Cardiovascular System, Lecture 7

Question 1

Arterioles

Answer 1

blood pressure
- starting to see pressure drop with distance from heart and resistance encountered
total cross sectional area
* significant branching - increasing area - increasing resistance
- results in slowing velocity of blood flow
- we want to slow it down because we are coming into the exchange with the capillaries, by design loss pressure as we go through network of capillaries
- there will be a drop in capillary coming in a bit higher (we can alter where we enter capillary bed - higher or lower)
- strong area of resistance regulation (vasconstriction/vasodilation)

Question 2

Arterioles - regulation

Answer 2

• arterioles have rich supply sympathetic (vasomotor nerves); little parasympathetic
• vasomotor tone: arterioles start in a state of partial vasoconstriction (level of sympathetic all the time)
• adjust sympathetic stimulation up or down to get desired level of vasoconstriction/vasodilation:
  ◦ recruit alpha receptors: smooth muscle contracts/ arteriole radius decreases (vasconstriction)
  ◦ recruit beta receptors: smooth muscle relaxes / arteriole radius increases (vasodilation)
• alter arteriole radius - alter resistance - alter organ blood flow
  ◦ Forgan = MAP/Rorgan-

Question 3

Arterioles - regulation - neural and hormonal

Answer 3

bind alpha receptors - arteriolar vasoconstriction - increases TPR
- neural: increase recruitment of vasomotor nerves (NE, epinephrine)
- hormanal: secretion of angiotensin ll/antidiuretic hormone (ADH)
bind beta - arteriolar vasodilation - decreases TPR
- neural: decrease recruitment of vasomotor nerves (NO (nitric oxide), epinephrine)
- hormonal: secretion atrial natriuretic peptide (ANP)
all arterioles have both alpha and beta but some have dominance in one
- alpha greater numbers in skin and kidneys
- beta greater numbers in skeletal muscle, respiratory passages

• epinephrine has the ability to bind to both alpha and beta receptors
• most arterioles are going to have both, but dominance in one will be shown
• skeletal muscle - more beta receptors (ex)
• hormonal will be covered more in urinary system (do not worry about details)

Question 4

Arterioles - regulation - local

Answer 4

• local - can often be dominant effect even when neural or hormonal oppose
  ◦ example: exercising skin has a lot of alpha receptors - neural or hormonal would lead to vasoconstriction - actually vasodilation due to local effects
• vasoconstriction
  ◦ stretch arteriole/cold environment
  ◦ if something wants to pull, respond with this
  ◦ out on real cold weather, take the blood from hands and take to core part of bodies
• vasodilation
  ◦ gas change (decreased oxygen)/elevated ions (potassium, hydrogen)/elevated osmolarity/endothelial cell release (nitric oxide, injury cytokines/hot environment)
  ◦ hot extremities, get the reverse, offload some of the heat so body temperature does not go too high

Question 5

Capillaries

Answer 5

blood pressure
- still decreasing pressure
- higher pressure as enter capillary bed and lower as exit capillary bed
total cross sectional area
* maximal branching - maximal area - maximal resistance
- thin walls so short distances for exchange (moving nutrients and oxygen to tissues that need them)
- slowest velocity of blood flow - ideal for exchange

Question 6

Capillaries - regulation

Answer 6

2 factors in regulation:
- arteriole: into capillaries
- vasoconstricted: less capillary blood flow
- vasodilated - greater capillary blood flow
- precapillary sphincters - within capillaries
- constricted: less of capillary bed open (most blood in metarteriole (largest passage way))
- relaxed: more capillary bed open
- exercise
- local changes promote precapillary sphincter relaxing
- improves with training to better match capillary blood flow to metabolic needs

Question 7

Capillaries - exchange - Starling Forces

Answer 7

4 starling forces:
- 2 hydrostatic pressure - blood (Pc)/interstitial fluid (Pif)
- 2 osmotic pressure - blood (πc)/interstitial fluid (πif)
◦ combined: net filtration pressure (NFP) = Pc + πif - Pif - πc
- capillaries to interstitial fluid if filtration (Pc and πif) > absorption (Pif and πc)
◦ true at arterial end: NFP = (35 + 3) - (0 + 28) = +10 filtration
- interstitial fluid to capillaries if absorption (Pif and πc) > filtration (Pc and πif)
◦ true at venous end: NFP = (15 + 3) - (0 + 28) = -10 absorption
- filtration > absorption by ~4 liters/day (excluding kidney capillaries)
- “excess” filtration enters lymphatic system eventually to be returned to bloodstream (see with immune system)
- if you are moving from capillary to interstitial fluid -> filtration
- interstitial fluid to capillary -> absorption (back into blood)
- if we get a negative number it favours absorption
- if we get a positive number it favours filtration

Question 8

Capillaries - exchange - arteriolar change

Answer 8

NFP = Pc + πif - Pif - πc
- arteriolar vasodilation/vasoconstriction alters blood hydrostatic pressure (Pc):
◦ vasodilation - higher Pc coming into capillaries - more filtration, less absorption
‣ example: Pc from 35 to 50 mmHg (other equation values same)
◦ vasoconstriction - lower Pc from 35 to 18 mmHg (other equation values same)
* while vasodilation is the widening of your blood vessels, vasoconstriction is the narrowing of blood vessels