Unit 3 - Kidneys & Osmoregulation

Question 1

How is our body fluid compartments divided?

Answer 1

intracellular fluid (2/3 of body fluid)
extracellular fluid (1/3 of body fluid)
- plasma (1/4 of ECF)
- interstitial fluid (3/4 of ECF)

Question 2

what is the difference between osmolarity and osmolality?

Answer 2

osmolarity = osmoles/ 1L solution
osmolality = osmoles/ 1 Kg solvent

Question 3

what is tonicity?

Answer 3

ability of an extracellular solution to move water into/out of cell by osmosis

Question 4

how is free movement of water controlled?

Answer 4

osmotic and hydrostatic pressure

Question 5

if there is a higher oncotic pressure than hydrostatic pressure what will happen?

Answer 5

absorption of fluid from interstitium into capillaries

Question 6

what is oncotic pressure?

Answer 6

colloid osmotic pressure -pressure exerted by proteins in blood = albumin

Question 7

what regulates the distribution of water between cells and interstitial fluid?

Answer 7

osmotic pressure

Question 8

what is the GFR?

Answer 8

glomulerular filtration rate - rate at which kidneys form glomerular filtrate (ultrafiltration)
GFR =Kf x net filtration pressure (usually 180L/day)
GFR = (concentration of urine x amount of urine produced per minute)/ plasma concentration

Question 9

what is net filtration pressure?

Answer 9

glomerular hydrostatic pressure - bowman’s capsule pressure - glomerular oncotic pressure
(approx 10mmHg)

Question 10

what is Kf?

Answer 10

filtrate coefficient - measure of membrane’s permeability to water determined by surface area and hydraulic pressure

Question 11

how can GFR be measured clinically?

Answer 11

cannot measure directly so can measure rate of excreted substances which are not secreted/ absorbed by tubules= filtered load

• inulin clearance - not convenient does not have steady conc
• creatinine clearance - most common (<100 healthy)

Question 12

how can GFR be altered?

Answer 12

• decrease in afferent tone - increases diameter of afferent arteriole/ increase in efferent tone - decrease diameter of efferent arteriole = increases glomerular hydrostatic pressure - more filtration
• change in Kf
• change in oncotic pressure

Question 13

how does kidney sense changes in GFR?

Answer 13

tubuloglomerular feedback

• decrease in arterial pressure = decrease in GFR (detected by baroreceptors in carotid sinus)
• decreased filtrate NaCl concentration to macula densa stimulates juxtaglomerular cells to release renin
• renin cleaves decapeptide -angiotensin I which is converted to angiotensin II
• increase in efferent tone - vasoconstriction - increase in hydrostatic pressure = increase in GFR

Question 14

what else does angiotensin stimulate?

Answer 14

• release of aldosterone from zona glomerulosa: increases NaCl and water reabsorption - increas in blood volume - increase in blood pressure
• release of ADH: - stimulates water reabsorption from final 1/3 of distal tubule and collecting ducts of kidneys (into plasma) urine volume decreases and is more concentrated = urine osmolality increases and plasma osmolarity decreases

Question 15

which hormone opposes the effect of aldosteron and ADH?

Answer 15

ANP/ANF - atrial natriuretic peptide - released from heart during atrial stretch
acts on receptors that increase GFR to decrease NaCl reabsorption in distal nephron so decrease in blood pressure - decrease volume - increased sodium excretion - smaller cardiac output

Question 16

what are the three components that restrict the passage of fluid from glomerular capillaries?

Answer 16

• capillary wall contains fenestrae
• basement membrane - fibrils of negatively charged proteins
• podocytes extend pedicels over basement membrane - provides filtration slits

Question 17

why does albumin retain inside capillaries?

