Physiology Flashcards
What is osmolarity?
the concentration of osmotically active particles present in a solution with units mosmol/l
How is osmolarity calculated?
using molar concentration of solution and number of osmotically active particles present
What is the difference between osmolarity and osmolality?
- osmolality is osmol/kg
- osmolarity is osmol/l
they are used interchangeably for body fluids
What is tonicity?
the effect that a solution has on cell volume
What are the different tonicities of fluid?
- isotonic= no change
- hypotonic= increase in cell volume
- hypertonic= decrease in cell volume
What does tonicity also take into account?
the ability of a solute to cross a cell membrane
How much is total body weight in a male or female?
male is 60% of body weight
female is 50% of body weight
(differences is due to the way fat is stored)
Where is body water stored?
- ICF: 2/3rds
- ECF: 1/3rd
What is the breakdown of where water is in the ECF?
- 80% interstitial fluid
- 20% plasma
- negligible% lymph and transcellular fluid
What does TBW equal?
ICF + ECF
What can be used to measure body fluid compartments?
tracers (ECF and TBW are calculable)
How can the volume of distribution be measured?
using a small amount of sample of a large container
What is the equation for distribution volume (l)?
(Qx(mol) of tracer) / X
the bottom is the equilibration volume of tracer in body
What are insensible losses of water from the body?
from skin and lungs
What are sensible losses of water from the body?
sweat, faeces and urine
How is water imbalance manifested in the body?
as changes in blood fluid osmolarity
Which ions are more concentrated in the ECF?
Na+
Cl-
HCO3-
What ion is more concentrated in the ICF?
K+
What are the osmotic concentrations of the ECF and ICF?
they are identical at around 300mosmol/l
How does fluid shift happen?
by movement of water between ECF and ICF in response to an osmotic gradient
What is tightly intertwined with fluid balance?
electrolyte balance
What causes changes in fluid osmolarity?
gain or loss or water
gain or loss of NaCl
Where is Na+ excluded from?
the ICF
What happens if there is NaCl gain in the ECF?
- increased ECF
- decreased ICF
What happens if there is NaCl loss in the ECF?
- decreased ECF
- increases ICF
What changes the composition and volume of the ECF?
the kidneys
What does regulation of ECF volume cause?
long term BP regulation
Why is electrolyte balance important?
- it determines water balance
- concentrations of Na+ and K+ can affect cell function
Which electrolyte determines ECF volume?
Na+ as water follows salt
What does K+ mainly do?
establishes membrane potential
What can changes to K+ cause?
paralysis and cardiac arrest so this must be monitored by the kidneys
What are the main kidney functions?
- water balance
- salt balance
- maintenance of plasma volume
- maintenance of plasma osmolarity
- acid-base balance
- excretion of metabolic waste products
- excretion of exogenous foreign compounds eg drugs
- secretions of renin (controls aBP)
- secretion of erythropoietin
- conversion fo vit D to active form (calcitriol which then does Ca2+ absorption from GI tract)
What is the nephron surrounded by?
peritubular capillaries
Is the nephron in the medulla or the cortex?
- loop of Henle and the end of the collecting duct are in the medulla
- the rest of the nephron is in the cortex
What are the two types of nephron?
- juxtamedullary (20%)
- cortical (80%)
What are the features of juxtamedullary nephrons?
- dip deep into the inner medulla
- long loop of Henle into medulla
- single capillary structure called the vasa recta
- these concentrate urine
What are the features of cortical nephrons?
- glomeruli in outer cortex
- short loops of Henle dipping into outer medulla
What is the inner layer of the Bowman’s capsule?
podocytes
What is the juxtamedullary apparatus?
the afferent and efferent arterioles run either side of the distal convoluted tubule which has a macula dense where it touches them
How much of the plasma is filtered in glomerular filtration?
20%
What does the rate of excretion equal?
rate of filtration (GF) + rate of secretion (TS) - rate of reabsorption (TR)
What are the two processes out of the four that are easy to measure?
- rate of filtration of X= [X]plasma * GFR
- rate of excretion of X= [X]urine * Vu (urine production rate)
How is net reabsorption calculated?
net reabsorption (when filtration is greater than excretion) is calculated by rate of filtration - rate of excretion
How is net secretion calculated?
net secretion (when filtration is smaller than excretion) is calculated by rate of excretion - rate of filtration
What can be assumed if filtration > excretion?
net reabsorption
What can be assumed if excretion > filtration?
net secretion
What are the filtration barriers in the glomerulus?
