Cardiovascular: Venous Blood flow L10 Flashcards
Define vascular compliance, give an example.
Compliance is a measure of how much a vessel can expand in response to pressure. Thin-walled vessels (veins) have a higher compliance than thick-walled vessels (arteries).
Give the equation for compliance.
Compliance = Change in volume/Change in pressure
Describe compliance in arteries.
If there is a small change in volume for a given change in pressure, this gives a small value for compliance. Arteries have high smooth muscle content and are relatively thick. This makes them ‘stiff’ vessels, the opposite of compliant. The arteries can hold their shape even if there is no blood and the pressure is 0mmHg. However, arteries can expand e.g the aorta pulsates as it receives a new load of blood following a heart beat. The volume vs pressure graph for arteries is linear. This tells us that compliance (ratio between V and P) is constant at all pressures.
Describe compliance in veins.
A small change in pressure can cause a large change in volume. This means veins have high compliance. At 0mmHg, veins are collapsed (complete collapse occurs when a vacuum is created [negative pressure] is created, as can happen when blood is drawn from a person’s vein). As we increase pressure from 0mmHg, the veins will expand. Initially this occurs rapidly, there is a huge change in volume with a small change in pressure. In this initial phase, compliance is very high. However, unlike arteries, compliance is not constant. After the initial phase, there is a drop in expansion of the veins for a given pressure, there will be a smaller change in volume. Compliance is thus lower than the initial phase, but still higher than compliance in arteries.
How does venous pooling occur?
If a person is supine (lying down), there will be a greater volume of blood in the venous system, but it will be uniformly distributed. If a person is standing up, gravity will pull at this blood, and the blood will tend to pool at their feet. This can occur because veins have high compliance, they can easily stretch to accommodate the extra blood. This effect is referred to as ‘venous pooling’.
How is venous pooling a problem?
More blood in the lower body means less output to the heart and brain. This may make a person prone to fainting. This may also be the case with people with low general muscle tone.
What are some ways that we counteract venous pooling? (3 ways)
- Venous valves are generally bicuspid, and are staggered throughout veins to create a discontinuous column. These help to prevent backflow of blood to the feet with gravity, and to promote one-way (unidirectional) flow back to the heart.
- The tone (tension) of the tissues surrounding veins make it more difficult for the veins to expand. This makes them stiffer (less compliant), and works to prevent venous pooling.
- Skeletal muscles are particularly effective at causing this because they may contract and place more pressure on the veins, further counteracting their tendency to expand. Some skeletal muscles are part of the skeletal muscle pump. They actively contract to squeeze blood through the venous valves and promote return of blood back to the heart. The soleus muscle is an especially important part of this pump. The skeletal muscle pump is useful during exercise when cardiac output increases and blood flow back to the heart must increase accordingly.
What is a scenario where blood pressure drops quickly?How do veins promote survival?
A scenario of veins promoting survival would be a puncture of an artery (e.g from trauma), causing a lot of blood to be lost quickly. Blood pressure drops quickly, and vital tissues may not receive oxygen and nutrients they need. This is life-threatning. The veins contain 70% of the body’s blood supply and it is this supply that is used as an ‘internal blood transfusion’ to replace lost blood quickly. The recovery is rapid as the whole venous system is aligned with sympathetic nerves that will rapid communicate a strong venoconstriction to send more blood back to the heart than normal, approximately 0.5L will be replaced, keeping the mean arterial pressure in a normal range.
How does venous return increase heart contractility?
The strength of contraction of the heart depends on the length of its muscle fibres at rest. Increasing the length of fibres increases the strength of contraction.
Increasing venous return increases the end diastolic volume (EDV) and stretches the ventricles. Thus, increasing venous return increases the length of muscle fibres in the resting state, and will increase heart contractility. More blood = more stretch = more contraction.
What is preload or ventricular filling pressure?
Increasing the end diastolic volume (EDV), increases the pressure in the ventricles before the heart contracts. This pressure is called preload/ventricular filling pressure.
How does venous return affect heart performance?
Because more venous return means more blood going back to the heart, increases EDV, lengthens out muscle fibres in the resting state, causing stronger contractions, a higher preload, this means stroke volume is also affected by venous return and thus cardiac output: a stronger beating heart ejects more blood. This is described by Starling’s law of the heart. Because stroke volume determines how much blood is pumped around the body, we use it to describe performance of the heart.
What is the relationship between stroke volume and preload called?
Starling’s Law of the heart.
What does Starling’s Curve show? How do you interchange between values on the curve?
It shows stroke volume increasing with preload until a plateau.
In the supine position there will be a higher preload because there is no work done against gravity and blood can easily reach the heart, SV is therefore high.
When standing upright, work must be done against gravity so theres a lower preload and thus a lower SV.
Exercise can increase the venous return, end diastolic volume and stroke volume. Responsiveness depends on fitness.
What happens if there is too much stretch?
Too much stretch (from increased preload) will lead to too much stretch of the sarcomeres ultimately decreasing stroke volume.