Answer 17

barrier more permeable to neutral/ positively charged molecules
albumin is negatively charged so retained due to mutual repulsion as well as its size

Question 18

what is the function of the PCT? describe the cells

Answer 18

function: selective reabsorption of water, glucose and amino acids
simple cuboidal epithelial cells - tight junctions between cells
microvilli on surface: brush border - increase SA

Question 19

what is the function of the ascending limb? describe the cells

Answer 19

function: concentration gradient between cortex and medulla

squamous epithelial cells with few mitochondria in thin limb/ cuboidal rich in mitochondria in thick limb

Question 20

what is the function of the distal convoluted tubule? describe the cells

Answer 20

function: ion balance

cuboidal wall - no brush border but initially has macula densa where distal tubule meets ascending limb and glomerulus

Question 21

what is the function of the collecting duct? describe the cells

Answer 21

function: concentrating urine/ water absorption/ urea
P cells - regulate sodium balance
I cells - acid-base balance

Question 22

what are the three main buffers in the body?

Answer 22

bicarbonate buffer
haemoglobin buffer
phosphate buffer

Question 23

what is the isohydric principle?

Answer 23

when a buffer reacts with H+ ions the change in H+ affects other buffer reactions in the same body compartments

Question 24

what is acidaemia and alkalaemia? why do they occur

Answer 24

pH <7.35 - due to respiratory/ metabolic acidosis

ph >7.45 - due to respiratory/ metabolic alkalosis

Question 25

what are the causes of respiratory acidosis and how does it affect pH?

Answer 25

• impaired gas exchange/ hypoventilation
• inhalation of CO2
• decreased respiratory drive

increased pCO2 in plasma = increased carbonic acid so increased H+ and HCO3- = decreased pH

Question 26

what are the causes of respiratory alkalosis and how does it affect pH?

Answer 26

• hyperventilation
• hypoxia
• increased respiratory drive

decreased pCO2 in plasma = decreased carbonic acid so decreased H+ and HCO3- = increased pH

Question 27

what are the causes of metabolic acidosis?

Answer 27

loss of base from gut (due to diarrhoea)
diabetic ketoacidosis
impaired acid secretion from renal tubules

Question 28

what are the effects of metabolic acidosis?

Answer 28

decreased pH leads to decrease in bicarbonate

• ketoacidosis - production of acetoacetic and B-hydroxybutyric acid (non-volatile acid) which decrease pH so H+ ions react with bicarbonate and CO2 is excreted
• in bony skeleton shift in pH ions from crystallites displaced by protons so crystallities used for buffering - ions excreted in urine - significant loss in calcium=bone dissolution

Question 29

what are the causes of metabolic alkalosis and how does it affect pH?

Answer 29

• loss of HCl from vomiting
• excess base ingestion
• aldosterone excess

low pH causes increase in non-volatile base (bicarbonate)

Question 30

what renal compensation occurs in the proximal tubule when chronic respiratory acidosis?

Answer 30

increased H+ excretion via NHE3: Na+-H+ antiporter
more carbonic acid which dissociates and then increase in pCO2 so reforms in tubular cells
bicarbonate formed leaves basolateral membrane (reabsorbed) in exchange for chlorine (excreted)

Question 31

what renal compensation occurs in the distal tubule when chronic respiratory acidosis?

Answer 31

excess h+ excretion by type A I cells via H+-ATPase against pH gradient so increases HCO3- pH returns to normal

Question 32

what renal compensation occurs in the proximal/distal tubule when chronic respiratory alkalosis?

Answer 32

reverse of acidosis

proximal: decreased H+ excretion/ increased bicarbonate excretion
distal: decreased H+ excretion by type A I cells/ increased bicarbonate excretion by non-type A intercalated cells

Question 33

what compensation occurs for metabolic acidosis?

Answer 33

respiratory compensation - increased respiration to decrease pCO2 which returns pH almost to normal but not exact because as it approaches 7.3 there is less stimulus to central/ peripheral chemoreceptors so pH is within 0.1 of range
renal compensation - corrects pH via ammoniagenesis= excretion of ammonia (slow adjustment)

Question 34

what compensation occurs for metabolic alkalosis?

Answer 34

similarly respiratory compensation -as high pH impairs respiration so higher pCO2 decreases pH but not to exact pH so renal compensation occurs via retention of Cl- and excretion of bicarbonate