Formation of urine Flashcards
what are the 4 stages of urine formation and where do they occur?
1) ultrafiltration (bowmans capsule)
2) selective reabsorption (PCT)
3) water reabdorption (Loop of Henle)
4) osmoregulation (DCT and collecting duct)
describe the process of ultrafiltration
- blood fows into the glomerus from the afferent arteriole which is wider than the efferent arteriole which carries blood away
- increases pressure in glomerus compared to pressure in the bowman’s capsule which forces blood plasma through the pores in the blood capillary, through the basement membrane and epithelial cells of the renal capsule forming glomerular filtrate
how are endothelial cells adapted for ultra-filtration?
narrow gaps called fenestrations allowing blood plasma through
(found in capillary wall)
how is the basement membrane adapted for ultra filtration?
- mesh of collagen and glycoprotein fibres which act as the filter only allowing molecules through with a relative molecular mass less than 69000
- this means large proteins and cells are not filtered out. The presence of proteins lowers the water potential and ensures some water is retained
(found in bowmans capsule)
how are epithelial cells of the renal capsule (podocytes) adapted for ultra filtration?
have finger like projections, fluid passes between these processes into the lumen of the bowman’s capsule
what substances form the glomerular filtrate?
- water
- amino acids
- glucose
- urea
- inorganic ions (Na+, Cl-, K+)
explain why the concentration of proteins in the blood plasma is so much larger than the concentration in glomelular filtrate when other substances such as glucos, water, urea etc is the same in both?
glucose, urea etc all have a relative molecular mass less than 69000 and are therefore small enough to go through all filotraring steps
however proteins have a relative molecular mass higher than 69000 therefore they are too big to fit through the basement membrane
describe the process of selective reabsorption
- sodium potassium opump actively transports sodium from the cells lining the proximal convuluted tubule into hte tissue fluid by active transports using ATP. This lowers the concentraion of sodium in cell cytoplasm
- sodium ions are transported into cell of the PCT along with glucose and aminoa cids by a cotransporter by facilitated diffusion
- glucose and amino acids then diffuse down concentration gradient out of the epithelial cells into the tissue fluid and then into blood capillary
(some water will also pass in this direction due to the water potential gradient)
how much of the substances is reabsorbed in selective reabsorption?
ALL of the glucose
ALL of the amino acids
SOME salts
MOST of the water
what are adaptions of the epithelial cells of the PCT for absorption?
- folding of membrane on capillary side to increase surface area and presence of sodium potassium pump for active transports
- mircrovilli of the epithelia cells of the PCT provide a large surface area for absorption
- membrane on lumen side contains many co-transporter proteins for facilitated diffusion
what are the properties of the ascending and descending limb of the loop of henle?
ASCENDING
- impermeablke to water
- actively transports sodium and chloride ions out
- sodium and chloride ions diffuse out near the bottom of the ascending limb
DESCENDING
- permeable to water
- sodium and chloride ions diffuse into the descending limb
describe the process of water reabsorption
- sodium and chloride iosn are actively transported out of the ascending limb into the tissue fluid in the medulla. The ascending limb is imperable to water, so the water stays inside the tubule
- because there is a lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. The water in the medulla is then reabsorbed back into the blood through the capillary network
- some sodium and chloride ions diffuse back into the descending limb
- near the bottom of the ascening limb, sodium and chloride ions diffuse out into the medulla, further lowering the water potential in the medulla
- the water potential gradient int he medulla also causes water to move by osmosis from the DCT and collecting duct into the medulla, and then enter the blood capillaries
what is a hairpin countercurrent multiplier?
it describes the arrangement of the tubule in a sharp hairpin, so that one part of the tubule passes close to the other part with fluid flowing in the opposite direction.
this allows the exchange of material and allows the increase in concentration of ions
why do desert animals have a long loop of henle
To conserve water
The longer the loop of henle, the more sodium and chloride ions are actively transported out of the ascending limb. Therfore the water potential in the medulla is even lower so then more water moves by osmosis from the descending limb, DCT, and collecting duct into the medulla, and then reabsorbed back ito blood
Describe the process of osmoregulation?
- water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain
- when water potential of blood decreases, osmoreceptors cells lose water by osmosis, they lose volume and send a signal to the posterior pituitary gland causing it to release a hormone called ADH
- when ADH is released into bloodstream, it is transported to the distal convoluted tubule and collecting duct where it binds to membrane bound receptors and causes protein channels called aquaporins to move into the membranes. This makes the walls of the DCT and collecting duct more permeable to water, as this allows more water to move out of the DCT and collecting duct by osmosis back into the blood causing the urine to be more concentrated
what is ADH and what does it do?
it is an anti-diuretic hormone and it controls water levels in the body
what are osmoreceptors?
cells in the hypothalamus which are sensitive to changes in blood water potential
what happens when someone is dehydrated?
dehydration is what happens when uyou lose water so the water content of the blood needs to by increased:
- water content of blood drops, so water potental drops
- detected by osmoreceptors in the hypothalumus
- posterior pituitary gland is stimulated to release more ADH into the blood
- more ADH means that the DCT and collecting duct are more permeable, so more water is absorbed into the blood by osmosis
- a small volume of highly concentrated urine is produced and less water is lost
what happens when someone is hydrated?
water content of the blood needs to be reduced:
- if water content of blood rises, water poential rises
- detected by osmorecpetors in the hypothalumus
- posterior pituitary gland releases less ADH into the blood
- less ADH means that the DCT and collecting duct are less permeabkle, so less water is absorbed into the blood by osmosis
- large volume of dilute urine is produced and more water is lost
what is happening at the 4 stages on the graph?
- glucose is actively transported out of the tubule
- sodium ions enter desceding limb and leave ascending limb
- urea concentration rises as water is removed from tubule
- water is absorbed from the tubule, increasing the relative concentration of sodium ions and potassium ions are actively transported into the tubule to be removed in urine
