renal physiology Flashcards
What is osmolality?
Particles per kilo of solution
-a high osmolality is a highly concentrated solution
What is osmolarity?
proportional to the number of particles per litre of solution; it is expressed as mmol/L.
-a high osmolarity is a highly concentrated solution
What is:
- hypotonic
- hypertonic
- isotonic
hypotonic: solution has a lower conc. than cell
Hypertonic: solution has a higher conc. than cell
isotonic: solution has the same conc. as cell
what compartments make up total body water?
Intracellular fluid: 67%
Extracellular fluid: 33%
- plasma 20%
- interstitial fluid 80%
- lymph and transcellular fluid
How are body fluid compartments measured?
using tracers - obtain distribution volume from tracer
What are the tracers for:
- total body water
- extracellular fluid
- plasma
TBW: 3H2O
ECF: inulin
Plasma: labelled albumin
How are body water compartments measured?
Volume in litres = dose (D) / sample conc
e.g. 42mg/1mg per litre = 42litres
e.g. for tracer X
distribution volume = quantity X (mol)/ equilbrium of X in body (mol/l)
what are the main ions in extracellular fluid?
Na+ CL- HCO3-
what are the main ions in intracellular fluid?
K+ Mg2+ and -vely charged proteins
Describe the four ways that fluid homeostasis is challenged? how does it affect intracellular vs extracellular fluid?
1: gain or loss of water - change to both ICF and ECF similarly
2: NaCl gain - water is drawn out of cells into the extracellular fluid
3: NaCl loss - water is drawn into cells out of the extracellular fluid
4: gain or loss of isotonic solution - ECF changes only as this does not affect tonicity
Which electrolyte is the main determinant of extracellular fluid?
Na+, therefore it is vital that this is regulated
Which electrolyte plays a key role in establishing membrane potential?
Potassium: more than 95% K+ is intracellular and small leakages or increased cellular uptake may affect conc. plasma K+
=muscle weakness
=cardiac irregulations
what are the ten kidney functions?
1 - water balance 2 - salt balance 3 - plasma vol. maintenance 4 - plasma osmolarity balance 5 - acid-base balance 6 - excretion metabolic waste products 7 - excretion exogenous foreign compounds 8 - secretion renin 9 - secretion EPO 10 - activated vit D
What is the functional unit of the kidney? what are their 3 functions? what two different types do you get and what is the difference? which is the most common type?
Nephron
functions:
1 - filtration
2 - reabsorption
3 - secretion
Juxtamedullary nephrons (20%) have a longer loop of henle which dips far into the medulla
Cortical nephrons (80%) only have a small loop of henle
what are the three basic renal processes?
1: glomerular filtration (20% of plasma that enters the glomerulus is filtrated
2: Tubular reabsorption
3: Tubular secretion
How is rate of excretion calculated?
Rate of excretion = rate of filtration + rate of secretion - rate of absorption
how is filtation rate calculated?
Rate of filtration of X = mass of X filtered into bowmans capsule per unit time
Rate of filtation of X = [X]plasma X GFR
How is excretion rate calculated?
rate of excretion of X = mass X excreted per unit time
= [X]urine X urine flow rate
How is reabsorption rate measured?
=rate filtration of X - rate excretion X
if so much X has been filtrated but only a smaller amount has been excreted - must have been reabsorbed
How is secretion rate measured?
= rate of excretion X - rate of filtration X
more X has been excreted than filtrated and therefore it must have been secreted
Describe the 3 filtration barriers in the glomerulus that prevent RBCs/plasma proteins from being filtrated??
glomerular capillary endothelium provides a barrier to RBCs
Basement membrane and slit processes of podocyte provides a barrier to plasma proteins
What two factors are involved in net filtration rate in the capillary?
hydrostatic and oncotic pressures in the capillary and in bowmans capsule (although no oncotic pressure in bowmans capsule as no plasma proteins here)
- usually this is about 10mmHg
What is GFR and how is this calculated?
rate at which protein free plasma is filtrated to bowmans capsule per unit time
-Kf X net filtration rate
(Kf is filtation coefficient: how holey is glomerular membrane?)
How is GFR regulated in the body?
Extrinsic: baroreceptor reflex
Intrinsic: autoregulation in the kidney
Describe autoregulation in the kidney?
