plasma and tissue fluid?

Answer 1

We need optimum concentration of water and salts

Question 2

What is osmoregulation?

Answer 2

Homeostatic control of water potential of blood

Question 3

Why is the kidney needed?

Answer 3

• Water: cells only function in isotonic solution; required for metabolic reactions
• Ions: required for cellular processes and osmotic balance
• Urea: toxic product of amino acid breakdown so could damage cells

Question 4

What does the breakdown of amino acids look like?

Answer 4

Amino acids –> ammonia –> urea (happens in the liver) –> urine

Question 5

What is the structure of the kidney?

Answer 5

• Fibrous capsule
• Cortex
• Medulla
• Renal pelvis funnel
• Ureter
• Renal artery
• Renal vein

Question 6

What is the fibrous capsule?

Answer 6

Outer membrane protects kidney

Question 7

What is the cortex?

Answer 7

Made up of renal bowman’s capsules, convoluted tubules and blood vessels

Question 8

What is the medulla?

Answer 8

Made up of loops of Henle, collecting ducts and blood vessels

Question 9

What is the renal pelvis funnel?

Answer 9

Shaped cavity that collects urine into the ureter

Question 10

What is the ureter?

Answer 10

Tube carries urine to the bladder

Question 11

What is the renal artery?

Answer 11

Supplies kidneys with blood from heart via aorta

Question 12

What is the renal vein?

Answer 12

Returns blood to heart via vena cava

Question 13

What are the functions of the kidneys?

Answer 13

• Blood is first filtered (ultrafiltration)
• Useful substances are selectively re absorbed
• Toxic urea and excess ions and water are removed as urine

Question 14

What is the nephron?

Answer 14

• Functional unit of the kidney
• Narrow tube 14mm long
• Around one million in each kidney

Question 15

What is the Bowman’s capsule?

Answer 15

Ultrafiltration of blood plasma into the Bowman’s capsule cup shaped and surrounds glomerulus. Inner layer is made of podocytes

Question 16

What is the proximal convoluted tubule?

Answer 16

Series of loops surrounded by blood capillaries. Walls made of epithelial cells which have microvilli

Most water reabsorbed by osmosis; glucose and ions reabsorbed by facilitated diffusion and then active transport

Question 17

What is the descending limb of loop of Henle?

Answer 17

Long loop extends from cortex into medulla and back again.
Surrounded by blood capillaries.
Water reabsorbed by osmosis according to the concentration of sodium ions in the interstitial space.
- Highly permeable to water

Question 18

What is the ascending limb of loop of Henle?

Answer 18

Sodium ions reabsorbed by facilitated diffusion and then active transport
- Impermeable to water

Question 19

What is the distal convoluted tubule?

Answer 19

Small amounts of water and ions reabsorbed to control blood pH

Question 20

What is the collecting duct?

Answer 20

Tube into number of distal convoluted tubules empty. Lined by epithelial cells and gets wider as get closer to pelvis.
Water reabsorbed depending on the presence of ADH hormone

Question 21

What is the afferent arteriole?

Answer 21

Ultrafiltration of blood plasma into the Bowman’s capsule; blood cells and proteins remain in the glomerulus capillary

Question 22

What is the glomerulus?

Answer 22

Knot of capillaries from which fluid is forced out of the blood they recombine to form the efferent arteriole

Question 23

What is the efferent arteriole?

Answer 23

Tiny vessel that leaves renal capsule. Smaller diameter than afferent so causes an increase in blood pressure within the glomerulus.
It later branches to form blood capillaries

Question 24

What is the blood capillaries?

