Regulation of ECF Volume - Pt 1

Question 1

What are the major ECF and ICF osmoles?

Answer 1

ECF - Na+ and Cl-
ICF - K+

Question 2

What is regulation of ECF volume equal to?

Answer 2

Regulation of body Na+

Question 3

What is the distribution of total body water?

Answer 3

ECF - 14l (1/3) with 3l in plasma and interstitial fluid is 11l
ICF - 28l (2/3)
Total 42l

Question 4

What does changes in Na content of the ECF lead to?

Answer 4

Changes in ECF volume and will affect volume of blood perfusing the tissues - effecting circulating volume and BP

Question 5

What is regulation of Na basically dependant on?

Answer 5

High and low pressure baroreceptors

Question 6

Describe the renal response to decreased ECF volume (hypovolaemia)

Answer 6

Increase salt and H2O loss (vomiting, diarrhoea or excess sweating) - Decreases PV, venous pressure, VR, atrial P, EDV, SV, CO then BP which decreases carotid sinus baroreceptor inhibition of sympathetic discharge

Question 7

What does increase sympathetic discharge lead to?

Answer 7

Increase vasoconstriction, total peripheral resistance and increase BP towards normal
By increased renin and renal constriction

Question 8

What does decreased atrial pressure and carotid sinus baroreceptor discharge lead to?

Answer 8

Increase ADH release - may cause hypo-osmolarity

Question 9

What does increased renin lead to?

Answer 9

Increased angiotensin II - increased proximal tubule NaCl and H2O reabsorption
Increased aldosterone - increased distal tubule NaCl and H2O reabsorption

Question 10

What is the effect of sympathetic discharge specifically on the kidneys?

Answer 10

Increased renal VC nerve activity then increased renal arteriolar constriction and an increase in renin

Question 11

What is the effect of increased renin specifically on the kidneys?

Answer 11

Increased angiotensin II - decreased peritubular capillary hydrostatic pressure (increased oncotic pressure) and increased Na reabsorption from proximal tubule so less Na excreted

Question 12

What are changes in proximal tubule Na reabsorption due to?

Answer 12

Changes in rate of uptake by peritubular capillaries determined by oncotic pressure
greater absorptive forces in peritubular capillaries

Question 13

What happens to oncotic pressure when NaCl and H2O is lost?

Answer 13

Oncotic pressure forces increase even more than normal so can reabsorb 75% of the filtrate in the proximal tubule

Question 14

How does GFR remain largely unaffected?

Answer 14

Autoregulation maintains GFR and VC of afferent and efferent means little effect on GFR until volume depletion causes decrease MBP

Question 15

What 2 things mainly maintain GFR?

Answer 15

Construction of afferent due to to sympathetic VC coupled with angiotensin II mediated constriction of efferent arterioles

Question 16

What controls the regulation of distal tubule Na reabsorption?

Answer 16

Aldosterone - adrenal cortical steroid hormone

Question 17

What is aldosterone secretion controlled by?

Answer 17

Reflexes involving the kidneys themselves
Juxtaglomerular cells (JG) at macular densa

Question 18

What are juxtaglomerular cells?

Answer 18

Epithelial cells with plentiful granules which are on smooth muscle of afferent arteriole before entering the glomerulus where they become specialised

Question 19

What is the macula densa?

Answer 19

Histologically specialised loop of distal tubule

Question 20

What do JG cells produce?

Answer 20

Renin which acts on large protein in alpha2 globulin fraction of plasma proteins - angiotensinogen

Question 21

Where is angiotensin produced?

Answer 21

Liver constantly produces angiotensinogen in the plasma

Question 22

What contains ACE enzyme?

Answer 22

Blood vessel endothelium - converts ANGI to ANGII in plasma
Greatest conversion occurs as blood passes through pulmonary circuit

Question 23

What does angiotensin II stimulate?

Answer 23

Aldosterone - secreting cells in zona glomerulosa of adrenal cortex

Question 24

What is the rate limiting step of aldosterone?

Answer 24

Release of renin as angiotensinogen is always in the plasma

Question 25

What controls the increase of renin release?

Answer 25

Pressure in afferent arteriole at level of JG cells decrease - renal baroreceptors so less distention, more secretion Sympathetic nerve activity via B1 effect Decreased NaCl at macula densa delivery increases renin

Question 26

What controls the inhibition of renin release?

Answer 26

Angiotensin II feeds back ADH inhibits renin release by osmolarity control

Question 27

What provides the basis of tubuloglomerular balance?

Answer 27

Close relationship between afferent arteriole with JG cells and macula densa

Question 28

What happens in hypovolaemia?

Answer 28

Increased proximal and distal tubule Na reabsorption together with osmotic equivalents of H2O - helps restore volume deficits mediated by CV reflexes

Question 29

Why is angiotensin II important in body's response to hypovolaemia?

Answer 29

Stimulates aldosterone - NaCl and H2O retention Vasoconstriction - increases TPR Acts on hypothalamus to stimulate ADH secretion - increase H2O reabsorption from CD Stimulates thirst mechanism and salt appetite

Question 30

What does aldosterone directly effect?

Answer 30

Increase Na reabsorption which increases volume and maintain osmolarity

Question 31

What does GFR increase lead to?

Answer 31

Flow through tubule increases so flow past macular densa increases - paracrine from macular densa to afferent arteriole - afferent constricts - resistance increases - hydrostatic pressure in glomerulus decreases so GFR decreases

Question 32

What contributes to GFR constancy?

Answer 32

Tubuloglomerular feedback

Question 33

What happens if a patient has lost 3l of salt and water from ECF and drinks 2l of pure water?

Answer 33

Opposing inputs to ADH secreting cells Decreased ECF osmolarity means inhibition of ADH via osmoreceptors Decreased ECF volume means increase ADH via baroreceptors

Question 34

What has primary effect is ECF osmolarity and volume is decreased?

Answer 34

Volume considerations ADH will be increased as baroreceptors

Question 35

What happens when volume in restored in hypovolaemia?

Answer 35

Osmolarity will be normalised then again be the main determinant of ADH So if loss salt and water - replace with salt and water