Homeostasis Flashcards
Homeostasis
- the maintenance of a constant internal environment by negative feedback
- in a state of dynamic equilibrium
- maintains glucose conc, pH, core temp, solute potential
= cells function normally, reactions occur normally
negative feedback
- receptor detects deviation from set point, sends instructions to co-ordinator
- co-ordinator communicates with effectors = corrective responses
- returns to normal = effectors stop
- condition restored to optimum levels
negative feedback examples
- glucose regulation of blood via insulin
- thermoregulation of core body temp at 37c
- water potential by ADH
positive feedback
- enhances the size of a stimulus
- amplifies the change
positive feedback examples
- oxytocin stimulates uterus contractions
- platelets adhering to a cut attract more
egestion
removal of undigested semi-solid waste (faeces)
excretion
removal of waste produced by body due to metabolism
ammonia
- highly toxic
- water soluble
- large volume of water to dilute = non-toxic conc
- quickly diffuses out gills (large SA) of freshwater animals
urea
- excreted by mammals
- water soluble, less toxic
- energy used to convert excess amino acids/ nucleic acids to urea
urea production
- excess amino acids are deaminated in the liver
amino acid ~ a-keto acid + ammonia ~ urea - amine group converted into urea
uric acid
- low toxicity
- low water solubility (little water needed)
- lots of energy to produce
- excreted by reptiles, birds, insects
- advantageous in scarce environments
osmoregulation
control of water potential of the body’s fluids
kidney structure
- renal vein (away)
- renal artery (towards)
- medulla (reabsorption of water)
- cortex (ultrafiltration, selective reabsorption)
- pelvis (empties urine into ureter)
- urethra (urine out of body)
- ureter (urine to bladder)
- bladder (stores urine)
nephrons
collectively provide a large SA for exchange of materials
ultrafiltration
filtration under high pressure
- bowman’s capsule and glomerulus
ultrafiltration steps
- blood enters at high pressure glomerulus via afferent arteriole (wide), leaves via efferent (narrower)
- small molecules and ions are forced through
leaving the glomerular filtrate
ultrafiltration layers
- fenestrations of endothelial cells (capillary walls)
- selective molecular filter of basement membrane (blood cells, platelets and protein too large to pass)
- filtration slits of pedicels (podocyte extensions)
glomerular filtrate composition
- water
- glucose
-urea - amino acids
- salts
glomerular filtration rate
rate at which fluid passes from the blood in the glomerulus to the Bowmans capsule
- determined by difference in water potential
selective reabsorption
useful substances (glucose, Na+, amino acids) are reabsorbed from the glomerular filtrate back into the blood
- in the PCT by FD and AT
- all glucose, most of the water + mineral ions are reabsorbed
epithelial cuboidal cells of PCT adaptions
- microvilli = large SA
- many mitochondria provide ATP for AT
- folded basement membrane / basal channels = large SA
- close association with capillaries
- tight junctions between cells = prevent molecules diffusing between adjacent cells or back into glomerular filtrate
selective reabsorption steps
- glucose and amino acids enter the cell by co-transport with Na+ ions
- chloride ions enter by facilitated diffusion
- water enters by osmosis
- they diffuse across the cytoplasm (down conc gradient), provided energy for secondary AT of glucose against conc gradient
- glucose leaves by FD (carrier) and secondary active transport (pump)
- Na+ AT out via a sodium-potassium pump
- amino acids and Cl- leave by FD
- water leaves by osmosis
the glucose threshold
glucose conc in filtrate too high = too few transport molecules in PCT cell membranes to absorb it all
= glucose passes into loop of Henle
= lost in urine
why glucose in urine?
- pancreas secretes too little insulin (type 1 diabetes)
- response of liver cells to insulin is reduced due to damaged insulin receptors (type 2 or gestational diabetes)
reabsorption of water by loop of Henle steps
- filtrate leaves PCT, entering descending limb
- descending limb = permeable to water = water leaves filtrate by osmosis, down water potential gradient
- Na+ and Cl- diffuse into descending limb from medulla
- water leaving = filtrate more concentrated = max conc at apex of loop
- medulla has a low water potential
- ascending limb = impermeable to water, but Na+ and Cl- leave by FD then AT into tissue fluid of medulla
- medulla = low water potential = water moves out by osmosis, then readily removed by blood in vasa recta
counter current multiplier
- filtrate flows in opposite directions
- the conc of solutes in the filtrate increases towards the apex (maximised)
variation in loop of henle length
- long loop = adapted to dry environments
- short loop = adapted to freshwater environments
longer loop = more ions can be pumped into the medulla, lower the water potential of the medulla = more water reabsorbed into bloodstream
osmoregulation
the control of body fluid water potentail by negative feedback
- affects DCT and collecting duct
osmoregulation steps - dehydration
- low water levels in blood / high osmotic pressure
- detected by osmoreceptors in hypothalamus
increases amount of ADH secretion from posterior pituitary gland into blood stream - binds to receptor proteins on walls of DCT and collecting duct
- increases permeability to water ; aquaporins are added to cell membranes of effectors = more water reabsorbed by osmosis
- more water reabsorbed into blood from filtrate, due to low water potential in medulla
- small volume of concentrated urine produced
- water potential increases in blood; back towards set point
- info fed back to hypothalamus = less ADH produced
- thirst encourages drinking of water
dialysis
method of replacing kidney function
cleans blood
blood and dialysis fluid separated by a selectively permeable membrane
haemodialysis
- blood taken from artery, travels through fibres of dialysis tubing
- tubing surrounded by dialysis fluid
- small ions/molecules in blood diffuse down conc gradient into dialysis fluid
- counter current mechanism = conc gradient maintained
- heparin added as a blood thinner
composition of dialysis fluid
- glucose and ion conc similar to normal blood levels
- normal salt conc = corrects any imbalance
- no urea
continuous ambulatory peritoneal dialysis
- dialysis membrane is the patients own peritoneum, accessed via catheter
- dialysis fluid passes from peritoneum into abdomen
- peritoneum has rich blood supply allowing exchange of materials
kidney transplant stages
- donor must be living or in circulatory death / brain stem injury
- donor / recipient in blood groups and human leukocyte antigens
- cells with foreign antigens invoke an immune response = rejection
- recipient takes immunosuppressant drugs forever = more susceptible to infection
kidney transplant advantages
- permanent solution
- allows for a less disruptive lifestyle
- no diet restrictions
kidney transplant disadvantages
- immunosuppressants = increased risk of infection
- may only last a few years
- long waiting list
- chance of rejection
- surgery poses risks
dialysis advantages
- started straight away (no waiting list)
- readily available
- no risk of rejection
dialysis disadvantages
- requires frequent hospital visits
- feel unwell between sessions due to toxic waste build up
- diet restrictions (low protein)
CAPD advantages
- portable, completed at home and can travel
- fewer diet restrictions than normal dialysis
- cheaper than using machines
CAPD disadvantages
- needs to be done several times a day
- must drink little
- must avoid potassium rich foods
treatments for kidney faliure
- reduce protein intake = less urea formed
- drugs to reduce b.p. (ACE, ARBs, calcium channel blockers dilate blood vessels, beta blockers)
- glucose and insulin = treat high potassium conc
- biphosphates = treat high calcium conc
- dialysis
- transplant
