Unit 2 Lecture 13: Veins

Question 1

Where are the pressures lowest in the vasculature system?

Answer 1

Veins; they are not really strong due to low pressures

Question 2

Unique fact about veins?

Answer 2

60% of blood volume is in veins
* Blood pools in veins
* Move against gravity
* Very compliant which allows pooling
* Large diameter

Question 3

Describe the structure of veins

Answer 3

• Thin walled
• Have minimal smooth muscle that is not contractile
• Valves that prevent backflow

Question 4

What is the muscle pump?

Answer 4

Under situations of very low pressure, one thing that propels blood forward is skeletal muscle pump effect

Question 5

What is deep vein thrombosis? How is it connected to the muscle pump effect?

Answer 5

Formation of a blood clot due to blood pooling in the veins
* If the skeletal muscle pump does not help move blood through the vein and the blood pools it will form a clot

NOTE: Pooling of blood will form a blood clot

Question 6

Sympathetic stimulation & inhibition does what to the veins?

Answer 6

Stimulation induces venoconstriction increasing pressure while inhibition decreases pressure; volume change in the veins is significant compared to the arteries

Stimulation decreases compliance

Question 7

Explain how vasoconstriction and volume change works with the arteries?

Answer 7

• When you have stimulation of the sympathetic nerves, the arteries vasoconstrict, which prevents the windkessel effect, so the volume change decreases

Question 8

Why does the vein change so much in volume for a small change in pressure?

Answer 8

The veins are very compliant so it means that more volume can be taken up for a small change in pressure
* Minimal matrix proteins (elastin); less recoil due to not enough elastin

REMEMBER: Compliance = ΔV/ΔP

Question 9

Flow/Venous return at the level of the veins is determined by what?

Answer 9

P1-P2/R
* P1 is the starting the pressure (pressure entering the vein)
* P2 is the resistive pressure (what is blocking blood flow; essentially regulates blood flow)

Question 10

Venous pressure determines venous return. That being said, what causes changes in P1 and P2?

Answer 10

P1: ↑ sympathetic activity
↑ blood volume
↑ muscle pump
P2: ↑ inspiration

• Sympathetic activity increases venoconstriction which increases pressure gradient from P1 to P2
• Increased blood volume like through capillary reabsorption; we increase volume in a confined space increasing venous pressure
• Increased muscle pump puts more pressure on the vein increasing driving pressure

Question 11

How does inspiration reduce resistive pressure (P2; right atrium)

Answer 11

• When we breathe in there is a compression of the thoracic cavity, we decrease the pressure being exerted on the vein by the diaphragm
• This encourages blood flow through the vein as it reduces resistive pressure that blood goes through into the right atrium
• Increase inspiration there is more room for thorax but this decreases the pressure; by decreasing pressure that encourages veins to have less pressure
• Increasing inspiration on P2 decreases venous pressure due to volume increasing - Key Takeaway
• When you expire you put more pressure on the veins of P1

When we increase inspiration on P2 we decrease the venous pressure so the overall pressure gradient increases or when we expire

Question 12

When you increase pressure gradient what happens to the venous return, atrial pressure, EDV and Stroke volume?

Answer 12

Everything increases essentially and when you have a higher stroke volume that is correlated to increased blood volume in ventricles which mean more stretch (Frank-Starling Mechanism)

Question 13

What are all the factors that affect venous return positively?

Answer 13

1. Increase in blood volume (water and salt retention; extract more liquid to get released into capillary from organ; absorption>filtration)
2. Respiratory pump (inspiration; decrease pressure in chest veins (P2) and increase pressure gradient
3. Skeletal Muscle pump
4. Increased sympathetic vasoconstriction activity (activated venoconstriction)
5. Pressure imparted to blood by cardiac contraction (Left ventricle pumping harder and increasing driving pressure by increasing MAP)
6. Cardiac suction effect
7. Venous valves (one-way valves preventing backflow and increasing venous return)

Question 14

What is the cardiac suction effect?

Answer 14

When ventricles relax, it sucks blood from atria to ventricle which means it’s also sucking blood from venous system
* Decrease pressure in heart increases pressure gradient

This is P2 (Reduces resistive pressure)
Venous pressure (P1) is high but by reducing that with the suction effect we have less resistive pressure (P2)

Question 15

What is the baroreceptor reflex?

Answer 15

Similar to a stretch reflex which occurs in the carotid artery
* It is dictated by changes in blood pressure

Located in the carotid and aortic body

Question 16

What happens in the baroreceptor reflex when you have increased blood pressure? What about during decreased blood pressure?

Answer 16

• When blood pressure increases this causes an increased stretch which is detected by the baroreceptor which then tries to dampen the increased MAP by activating PNS and inhibiting SNS activity (decreases SV)
• When blood pressure decreases it has an inhibitory effect on the baroreceptor reflex

NOTE:
When there is no mechanoreceptor reflex occuring due to stretch, the baroreceptor reflex will not work

Question 17

How do baroreceptors send signals once they detect a stretch?

Answer 17

• Baroreceptors have afferent nerves (sensory neurons) which send AP to the brain
• This will then go to the cardiovascular control centre (in the medulla oblongata)

CV centre is known as the integrating centre

Question 18

Where is the cardiovascular control centre located and what does it do once the signal is received from the baroreceptor and brain?

Answer 18

It is located in the medulla oblongata in the brainstem; it will decrease SNS (downstream) and increase PNS which will ultimately lead to dampening MAP

SNS activity: Decreases Heart Rate (↓CO) and Blood Vessel (↓TPR) ==> ↓MAP
PNS activity: Decreases Heart Rate (↓CO) ==> ↓MAP

Question 19

Why does the SNS activity have more influence than the PNS on the MAP?

Answer 19

Overall it dominates more than PNS activity in ↓CO and ↓TPR