Peripheral vasculature

Question 1

What is venous return?

Answer 1

rate at which blood returns to the thorax from the peripheral vascular beds e.g. blood entering the central venous pool

Question 2

What is the central venous pool?

Answer 2

the volume enclosed by the right atrium and the great veins in the thorax

Question 3

What is cardiac output?

Answer 3

rate at which blood leaves the central venous pool and is pumped out of the heart *inflow and outflow influence central venous pressure

Question 4

What is the relationship between venous return and cardiac output under normal steady state conditions?

Answer 4

they are equal it’s a closed system!

Question 5

What is mean circulatory pressure?

Answer 5

mean pressure when cardiac output stops and the pressure within the vascular system redistributes *influenced by the volume of the circulating blood and the smooth muscle venous tone

**measures ‘fullness’ of the system normal: 7mm Hg

Question 6

What does smooth muscle venous tone determine?

Answer 6

the capacity of the system! it can change size

Question 7

What happens when the central venous pressure equals the mean circulatory pressure?

Answer 7

no more gradient! flow ceases

Question 8

What happens in a normal heart if you increase cardiac output?

Answer 8

decreases the CVP (putting in more blood!) increases the pressure gradient -> increases venous return

Question 9

Why is severe hemorrhage dangerous?

Answer 9

u get a negative CVP -transmural pressure collapses the large veins

=zero venous return

Question 10

Why is venous return important?

Answer 10

it determines the end diastolic volume which determines stroke volume which helps determine cardiac output

Question 11

So what factors influence venous return?

Answer 11

*they all alter the pressure gradient b/w peripheral and central venous pressure

• increased sympathetic venoconstriction
• skeletal leg muscle pump
• incr blood volume
• cardiac contraction
• incr respiratory pump (decr intrthoracic P)
• cardiac suction
• venous valves

Question 12

Which factors increase peripheral venous pressure?

Answer 12

all these push more blood back to the heart!

• increased sympathetic venoconstriction: decr compliance which prevents from expanding -> when blood enters, it will be redirected to the heart
• muscle pump: push blood to the pool! has valves to prevent retrograde
• incr blood volume: e.g. transfusion, it will incr the P gradient -

cardiac contraction: driving force for venous return

Question 13

What factors decrease CVP?

Answer 13

• increasing the respiratory pump: pulls more blood to the heart due to the negative pressure
• cardiac suction: diastole sucks extra blood

Question 14

What is the function of venous valves?

Answer 14

to maintain pressure gradient between the peripheral and central venous pool in the face of gravitational forces

-breakup the column of fluid

Question 15

What do you get when you plot CVP and cardiac output?

Answer 15

intersection: equilibrium @ 2mmHg and 5 L/min

Question 16

What happens when you give a blood transfusion?

Answer 16

you shift the venous return curve up and to the right so get more CO but at at higher pressure *u can also get this by venoconstriction (incr fullness)

Question 17

What happens in hemorrhage?

Answer 17

you shift the curve down and to the left so get less CO and a lower pressure

*u can also get this by vasoconstriction (decr fullness)

Question 18

What are the factors that influence venous function?

Answer 18

1. blood volume: transfusion or hemorrhage -increase blood volume, increase venous return and vice versa 2. venous tone: sympathetics or drugs -increase tone (venoconstriction), increase venous return and vice versa

Question 19

What happens if u get sympathetic stimulation?

Answer 19

the cardiac function curve shifts up! so get more CO at a lower pressure :)

Question 20

What happens when you have CHF?

Answer 20

the cardiac function curve shifts down so get less CO at a higher pressure BAD :(

Question 21

What does the body do to compensate for CHF?

Answer 21

it increases volume! (hypervolemia) so this lets u get back ur CO :) but at higher pressure :/

Question 22

What happens when the CHF gets worse??

Answer 22

BAD! the cardiac function curve goes even lower so now u get lower CO and even higher P >:(

Question 23

Okay so what happens during hemorrhage and how does the body compensate?

Answer 23

@ first: normal- CO=5L/min P: 2mm Hg

1. Hemorrhage: lose blood! so venous return curve shifts down to left- decr CO & P
2. sympathetics kick in: incr HR, incr contractility -shift cardiac function up to left: CO incr but P decr
3. incr venous tone (also sympathetics):
   - incr venoconstriction (incr fullness: as if giving more blood but just making container smaller)
   - shift venous return curve up to right
   - Restore CO and incr P a bit still lower than normal *note: everything happens at same time so would go from normal to the end point (A to D)
   - automatically get back the CO but at a lower P by increasing contractility/HR/venoconstriction