2.8B. Venous circulation, factors determining venous pressure and flow. Control of capacity vessels. Flashcards
I. Venous circulation
1. What are the characteristics of venous circulation?
- Venous circulation is the way out of the microcirculation
- Venous system returns the blood to the heart from the tissues
I. Venous circulation
2. Mechanism of venous circulation
By the time the blood reaches the venules + veins, the pressure is less than 10mmHg
-> decreases even further to nearly 0mmHg in the SVC/IVC + right atrium = this hydrostatic pressure is called the CVP (central venous pressure)
I. Venous circulation
3. What are the characteristics of veins? (Control of capacity vessels.)
- Veins are often referred to as capacitance vessels
-> High compliance (distensibility of vessel) of the veins allows them to accommodate a large volume of with less buildup of pressure (C=V/P)
-> 65% of total blood volume is kept in the veins, which makes them a large reservoir - Reservoir function allows them to control the volume of venous return to heart, which in turn determines the cardiac output (CO)
II. Venous pressure and flow
2. What is the value of Hydrostatic pressure in the postcapillary venule?
- Hydrostatic pressure in the postcapillary venule is about 20mmHg
II. Venous pressure and flow
3. How can Central venous pressure (BP in the venae cavae) affect Venous pressure and flow?
about 0 – 5mmHg
- Increased when: heart failure, increased volume
- Decreased when: volume loss
II. Venous pressure and flow
4. How can distensibility and resistance of veins affect Venous pressure and flow
Veins are distensible and have low resistance to blood flow
- Compliance varies within the body
- Ex: lower limbs are less compliant than those at or above the level of the heart
II. Venous pressure and flow
5. How can compliance of veins affect Venous pressure and flow
- Decrease in compliance (and vascular thickening) occurs with age
-> Elastic content of the vascular wall and its collagen content decreases
II. Venous pressure and flow
6. What are the 3 Factors influencing venous pressure?
- Cardiac cycle
- Respiratory cycle
- Muscle contraction
II. Venous pressure and flow
6A. How can cardiac cycle affect venous pressure?
retrograde effect of the heart: the arterial contraction, ventricular contraction and atrial filling increase the venous pressure
II. Venous pressure and flow
6B. How can respiratory cycle affect venous pressure?
during inspiration, pressure in the jugular vein falls (intrathoracic P becomes negative, which sucks blood into the thoracic cavity) while pressure in the femoral vein rises (as intra-abdominal P increases).
II. Venous pressure and flow
6C. How can Muscle contractions affect venous pressure?
as the person begins to walk, venous compression exerted by contracting leg muscles, and venous valves operate to force blood towards the heart.
=> muscular contraction lowers the mean venous P and lowers capillary hydrostatic pressure.
III. Relationship between CO, venous return and atrial pressure
1. Describe Cardiac function curve?
- Represents relationship between CO and right atrial P (related to venous return)
- when the right atrial pressure increases (via higher venous return), CO increases due to higher EDV/diastolic fiber length
III. Relationship between CO, venous return and atrial pressure
2. Describe the Vascular function curve
- Represents relationship between venous return and right atrial pressure.
- Blood flow is driven by a pressure gradient.
+) The lower the pressure in the RA, the higher the pressure gradient and the greater the venous return.
+) So increase right atrial pressure would decreases venous return. - When right atrial P is negative, the curve is flat because veins would collapse in this situation so even with high pressure gradient, this collapse of veins will impede venous return.
III. Relationship between CO, venous return and atrial pressure
3. Explain Cardiac contractile force / inotropic effect?
- Increased inotropy leads to increased CO, decreased atrial pressure (because less blood remains in the atria) and increased venous return
- Opposite effects with decreased inotropy
III. Relationship between CO, venous return and atrial pressure
4. How can Total Peripheral Resistance affect Venous return?
- Increase in TPR (SYM activity, α1) via increased vasoconstriction of arterioles
-> increased arterial pressure (less blood pumped out of heart)
-> increased afterload that the heart has to pump against
-> decreased CO
-> cardiac function curve shifts downward
-> less blood returns to the heart
-> decreased venous return
=> Increased TPR (contraction of arterioles) -> decreased venous return
=> Decreased TPR (dilation of arterioles) -> increased venous return
III. Relationship between CO, venous return and atrial pressure
5. How can Increased blood volume (venoconstriction) affect Venous return?
- Increasing the blood volume (or constriction of veins thus forcing blood out of ‘’storage’’) will increase CO, venous return and atrial pressure
III. Relationship between CO, venous return and atrial pressure
6A. How can gravity affecting venous return?
Gravitational force affects blood pressure depending on the body position
- Standing: blood pools in the lower extremities and distend both the arteries and veins
-> Distension occurs more on the venous side because venous compliance is much greater than arterial compliance
- Lying down: gravity affect both arteries and veins equally, so the pressure difference between them is unaltered
III. Relationship between CO, venous return and atrial pressure
6B. How can Muscular activity and valves affecting venous return?
- Skeletal muscle pump: their activity (contraction + relaxation) increases venous return by compressing and decompressing the veins located nearby
- Valves prevent venous blood from flowing downward toward the feet