- glomerular capillary endothelium (barrier to RBC)
- basement membrane/basal lamina (plasma protein barrier)
- slit processes of podocytes (plasma protein barrier)
What is the path of fluid during glomerular filtration?
- from lumen of glomerular capillary
- through large pore
- into basement membrane
- through filtration slit between podocytes
- into lumen of Bowman’s capsule
What are the filtration pressures of the glomerulus?
- glomerular capillary blood pressure (favours)
- glomerular capillary oncotic pressure (against)
- Bowman’s capsule hydrostatic pressure (against)
- Bowman’s capsule oncotic pressure (favours)
What is GFR?
rate at which protein-free plasma is filtered from glomeruli into Bowman’s capsule per unit time
What does GFR equal?
GFR = Kf (how ‘holey’ the membrane is) * net filtration pressure
What is normal GFR and what is the main determinant of its value?
125ml/min
glomerular capillary blood pressure
What is the intrinsic mechanism of GFR?
autoregulation
- myogenic mechanism
- tubuloglomerular feedback mechanism
What is the extrinsic mechanism of GFR?
sympathetic control via baroreceptor reflex
What happens when arterial BP increases to GFR?
- there is more blood flow into glomerulus
- increased glomerular capillary BP
- increased net filtration pressure
- increased GFR
What do vasoconstriction and vasodilation do to GFR?
vasoconstriction of afferent decreases GFR
vasodilation of afferent increases GFR
What does a fall in blood volume do to urine volume?
- fall in blood volume and decrease in aBP
- detected by aortic and carotid sinus baroreceptors
- increased sympathetic activity
- generalised arteriolar vasoconstriction
- constriction of afferent arterioles
- decreased BPgc
- decreased GFR
- decreased urine volume (compensates for decreased blood volume)
What does auto regulation go to GFR?
stops short-term changes in systemic arterial pressure affecting GFR
What is involved in myogenic auto regulation of GFR?
vascular smooth muscle stretched (increased arterial pressure) and it contracts so the arteriole is constricted
What is involved in tubuloglomerular feedback?
involves the juxtaglomerular apparatus, if GFR rises, more NaCl flows through tube so constriction of afferent arterioles (macula densa in distal tubule senses the NaCl tubular fluid content)
What do kidney stones do to GFR?
decrease GFR due to increased Bowman’s capsule fluid pressure
What does diarrhoea do to GFR?
decreases GFR due to high capillary oncotic pressure
What happens to GFR in severely burned patients with a low capillary oncotic pressure?
high GFR
What happens if there is a decrease in surface area available for filtration?
low GFR
What values contribute to the normal net filtration of 10?
——————————-> normal net filtration 10
——–>glomerular capillary fluid pressure 55
BC oncotic pressure 0
What is plasma clearance?
the volume of plasma completely cleared of a particular substance per minute (each substance has its own specific plasma clearance)
What is does clearance of X equal? (important)
([X]urine * Vurine) / [X]plasma
What is special about inulin?
clearance = GFR as there is no secretion or absorption
What does the amount of inulin filtered equal?
the amount of inulin excreted
What does GFR of inulin equal?
([inulin]urine * Vurine) / [inulin]plasma = GFR= clearance
What substances have a clearance of 0?
eg glucose which are filtered and then completely reabsorbed and not secreted
What happens with substances which are filtered, partly reabsorbed and not secreted?
clearance < GFR eg urea
What happens with substances that are filtered, secreted and not reabsorbed?
clearance > GFR eg H+
What is the overall outcome in these situations?
a) clearance < GFR
b) clearance = GFR
c) clearance > GFR
a) net reabsorption
b) secretion and reabsorption cancel out or don’t exist
c) net secretion
What is the special feature of PAH?
para-amino hippuric acid is completely cleared from the capillaries by filtration and secretion so is used to calculate RPF
What should a clearance marker be?
non-toxic
inert
easy to measure
What should a GFR marker be?
filtered freely and not secreted or reabsorbed
What should a RPF marker be?
filtered and completely secreted
normal RPF is 650ml/min
What is filtration fraction?
fraction of plasma flowing through the glomeruli that is filtered into the tubules
What does the filtration fraction equal?
