control of blood water potential

Question 1

what is the function of the kidneys

Answer 1

to excrete waste products such as urea and regulate the water potential of blood

Question 2

define osmoregulation

Answer 2

control of the water potential of the blood via homeostatic mechanisms

Question 3

describe ultrafiltration

Answer 3

as blood passes through capillaries in the cortex of kidneys (glomerulus) substances are filtered out of the blood and into long tubules that surround the capillaries

Question 4

describe selective reabsorption

Answer 4

• useful substances such as glucose and the right volume of water are reabsorbed back into the blood
• the unwanted substances pass along to the bladder and are excreted as waste

Question 5

describe the gross structure of a mammalian kidney

Answer 5

1. Fibrous capsule: protects kidney.
2. Cortex: outer region consists of Bowman’s capsules, convoluted tubules, blood vessels
3. Medulla: inner region consists of collecting ducts, loops of Henle, blood vessels.
4. Renal pelvis: cavity collects urine into ureter.
5. Ureter: tube carries urine to bladder.
6. Renal artery: supplies kidney with oxygenated blood.
7. Renal vein: returns deoxygenated blood from kidney to heart.

Question 6

describe the structure of a nephron

Answer 6

1. Bowman’s capsule at start of nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes.
2. Proximal convoluted tubule (PCT): series of loops surrounded by capillaries, walls made of epithelial cells with microvilli.
3. Loop of Henle: hairpin loop extends from cortex into medulla.
4. Distal convoluted tubule: similar to PCT but fewer capillaries
5. Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney.

Question 7

describe the blood vessels associated with a nephron

Answer 7

1. afferent arteriole - enters renal capsule and forms glomerulus, delivers blood to the glomerulus
2. efferent arteriole - smaller in diameter, higher pressure and takes filtered blood away from the glomerulus

Question 8

explain how glomerulus filtrate is formed

Answer 8

• via ultrafiltration in the Bowman’s capsule
• high hydrostatic pressure in the glomerulus forces small molecules out of capillary
• moves through the nephron tubules, capillary wall, basement membrane and into the Bowmans capsule
• basement membrane act as a filter
• blood cells and large molecules stay in the capillary

Question 9

how are cells of the Bowman’s capsule adapted for ultrafiltration?

Answer 9

• fenestrations between epithelial cells of capillaries
• fluid can pass between and under the folded membrane of podocytes

Question 10

explain why the efferent arteriole has a lower pressure than the glomerulus

Answer 10

• the efferent arteriole is smaller in diameter than the afferent arteriole
• so blood in the glomerulus is under high pressure

Question 11

explain how the loop of henle maintains a Na+ gradient

Answer 11

top of the ascending limb:
1. Na+ ions are pumped out into the medulla by active transport
2. The ascending limb is impermeable to water, so the water stays inside the tubule
3. This creates a low water potential in the medulla, because there’s a high concentration of ions.

descending limb:
1. water moves out of the descending limb into medulla via osmosis, water in medulla is reabsorbed into blood through capillary network

bottom of ascending limb:
1. Na+ ions diffuse out into the medulla via facilitated diffusion, lowering the water potential in the medulla

Question 12

describe the structure of the glomerulus

Answer 12

• endothelium of the capillary - each capillary endothellial cell is perforated by tiny membrane-line pores,
• basement membrane - made up of collagen and glycoprotein
• epithelium of Bowmann’s capusule - tiny finger like projections with gaps in between known as podocytes, only small molecules can pass through

Question 13

explain how a gradient of sodium ions is maintained in the medulla by the loop of Henle

Answer 13

• active transport of Na+ and CL- out of ascending limb
• water potential of interstitial fluid decreases
• osmosis of water out of descending limb (ascending limb is impermeable to water)
• water potential of filtrate decreases going dow descending limb and is lowest in the medullary region and highest at top of ascending limb

Question 14

explain the role of the distal convoluted tubule

Answer 14

• its role is reabsorption of water via osmosis and of ions via active transport
• its permeability of walls is determined by action of hormones

Question 15

explain the role of the collecting duct

Answer 15

reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins

Question 16

state what happens during selective reabsorption and where it occurs

Answer 16

• useful molecules from glomerular filtrate like glucose are reabsorbed into the blood
• occurs in proximal convoluted tubule

Question 17

outline the transport processes involved in selective reabsorption

Answer 17

• glucose from glomerular filtrate enters cells lining proximal convoluted tubule via co-transport with Na+ ions
• glucose in cell lining proximal convoluted tubule enters intercellular spaces via active transport
• glucose in intercellular spaces moves into blood capillary lining tubule via diffusion

Question 18

how are cells in the proximal convoluted tubule adapted for selective reabsorption?

Answer 18

• microvilli: large surface area for co-transported protein
• many mitochondria: ATP for active transport of glucose into intercellular space
• folded basal membrane: large surface area

Question 19

explain why it is important to maintain an Na+ gradient

Answer 19

• countercurrent multiplier: filtrate in collecting duct is always beside an area of interstitial fluid that has a lower water potential
• maintains water potential gradient for maximum reabsorption of water

Question 20

what might cause blood water potential to change?

Answer 20

1. level of water intake
2. level of ion intake in diet
3. level of ions used in metabolic processes or excreted
4. sweating

Question 21

explain the role of the hypothalamus in osmoregulation

Answer 21

• osmosis of water out of osmoreceptors from a high water potential to a low water potential in the blood of hypothalamus causes them to shrink
• this triggers hypothalamus to produce more antidiuretic homrone (ADH)

Question 22

explain the role of the posterior pituitary gland in osmoregulation

Answer 22

stores and secretes the ADH produced by the hypothalamus

Question 23

explain the role of ADH in osmoregulation

Answer 23

1. Makes cells lining collecting duct more permeable to water:

• Binds to receptor -› activates phosphorylase -› vesicles with aquaporins (water channels) on membrane fuse with cell-surface membrane.
• water diffuses from collecting duct to medulla via osmosis from high to low water potential

2. Makes cells lining collecting duct more permeable to urea:

• water potential in interstitial fluid decreases.
• more water reabsorbed = more concentrated urine.