Kidney Function Flashcards
what is the role of the kidney in water/ electrolyte homeostasis
intake/ loss must be in balance (over any significant period)
- approximate
- expenditure of a lot of metabolic energy
- temperature regulation
what are the conditions that normal intake/ output are measured in
adult
male
23 degrees + normal daily activities
what is the typical intake of water per day
water ~1200ml
food ~1000ml
metabolic ~300ml
total ~2.5L/day
what is the typical output of water per day
urine ~1500ml
sweat ~100ml
faeces ~200ml
insensible loss ~700ml
- respiratory loss
total ~2.5ml
what is the sweat loss /hour and /day during heavy exercise in hot/ humid conditions
> 2L/ hour
10L/ day
urine output may well be reduced in these conditions
how is the role of salt controversial
intake is hard to measure
excretion normally measured
UK recommendations is 6g/ day
American Guidelines are similar but drop to 3.75g/ day
- if you are >51, have ^ BP, diabetes, chronic kidney disease
what is renal blood flow
25% of cardiac output
- about 625 ml/ 100g/ min
what is the BP is glomerular capillaries
50-60mmHg
- renal artery is short and relatively large radius
what is the passage of blood through the kidney
afferent arteriole -> glomerular capillaries -> efferent arteriole -> tubular capillaries -> venuole
what are the two types of nephron
superficial and juxta-medullary
- superficial just dips into the medulla
- juxta-medullary extend to papilla
- water reabsorption more effective in the longer juxta-medullar nephrons q
what are the 4 sections of the nephron
PCT, loop of Henle, DCT, collecting duct
continuous layer of epithelium
the cell shapes in the wall are very different
- reflects activity; SA; ion pumping etc.
what are the three basic principles of the kidney
ultrafiltration
reabsorption
secretion
describe ultrafiltration in the kidney
driven by blood pressure in glomerular capillaries
- high renal blood flow
- high filtration rate (90-140ml/ min)
describe reabsorption in the kidney
in the PCT and DCT
active pumping from filtrate in tubules
- for substances to be retained: water, glucose, amino acids, electrolytes
describe secretion in the kidney
active pumping into tubules
for substances to be eliminated fast then filtration alone allows: H+, ammonia, uric acid, some drugs (e.g. antibiotics -> need to take them every few hours)
foreign, unusual substances
how are the pumping rates controlled in the kidney
hormones
e.g. aldosterone can adjust the rates of Na+ and K+ excretion
aldosterone promotes salt retention
what are podocyte cells
the gap between the podocyte processes determines what gets filtered out
if the molecule is small enough, it will be filtrated
what molecules are filtered by podocyte cells
filtration of water and small molecules through slits between the podocytes
- ions, urea, glucose, amino acids, small proteins etc.
- cut off at ~67KD -> the size of haemoglobin
what does the presence of haemoglobin in urine suggest
either kidney infection/ inflammation or ^BP
describe active reabsorption in the PCT
brush border
active reabsorption of glucose, amino acids, Na+ and K+ ions
Co-transporters, aqueous channels, membrane pumps
- lots of mitochondria to promote this
substantial water reabsorption
what is the result of absorption by the end of the PCT
complete reabsorption of glucose, amino acids
substantial reabsorption of Na+ an water
volume of filtrate reduced by 2/3rds
what is the structure of the loop of Henle
thinner wall during descent into the medulla
thicker wall during ascent from the medulla
what is the function of the loop of Henle
solute diffuses into descending tubule: counter-current mechanism ‘recycles’ solutes
ion pumping develops high osmotic pressure at the tip of the loop
- no net re-absorption here
- longer the loop, higher the osmotic pressure
what is the difference in structure of the DCT to the PCT
more solute reabsorption and secretion
DCT pumps are under hormone control
- fine tuning the product
less intense electrolytes and water reabsorption
DCT ion pumping can be controlled by hormones like aldosterone
what is the purpose of ADH/ AVP
CDs pass close to tips of the loop
- if CDs are permeable to water, then moves out of the duct to concentrate filtrate
ADH increases presence of aquaporins in the lumina membrane
- allows water movement
- quick response
describe process of control of blood volume
1- water intake restricted
2- plasma osmolarity ^
3- more ADH is secreted by the hypothalamus
4- ADH ^ water permeability of CDs
5- ^ water absorption
6- concentrated urine is produced
what is normal plasma osmolarity
300 mOsm (275-290 mOsm - UK value)
what is the maximum concentration of urine
~1200 mOsm
what is the minimum urine output
~1 ml/ min
describe control of blood pressure
renin/ angiotensin/ aldosterone system
hypo-filtration initiates secretion of renin by the juxtaglomerular apparatus
renin splits angiotensinogen
- angiotensin I produces -> converted to angiotensin II (powerful vasoconstrictor)
this system regulates renal blood flow and glomerular filtrations rate (low BP, low renal flow, hypofiltration)
sympathetic nerves enhance this action
describe control of salt balance
aldosterone ^ when electrolyte concentrations fall (secreted by golerulosa cells of the adrenal cortex)
aldonsterone ^ reabsorption of Na+ and Cl- ions from Loop, DCT and duct cells
- also ^ K+ secretion
when electrolyte reabsorption increases; water reabsorption increases
