Lecture 12 - Venous blood flow and the heart Flashcards
Gap junctions and actin and myosin
Gap junctions do not have anything to do with actin and myosin and therefore have nothing to do with cross bridges either
Distribution of cardiac output at rest
Systemic circuit is at play always - we leave the left side of the heart and we move that blood through the arteries and then it branches out and works with many circuits in parallel to deliver blood to many different organs and then comes out the other side and is recollected in the veins and is sent back to the heart
Where is most of the blood found?
Roughly 2/3 of the blood at any given moment is in the veins.
You can’t store extra blood in the heart because it is already filling itself basically as much as it can (no extra space), can’t be stored in the artery because of we pack extra blood here then the MAP will go up and this can’t happen, can’t be stored in the capillaries because they are tiny and therefore are this primarily for gas exchange, so the only place left is in the veins and they store the extra, large amount of blood
Veins and arterioles - pressure and volume
High volume of blood at a low pressure for veins
Low volume of blood at a high pressure for arterioles
Compliance
The extent to which a vessel allows deformation in response to an applied force
How easy is it to change the volume inside the structure based on the amount of pressure being put into it
Compliance in terms of veins and arteries
Veins have a thin wall and therefore are more compliant than an artery which has a thick wall and is therefore rigid (low compliance)
High compliance vs low compliance
Low compliance objects will not change shaped based on pressure i.e. they are rigid
Something has high compliance or is very compliant if volume gets larger much easier with the addition of pressure
Compliance equation
Compliance = ΔV/ΔP
What does compliance allow veins to do?
Allows veins to store much more extra blood
Veins have ‘survival’ valve
In an emergency situation, can transfer some of the blood from the veins to the arteries.
For example - arterial puncture, loss of arterial pressure (even a little is enough to drop MAP), life-threatening fall in arterial pressure. Venoconstriction occurs (under neural control) which pushes extra blood to the heart so that the extra blood can go back through the heart and get pumped back our into the arteries which increases the stroke volume out of the heart into the arteries, therefore MAP will bounce back (self blood transfusion/blood transfusion from venous to arterial system)
Pooling in veins
High vascular compliance means that blood tends to accumulate (‘pool’) in veins
Venous volume is larger than arterial volume. While supine (laying down), venous volume is uniform from head to toe. In the upright position, venous volume below the heart increases; whereas venous volume above the heart decreases. Extreme venous pooling in the legs and feet can occur (varicose veins)
What counteracts venous pooling?
Venous valves - allows for a discontinuous column with more even distribution of weight and helps with the fact that the veins are susceptible to gravity
Tone of surrounding tissue
Tone of surrounding tissue also counteracts venous pooling along with venous valves. Particularly the case for skeletal muscle, because it can alter its tensile state. Resting muscle tone varies between individuals. Muscle tone acts to stiffen the veins - makes them less compliant and prone to pooling. Some people prone to fainting have low muscle tone and excessive venous pooling
Low muscle tone and excessive venous pooling is often associated with …
The elderly - less skeletal muscle tone means that veins have less support which means that more pooling happens and there is less circulation through the system
What happens when skeletal muscle contracts in terms of veins?
Skeletal muscle contractions increase ‘venous return’ to the heart.
Valves superior to the contracting muscle open which allows blood to move towards the heart. Valve inferior to the contracting muscle are forced closed, preventing the back flow of blood to the capillaries.
