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)