Venous blood flow and the heart Flashcards
Where is the blood found?
Almost 2/3 of your blood in the body is found in the systemic venous system, the other 1/3 is divided up into the pulmonary circuit, heart, arteries and capillaries
How do veins store blood at lower pressure?
Veins store more blood at lower pressure as they have thinner walls than arteries meaning they are more compliant - allow deformation (can be stretched) in response to an applied force
Compliance = ∆V/∆P describes how much volume change can we get for a particular change in pressure
Compliance in veins vs arteries
Arteries have small volume hence they have large pressure - rigid, will hold their volume, they won’t change their size - very low compliance
Veins have large volume hence they have small pressure - compliant, will change in size hence can accommodate for extra volume
How are veins good for survival?
Arterial puncture will drop the pressure hence significant loss of arterial blood, life threatening fall in arterial pressure
Vanoconstriction is under neural control can transfuse blood from venous (from blood reserves) to arterial system
What happens when you’re lying down vs upright?
Venous volume is larger than arterial volume, while lying down venous volume is uniform from head to toe, in the upright position venous volume below heart increases and above heart decreases leading to extreme venous pooling in legs and feet
How do venous valves counteract venous pooling?
Having valves in your veins counteract venous pooling as they can open and close hence counter act gravity which allows for a more even distribution of weight
If there were no valves extreme pooling would form and blood would not be evenly distributed - very heavy at the bottom
How does ‘tone’ of surrounding tissue counteract venous pooling?
The surrounding tissue - its tone - can also counteract venous pooling, creating a splint. The muscles will splint the veins and stop them from expanding too much
People who have low muscle tone are prone to fainting and have excessive venous pooling (more at risk for varicose veins)
How do skeletal muscle contractions increase venous return to the heart?
Valves superior to contracting muscle open, allowing blood to move towards heart. Valves inferior to contracting muscle are forced to close, preventing back flow of blood to the capillaries
The tighter the muscle the more compact the vein allowing the muscle to squeeze blood up veins back to the heart -> muscle contractions increase venous blood flow
Starlings Law of the Heart
The more stretched muscle fibres are before a contraction, the stronger the contraction and stroke volume will be
Therefore venous return directly affects the stroke volume and performance of the heart