GFR / renal plasma flow which is 20% normally
What is the wall of the nephron made of?
single-layer of cuboidal epithelial cells
Where is renin made in the kidneys?
granular cells make renin at afferent arteriole where it meets the macula densa
What is the normal pH, HCO3- and arterial PCO2 of the blood?
pH= 7.4
[HCO3-]= 25
arterial PCO2= 35-45
What does acidosis and alkalosis lead to?
acidosis = CNS depression
alkalosis = overactive CNS and PNS
changes in [H+] also affects enzymes and K+ levels
Where is H+ added from?
- carbonic acid from CO2 and H2O (resp system)
- inorganic acids made during nutrient breakdown
- organic acids made from metabolism
What is the main equation for acid base balance?
CO2 + H2O H2CO3 H+ + HCO3-
lungs (carbonic anhydrase) kidneys
What is the simplified equation for pH of the body?
pH= kidneys/lungs
What can the kidneys do to the amount of HCO3-?
- variable reabsorption of filtered HCO3-
- kidneys can add new HCO3- into the blood
How does HCO3- enter the blood?
- HCO3- joins with H+ and then dissociates into H20 and CO2
- this enters cell
- CO2 passes through and together they join to make H2CO3 which dissociates inside cell
- H+ moves back into tubule against Na+
- HCO3- moves out into interstitium with Na+ and KNa makes the Na gradient
What is needed for HCO3- to be reabsorbed?
H+ is needed for reabsorption to prevent acidosis
What does an increase in CO2 do to H+ exchange at PCT?
- increase in H2CO3 so increase in secretion of H+ from tubular cell
- more CO2 breathed in = more H+ in tubular fluid
Normally, what is the situation with H+ and HCO3- in the PCT?
more H+ is secreted than HCO3- is filtered so all HCO3- is reabsorbed
What happens when HCO3- is low?
- H+ combines with phosphate to make H2PO4- which is expected and HCO3- is reabsorbed into the blood to add to buffer stores
- NH4+ is made from H+ which is excreted and new HCO3- is reabsorbed into the blood which uses glutamate from the liver
What is the difference between compensation and correction of acid base balance?
- compensation is restoration of the pH irrespective of the PCO2 and [HCO3-]
- correction is restoration of pH, PCO2 and [HCO3-] to normal
What are the causes of respiratory acidosis?
retention of CO2 by the body eg COPD, chest injury, airway restriction or respiratory depression
What does uncompensated respiratory acidosis look like?
high PCO2 and a low pH
What is the compensation and correction of respiratory acidosis?
- compensation= the kidneys secrete H+ in response to the CO2 retention (HCO3- rises as a result)
- correction= decreasing PCO2 by restoration of normal ventilation
What is the cause of respiratory alkalosis?
excessive removal of CO2 from the body eg altitude, hyperventilation or hysterical over breathing
What does uncompensated respiratory alkalosis look like?
low PCO2 and high pH
What is the compensation and correction of respiratory alkalosis?
compensation= kidneys lower the [HCO3-] so H+ is conserved correction= restoration of normal ventilation
What is the cause of metabolic acidosis?
COMMON
excess of H+ from other sources than CO2 eg ingestion fo acid, excessive metabolic production of H+ ie DM or diarrhoea
What does metabolic acidosis look like?
- decrease in [HCO3-] as a result of buffering for H+
- low pH
What is the compensation and correction for metabolic acidosis?
- compensation= CO2 is blown off by increase in ventilation due to low H+ which happens first
- correction= increase in acid load excretion and new HCO3- into. blood as H+ secreted and then the respiration can go back to normal
What is metabolic alkalosis caused by?
excessive H+ loss from the body eg vomiting, ingestion of alkali or aldosterone hyper secretion
What does metabolic alkalosis look like?
decrease in H+ and increase in [HCO3-] so increased pH
What is the compensation and correction for metabolic alkalosis?
compensation= CO2 is retained which shifts buffer to increase [H+] which also increases HCO3- correction= HCO3- excretion in the urine and after, the respiratory function can return to normal
What does the graph of all acid-base disturbances look like?
HCO3- against pH
top left= resp acidosis which goes diagonally up to comp and straight down to correct
top right= met alkalosis diagonally up to comp and straight down to correct
bottom left= met acidosis which goes diagonally down to comp and straight up to correct
bottom right= respiratory alkalosis which goes diagonally down tpo comp and straight up to correct
