renal Flashcards
What is the definition of a buffer
an acid-base buffer resists change in pH when an acid or base is added to it. It consists of a weak acid and its conjugate base
HA <-> H+ + A-
What is the buffer-titration curve
titration curve is a plot of pH vs the amount of acid or base added to buffer solution
useful for meauring pKa
pH on y asix and buffer composition on x
What are the ideal characterisitics of an ideal buffer
a good buffer solution must maintain a nearly constant pH when either acid or base is added.
Two features render this possible:
- range of buffer
- buffering capacity - ratio of concentrations of weak acid to conjugate base must remain faily constant so addition of acid or base doesnt affect it.
What are the physiological buffer systems in the body
The bicarbonate/carbonic acid buffer system - most important system
Despite it having a low pKa (6.1) relative to blood pH, it is effective due to the ready excretion of carbonic acid in the form of co2 by the lungs, and the continuous regeneration of bicarbonate by the kidneys.
It is more efficient at buffering acids since its efficiency increases as the pH falls.
It is the main buffer system in the blood due to the abundance of plasma bicarbonate. The production of carbonic acid is catalysed by the enzyme carbonic anhydrase, which is present in red blood cells, but not in plasma.
Haemoglobin - acts as blood buffer due to the imidazole groups of its histidine residues
Plasma and proteins
Phosphate
urinary buffering - occurs in PCT, DCT and CD
What effects do chronic kidney disease have on acid base balance
More acid is produced by metabolism than excreted. Depleting extracellular buffers and reduces plasma bicarb levels
reduced number of functioning nephrons
excess potassium causes intracellular alkalosis which inhibit H ion secretion
Bicarb reabsoption and regeneration reduced
Hb levels decreased due to reduced EPO
Excess acid may be buffered by calcium carbonate in bone contributing to renal osteodystrophy
How much of cardiac output does the kidney receive
20-25% of CO
90% of blood flow supplies the cortex via renal artery
10% goes to renal capsule and adipose tissue
Describe the autoregulation of renal blood flow
autoregulation describes the ability to maintain a constant RBF over a wide mean arterial pressure or tissue perfusion pressures from 90-200mmHg
Myogenic theory - most widely accepted brought about by direct contractile response of the afferent arteriolar smooth muscle to stretch. An increase in perfusion pressure causes increase in SM contraction and increase in renal vascular resistance - maintaining blood flow
sympathetic nerve stimulation - results in constriction of afferent arterioles reducing RBF
Renal prostaglandins attenuate sympathetic-induced vasoconstriction through vasodilation, thereby increasing RBF
Angiotensin 2 vasoconstricts the efferent arteriole more than afferent therefore maintaining GFR
What do you understand by GFR
Glomerular filtration rate is a unit of measure of the kidneys excretory function and can be defined as the volume of plasma cleared by of an ideal substance per unit time
usually expressed as ml/min and is usuaully 125ml/min or 180L/day
Values in women are suaully 10% lower than men
How can GFR be measured
GFR can be calculated by using plasma clearance of ideal substance using Fick method or estimated using prediction formulae
using fick principle
clearance (GFR) = urine conc x urine flow/plasma conc
eGFR is uses Cockroft and Gault equation or based on modification of diet in renal disease study (MDRD)
How and why is inulin used
Inulin is an exogenous polysaccharide with a molecular weight of 5200daltons
its freely filtered through the glomeruli
not reabsorbed nor secreted
not metabolised
not stored in the kidney
no effect on filtration rate
not toxic
easy to measure in blood and urine
however use is limited due to its expense and impracticalities
What factors effect GFR
same as those governing filtration across any capillary bed
- permeability of capillaries
- size of capillary bed
- hydrostatic and osmotic pressure gradients across the capillary wall (starlings forces)
