Venous system

Question 1

Venules

Answer 1

• In the microcirculation, blood flows from capillaries into venules - have collagen so stronger
• These converge to form veins that exit the organ
• They have liMle tone or resistance
• Communicate with arterioles, chemically, to match inflow & outflow

Question 2

Veins - blood return

Answer 2

\: a volume reservoir
Return blood toward the heart
 -  Largeradius
-  Low resistance
Compared to arterioles & arteries:
-  Less smooth muscle with little myogenic tone
-  Less elastin so little recoil

Highly compliant > huge storage capacity therefore called capacitance vessels

Question 3

veins at rest

Answer 3

At rest, veins store extra blood…
but it’s not stagnant!

When capillary beds are closed: blood by passes > enters the veins > they stretch > total CSA increases > blood moves more slowly

Question 4

veins 3 - reservoir capacity

Answer 4

When required, the capacity of the reservoir decreases to increase venous return to the heart

Question 5

Venous capacity

Answer 5

• When venous capacity decreases, more blood is pumped by the heart > increased effective circulating blood volume
• When venous capacity increases, less blood is pumped by the heart > reduced effective circulating blood volume

Question 6

Veins: low pressure but there’s still flow

Answer 6

• A lot of pressure has been lost by the time blood reaches the venous system
• But atrial pressure is near zero so there is still a driving pressure from heart contraction (it’s small, but enough)
• Recall,F is proportional to ΔP/R

Question 7

Veins: factors that increase venous return

Answer 7

In addition to the driving pressure from cardiac contraction, other factors promote venous return:

1. Sympathetic-induced vasoconstriction
2. Skeletal muscle pump
3. Venous valves
4. Respiratory pump
5. Cardiac suction
6. Increased blood volume

Question 8

1. Sympathetic-induced vasoconstriction:

Answer 8

• Not much smooth muscle or tone, but innervated
• Stimulation results in vasoconstriction > increases venous pressure > increases flow to right atrium à increases venous return
• Resistance isn’t really affected due to the large radius
• mean arterial pressue increase with increase sympathetic stimulation for both arteriole and vein

Question 9

sympathetic - vein

Answer 9

Stimulation increases flow due to decreased capacity i.e. ‘squeezing’ effect pushes blood out but large radius & low resistance is maintained

Question 10

sympathetic - arteriole

Answer 10

Arteriole
Stimulation decreases flow due to increased resistance i.e. ‘squeezing’ effect narrows the vessel & less blood can enter

Question 11

2. Skeletal muscle pump:

Answer 11

• Large veins in periphery sit between skeletal muscles
• Contraction of muscle compresses veins > increases venous pressure > increases flow to right atrium > increases venous return

Upon stance, the effects of gravity increase venous pressure below the heart (cardiac contraction + weight of column of blood)

Question 12

Consequences of skeletal muscle pump:

Answer 12

1. Increased hydrostatic pressure increases venous capacity > blood pools in lower-leg veins > reduces effective circulating blood volume
2. Increased capillary hydrostatic pressure forces excess fluid to filter out > oedema

Question 13

If we squeeze a vessel in the middle, what direction is fluid likely to flow?

Answer 13

both ways so have venous valves to allow blood to flow in 1 direction/prevent back flow

Question 14

3. Venous valves:

Answer 14

One-way valves prevent back-flow upon vasoconstriction muscle contraction & gravity

Question 15

Varicose veins:

Answer 15

venous valves cannot support the column of blood above. Most serious consequence is abnormal clot formation.