Excretion as an example of homeostatic control Part 2 Flashcards Preview

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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

1. sodium ions are actively pumped out of the cell lining the tubule
2. the concentration of sodium ions in cell cytoplasm decreases creating a concentration gradient
3. sodium ions diffuse into the cell through a cotransport protein, this carries glucose or an amino acid at the same time
4. water moves into the cell by osmosis
5. 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

1. glucose decreases in concentration as it is selectively reabsorbed from the proximal tubule
2. 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
3. the urea concentration rises as water is withdrawn from the tubule, urea is actively moved into the tubule
4. 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


How is he permeability of the collecting duct increased

- the cells in the walls of the collecting duct respond to ADH in the blood
- the cells have membrane bound receptors for ADH, the ADH binds to the receptors which causes a chain of enzyme controlled reactions inside the cell
- this ends with the vesicles containing water permeable channels to fuse with the cell surface membrane and makes the walls permeable to water
- as level of ADH rises the more water channels are inserted and this allows more water to be reabsorbed by osmosis into the blood, less urine is produced and the urine has a lower water potential


How is permeability of the collecting duct decreased

- the level of ADH in the blood falls and the cell surface membrane folds inwards in order to create new vesicles that remove water-permeable channels from the membrane
- this makes the walls less permeable and less water is reabsorbed by osmosis in the blood
- more water passes down the collecting duct to form a greater volume of urine which is more dilute


What monitors the water potential

the hypothalamus in the brain contains specialised cells called osmoreceptors, these are sensory receptors that detect the stimulus in the blood


What does osmoreceptors do when the water potential is low

- when the water potential of the blood is low then the osmoreceptor cells lose water by osmosis and shrink this stimulates the neurosecretory cells in the hypothalamus


What are neurosecretory cells

they are specialised neurones that produce and release ADH


How is the concentration of ADH in the blood adjusted when there is an increase in water potential

- this is detected by osmoreceptors in hypothalamus
- less ADH released from posterior pituitary
- collecting duct walls less permeable
- less water reabsorbed into blood and more urine produced
- decrease of water potential


How is the concentration of ADH in the blood adjusted when there is an decrease in water potential

- detected by osmoreceptors in hypothalamus
- more ADH released from posterior pituitary
- collecting duct walls more permeable
- more water reabsorbed into blood and less urine produced
- increase inn water potential of blood


How is ADH released

- neurosecretory cells release ADH,
- ADH moves down the axon to the terminal bulb in the posterior pituitary gland, where it is stored in the vesicles
- ADH is released b exocytosis
- ADH enters the blood capillaries and is transported around the body acting on the cells of the collecting ducts
- once water potential rises again less ADH is released


What is kidney failure

this is when the kidneys fail completely and they are unable to regulate the levels of water and electrolytes in the body or remove waste such as urea which can build up and lead to death


How can you assess kidney function

- can be assessed by estimating the glomerular filtration rate - measure of how much fluid passes into the nephrons each minute
- analyzing the urine for substances such as protein
- proteins in the urine indicate the filtration method has been damaged


what are the causes of kidney failure

- melitus - type 1 and type 2 sugar diabetes
- heart disease
- hypertension
- infection


what are the ways to treat dialysis

- renal dialysis - haemodialysis, peritoneal dialysis
- kidney transplant


How does haemodialysis works

- blood from an artery or vein is passed into the machine that contains an artificial dialysis membrane which is shaped to for artificial capillaries which increase the surface area for exchange
- Heparin is added in order to avoid clotting and the artificial capillaries are surrounded by dialysis fluid this flows in the opposite direction to the blood which improves the efficiency of exchange
- any bubble are removed before the blood is returned to the body via a vein
- two or three times a week for several hours each session, can carry it out at home


How does peritoneal dialysis work

- the dialysis membrane is the body's own abdominal membrane
- the surgeon implants a permanent tube in the abdomen and the dialysis solution is poured through the tube and fills the space between the abdominal wall and organs, after several hours the solution is drained from the abdomen
- can be carried out at home or work and the patient can walk around


what are the advantages and disadvantages of kidney transplant

+ freedom from dialysis
+ feeling physically fitter
+ improved quality of life able to travel
+ improved self image, no longer have a feeling of being chronically ill
- need to take immuosuppressant drugs
- need for major surgery under GA
- need for regular checks for signs of rejection
- side effects of immunosuppressant drugs


What can urine be tested for

- glucose for diabetes
- alcohol to determine blood alcohol levels
- recreational drugs
- anabolic steroids
- pregnancy


How does pregnancy testing work

- once an embryo is implanted in the uterine lining it produced a hormone called human chorionic gonadotrophin hCG, this can be found in the urine as early as six days after conception
- pregnancy tests use monoclonal antibodies


How does testing for anabolic steroids work

- anabolic steroids increase protein synthesis within cells, resulting in the build up of muscle and cell tissue
- they have a half life of 16 hours and remain in the blood for many days and they can enter the nephron easily