Excretion as an example of homeostatic control Part 2 Flashcards
what do the nephron process ensure
It ensures that the urine has a low water potential and therefore it has a higher concentration of solutes than what is found in the blood and tissue fluid
Describe the specialization of the cells lining the convoluted tubule which enables selective reabsorption
- this involves the active transport and cotransport
- cells that are involved are lining the proximal convoluted tubule
- the cell surface membrane that is in contact with the tubule fluid is folded to form microvilli which increases the surface area for reabsorption
- the cell surface membrane also contains special cotransporter proteins that transport glucose or amino acids in association with sodium ions from the tubule into the cell
- the opposite membrane of the cell, is close to the tissue fluids and blood capillaries is folded in order to increase surface area, this membrane has sodium and potassium pumps that pump sodium ions out of teh cell and potassium ions into the cell
- cytoplasm has mitochondria which indicates that energy is used which means that there is an active process requiring ATP
Describe the mechanism of selective reabsorption
- sodium ions are actively pumped out of the cell lining the tubule
- the concentration of sodium ions in cell cytoplasm decreases creating a concentration gradient
- sodium ions diffuse into the cell through a cotransport protein, this carries glucose or an amino acid at the same time
- water moves into the cell by osmosis
- glucose/ amino acids diffuse into the blood
- larger molecules that have entered the tubules are removed by endocytosis
Describe the loop of the Henle
consists of a descending limb that descends into the medulla and an ascending limb that ascends back out to the cortex
- known as a hairpin countercurrent multiplier system, this increases the efficiency of the transfer of mineral ions from the ascending limb to the descending limb in order to create a water potential gradient that is seen in the medulla
What does the arrangement of the loop of Henle allow
it allows mineral ions such as sodium and chloride ions to be transferred from the ascending limb to the descending limb, this increases the concentration of mineral ions in the tubule fluid which has a similar effect upon concentration of mineral ions in the tissue fluid giving the medulla a very low water potential
Describe how the loop of the Henle work
- mineral ions enter the descending limb causing the concentration of the fluid to rise
- this means the water potential decreases and becomes more negative the deeper the tubule descends into the medulla
- as the fluids rise up the ascending limb mineral ions leave the fluid, at the base this is done by diffusion but the higher up you go active transport isp used to move the mineral ions out, upper portion is ascending limb is impermeable to water
- the effect of these movements create a higher water potential in the fluid of the ascending limb and it decreases the water potential in the tissue fluid of the medulla and this causes the water potential of the tissue fluid becomes lower towards the bottom of the Henle
- the fluid passes down the collecting duct and passes through tissues with decreasing water potential this means that there is always a water potential gradient between the fluid in the collecting duct and that in the tissues therefore water can be moved by osmosis out of the collecting duct and into the tissue fluid
What happens in the collecting duct
- at the top of the ascending limb the tubule passes along a short distal convoluted tubule, this is where active transport is used to adjust the concentration of various mineral ions
- fluid flows into the collecting duct but at this stage the tubule fluid still has a lot of water and has a high water potential,
- the collecting duct carries the fluid down through the medulla to the pelvis
- as the tubule passes down the collecting duct water moves by osmosis from the tubule to the surrounding tissue and then enters the blood capillaries by osmosis
Describe the concentration changes in the tubule fluid
- glucose decreases in concentration as it is selectively reabsorbed from the proximal tubule
- sodium ions diffuse into the descending limb of the loop of Henle, this causes the concentration to rise, they are then pumped out of the ascending limbs so the concentration falls
- the urea concentration rises as water is withdrawn from the tubule, urea is actively moved into the tubule
- sodium ions are removed from the tubule but their concentration rises as water is removed from the tubule, potassium ions increase in concentration as water is remove, the potassium ions are also actively transported into the tubule to be removed in urine
What is osmoregulation
the control of water potential in the body
What does osmoregulation involve
- it involves controlling both the levels of salt and water in the body
why does the water balance need to be maintained
- to prevent water entering the cells and causing lysis or leaving cells and causing crenation
what are the sources of water for the body
- food
- drink
- metabolism
How is water lost from the body
- Sweat
- Urine
- water vapour
- faeces
Describe how water potential is balanced
- Kidneys use an effector to control the water content of the body and the salt concentration in bodily fluid
- on cool day when you drink lots of water the kidneys produce large volume of dilute urine whereas on a hot day where you drink little you produce a small volume of concentrated urine
Describe the mechanism of osmoregulation
- if you need to conserve less water the walls of the collecting duck become less permeable and less water is reabsorbed and a greater volume of urine is produced
- if you need to conserve more water, the collecting duct are made more permeable so that more water can be reabsorbed into the blood producing a smaller volume of urine
