Venous Blood Flow and The Heart Flashcards
Where is extra blood stored?
In the systemic veins.
Where arteries are high-pressure, low volume vessels, veins are the opposite. They have a massive volume of blood but it is under very low pressure.
How do veins store more blood at lower pressure?
Thin walls with less smooth muscle allows the veins to have compliance and stretch to accomodate the larger volume.
- Compliance is the extent to which a vessel allows deformation in response to an applied force (low compliance = not very stretchy)
- Compliance = change in volume / change in pressure
Describe venous vs arterial compliance
Arterial compliance is very low as it only changes a small volume in response to large pressure.
Venous compliance is very high as its volume changes a large amount under a small pressure.
Describe the veins ‘survival’ value
When an artery gets pierced, the veins constrict under neural control. This is called venoconstriction.
This pushes extra blood up to the heart so it can pump faster and avoid a life threatening drop in arterial pressure.
Describe how veins accomodate for the affects of gravity
To avoid extreme venous pooling in the legs and feet when upright, there are three different mechanisms the body uses.
1. Venous valves. These counteract venous pooling by not allowing blood to go backwards. When gravity acts, the blood just pools back against the valves rather than falling all the way back down.
2. Tone of surrounding tissue. Particularly for skeletal muscle, because it can alter its tensile state. Muscle tone acts to stiffen the veins (makes them less compliant and prone to pooling).
- Some people who are prone to fainting because they have low muscle tone and excessive venous pooling
3. Skeletal muscle contractions. These increase the venous return to the heart by squeezing the arteries when muscles contract. This makes blood go up to the heart agains gravity (but also down, however the valves stop it pooling).
- During excersise this increases the venous return to the heart because more skeletal muscle is contracting and therefore pushing on the veins.
What is Sterling’s Law of the Heart
The more stretched muscle fibres are before a contraction, the stronger the contraction will be.
(aka. with increased venous return then more blood that gets into the heart and the stronger the next contraction will be).
- Increased venous return means increased stroke volume because the heart stretches and then the sarcomeres have more room to contract, making a bigger contraction
Describe why SV decreases in the baroreflex
Because the blood went down with gravity as because he was strapped to a bed and tilted, he wasn’t activating any skeletal muscles which would have usually pushed on the veins to get the blood up. Therefore less blood goes back to the heart and SV decreases.