Answer 24

Network that surrounds PCT, loop of Henle and DCT.
Reabsorb mineral salts, glucose, water.
Merge to form renal vein

Question 25

Describe the process of ultratfiltration:

Answer 25

- Efferent arteriole has a smaller diameter than the afferent arteriole - Therefore pressure develops in glomerulus - Pushes small substances out of glomerulus -> Bowman's capsule eg/ glucose, water, urea, ions (glomerular filtrate) BUT large proteins/RBCs can't leave the glomerulus - Endothelial cells/podocytes act as a filter - have spaces therefore passes through space instead of cells - Passes through basement membrane

Question 26

What is the movement of the glomerulus filtrate resisted by? (ultrafiltration)

Answer 26

- Capillary endothelial cells - Connective tissue and endothelial cells of the blood capillary - Epithelial cells of the renal capsule - The hydrostatic pressure of the fluid in the renal capsule space - The low water potential of the blood in the glomerulus

Question 27

How is the PCT adapted for reabsorption of glucose and water?

Answer 27

- Microvilli - increase SA - Infoldings - increase SA, more co-transport proteins - Many mitochondria - ATP, active transport

Question 28

Describe the process of reabsorption of glucose and water by the PCT:

Answer 28

1. Na+ actively transported out of cell lining --> capillaries therefore conc. of Na+ in cell decreases 2. Na+ diffuse down a conc. gradient from PCT --> epithelial cell through co-transporter protein by facilitated diffusion 3. Carrier proteins are of specific types, each of which carries another molecule along with Na+ (co-transport) 4. Diffuse into blood therefore glucose, other valuable molecules are reabsorbed (as well as water)

Question 29

How is the gradient of sodium ions in maintained in the medulla maintained by the loop of Henle?

Answer 29

1. Na+ actively transported out of ascending limb 2. Low water potential in interstitial region. As ascending is impermeable to water no water is lost 3. Water leaves filtrate in descending limb (osmosis) into interstitial space. Water enters capillaries and moved away 4. Filtrate progressively looses water 5. At base of ascending Na+ diffuse out 6. In the interstitial space between collecting duct and ascending limb there is a water potential gradient. Highest water potential in cortex and lower water potential as move into the medulla 7. Collecting duct permeable to water - as filtrate passes down water moves out by osmosis into blood vessels 8. Water moves out down whole length of collecting duct. The COUNTER CURRENT MULTIPLIER ensures water potential gradient draws water out of tubule

Question 30

What is the loop of Henle and what is it responsible for?

Answer 30

- Hairpin loop extending into medulla - Responsible for water being reabsorbed from collecting duct - Urine becomes more concentrated (lower water potential compared to blood) - Longer loop = more concentrated urine can be

Question 31

What is the main role of the distal convoluted tubule?

Answer 31

- Have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate, by active transport. - Make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb.

Question 32

How do we gain water in the body?

Answer 32

1. Drinking 2. Eating 3. Respiration

Question 33

How do we lose water from the body?

Answer 33

- Urinating - Breathing - Sweating (also causes loss of ions)

Question 34

What happens when blood water potential falls...

Answer 34

- Osmoreceptor cells in hypothalamus lose water (osmosis and cell shrinkage) - Antidiuretic hormone (ADH) is produced by hypothalamus and passes to posterior pituitary gland then secreted into capillaries - ADH increases permeability of DCT and collecting duct walls to water - ADH bind to cell surface receptors on collecting ducts and DCT and activates enzyme phosphorylase - Activation of phosphorylase cause vesicles (aquaporin) to fuse with cell surface membrane - This increases number of water channels and permeability to water - ADH also increases permeability to urea which leaves and lowers water potential of surrounding fluid - Combined effect is more water is reabsorbed into blood so water is lost in urine - Reabsorbed water came from the blood - prevent water potential from getting lower - Osmoreceptors also send nerve impulses to the thirst centre of brain - Osmoreceptors in the hypothalamus detect the rise in water potential and send fewer impulses to the pituitary gland - Pituitary gland reduces the release of ADH and release permeability of the collecting duct (return to former state)

Question 35

When blood water potential rises...

Answer 35

- Osmoreceptor cells in hypothalamus gain water - Less ADH is produced - Less ADH is secreted - Walls of the collecting duct are less permeable to water - Less water is reabsorbed so more water is lost in urine