Myogenic:
-if vascular smooth muscle is stretched it contracts and constricts arteriole
Tubuloglomerular fneedback
-if GFR rises, more NaCl flows through which is detected by the macula densa in the juxtaglomerular apparatus
=constriction of afferent arteriole
=this ensures renal blood flow is maintained over a wide range of MAP to protect GFR
how could:
- renal stone
- diarrhoea
- severe burns
- change in surface area avail. for filtration
affect GFR
Renal stone:
-increase the hydrostatic pressure in the bowmans capsule = GFR decreased
Diarrhoea:
-increases capillary oncotic pressure (dehydration)
=GFR decreased
Severe burns:
-decreases the oncotic pressure of capillary = increases GFR
change in surface area avail. for filtration:
-decreases the filtration coefficient = decreases GFR
what is plasma clearance?
volume of plasma that is cleared of a particular substance per min
=rate of excretion/plasma conc.
=[X]urine + volume urine produced ml/min / [X] plasma
What substance is used as a clearance marker? what qualities does a substance have to have to be a clearance marker?
Creatinine clearance = GFR
-endogeous
(inulin can be used too)
Qualities: freely filtered but no secreted or reabsorbed
What is the clearance of: -glucose -urea -H+ -PAH (what can this be used for) compared to GFR
Glucose: all is reabsorbed and none is secreted (clearance = zero)
Urea: clearance < GFR (is partly reabs. and none secreted)
H+: clearance > GFR (not reabsorbed and some secreted)
PAH: freely filtered and is all secreted
(can use this to calculate renal plasma flow as this is the rate of clearance of PAH)
what is the filtration fraction of the kidney?
amount (% or fraction) of plasma that is filtered at the glomerulus
=GFR/renal plasma flow
Where in the nephron is the main site for water and ion balance regulation?
distal tubule and collecting duct
what regulates water and ion balance in the distal tubule and collecting ducts?
mainly hormones:
aldosterone - increases sodium reabsorption and increases H+/K+ secretion in the late distal tubule
ADH - increases the number of aquaporins in the luminal membrane of the late collecting duct to allow more water reabsorption
ANP: decreases sodium reabsorption
PTH: Ca2+ reabsorption increases, PO43- reabsorption decreases
How is: Filtration Secretion/Reabsorption Excretion Regulated?
Filtration: changes in blood pressure/changes in size of filtration slits
Secretion/reabsorption: changes in solute concentration e.g. ADH/Aldosterone
Excretion: bladder function under neural control
Describe how ADH is regulated, what does ADH cause
ADH is stimulated by: Hypertonic ECF, decrease in left atrial pressure, nicotine
ADH is inhibited by: hypotonic ECF, stretch receptors in upper GI tract, alcohol
ADH increases thirst, causes arteriolar constriction and increases water reabsorption in the late distal tubule
Describe salt balance in the body:
Amount of sodium reabsorbed is regulated by RAAS
Renin released from granular cells in the juxtaglomerular apparatus due to:
- decreased pressure in the afferent arteriole
- macula densa cells sense a decrease NaCl
- increased sympathetic activity due to a decrease in arteriolar BP
Aldosterone - stimulated by high potassium or low sodium in the blood or RAAS, acts to reabsorb sodium and and pump out potassium
Describe how RAAS is stimulated in heart failure?
Failing heart leads to a decreased cardiac output and decreased blood pressure which causes an increase in salt retention and water
What hormone inhibits RAAS? where is this produced and stored?
ANP - causes excretion of sodium
Produced by the heart and stored in the atria: if heart stretched (e.g. by high BP due to high salt) ANP is released
what is the difference between water diuresis and osmotic diuresis?
Water diuresis: this is an increased urine flow but no increased solute excretion
Osmotic diuresis: this is a primary increase in sodium excretion which causes a diuresis
How does the kidney regulate acid base balance by regulating H+ ions in plasma?
- in the proximal tubule HCO3- is reabsorbed
- if there is a lot of H+ ions in tubular fluid, kidneys make more HCO3- to regenerate buffer stores depleted by acid load
If the concentration of H+ ions in the tubular fluid increases - what does this cause?
- drives reabsorption of HCO3
- Forms acid phosphatase
- forms NH4+ (ammonium ion)
What is the difference between nephrogenic vs neurogenic diabetes insipidus?
Nephrogenic: inability of nephron to respond to ADH
Neurogenic: lack of vasopressin release from posterior pituitary - treat with desmopressin
