Cardiovascular Physiology 29 Flashcards
Which blood vessels are 60% of the blood volume contained?
The venous blood vessels
Where is 1/3 of the blood in the venous system found?
In the liver, bone marrow and skin
Why are veins considered high capacitance vessels?
Because they are able to store large volumes of blood
How does the composition of veins differ from arteries?
Veins have less smooth muscle than arteries and their walls have a higher collagen to elastin ratio than arteries
What is meant by the veins are highly distensible?
They expand very easily at low pressures but have little elastic recoil
What does constriction of the veins in the liver, skin and lungs do?
Redistributes 20% of the blood volume to delicate organs like the brain and skeletal muscle
What induces constriction of the veins in the liver, skin and lungs?
Sympathetic stimulation
What is the relative pressure of the venous system?
It is a low pressure system
What do flap like valves in veins do?
They close if blood begins to back up in a vein
What do Venous valves do?
Prevent the backflow of blood into the capillaries
What is meant by venous valves compartmentalize the veins?
They distribute the weight of the blood into compartments
What mechanisms help return blood from the veins to the heart?
- Contraction of venous smooth muscle
- The skeletal muscle pump
- The respiratory pump
What is the smooth muscle around the veins innervated by?
The sympathetic nervous system
What causes contraction of the venous smooth muscle?
The release of norepinephrine from sympathetic nerve endings
What happens when arterioles constrict?
The forward flow of blood is reduced
What happens when the veins constrict?
It increases venous pressure, driving more blood out of the veins into the heart increasing forward flow
How does the Skeletal pump increase Venous Return?
The Skeletal muscles contract which increases venous pressure forcing more blood back to the heart
How does the Respiratory pump increase venous return?
Inspiration increases venous return due to pressure changes between the abdomen and thorax which increase the pressure difference between the peripheral veins and the heart
What do any changes in venous return cause?
Changes in stroke volume and affects output through the frank starling mechanism
Which mechanisms increase venous return?
- Skeletal muscle pump
- Respiratory pump
- Increased sympathetic activity to the venous smooth muscle
How do the mechanisms for increasing venous return affect stroke volume through the frank-starling mechanism?
The increase in venous return causing an increase in atrial pressure increasing end-diastolic ventricular volume which increases the cardiac fiber length causing a greater force during contraction and a greater stroke volume
