Lecture 12: VENOUS BLOOD FLOW AND THE HEART Flashcards
Where is the most blood found in humans?
In the systemic veins
What do humans have in their circulatory system?
More blood than they need
How much blood is stored in the systemic veins?
Approximately 2/3
Why can’t the excess blood be in other areas of the circulatory system?
Cant be in the heart, pulmonary circuit (too short), arteries (maintaining MAP) or capillaries (too small)
What is the total blood volume in a 70kg human?
Approximately 5L
What is CO at rest?
Approximately 5L/min
What is arterial pressure at rest?
Approximately 100mmHg
What is arterial volume at rest?
Approximately 0.7L (13%)
What is venous pressure at rest?
Approximately 5mmHg
What is venous volume at rest?
Approximately 3.2L (64%)
What is the structure of arteries?
Thick and rigid tunica media so no way of allowing extra volume
What is the compliance of arteries?
Low
What is the structure of veins?
Thin and compliant tunica media so is able to allow extra volume
What is the compliance of veins?
high
What is compliance?
The extent to which a vessel allows deformation in response to an applied force
What is the formula for compliance?
change in volume/change in pressure
What happens in arteries?
Large change in pressure results in a small change in volume
What happens in veins?
Small change in pressure results in a large change in volume
What do veins have?
Survival value
What can veins do in an emergency situation?
Move blood across to arteries
What would happen after an arterial puncture?
Loss of arterial blood
What would happen after loss of arterial blood?
Life threatening fall in arterial pressure
What would happen after a life threatening fall in arterial pressure?
It will be sensed by the nervous system and venoconstriction will occur
What control is venoconstriction under?
Neural
What happens due to venoconstriction?
Veins constrict and push blood froward into the heart and arteries
What does high vascular compliance in veins mean?
That blood tends to accumulate (pool)
What volume is larger?
venous volume is larger than arterial volume
How is venous volume while supine (lying down)?
Uniform from head to toe
How is venous volume in the upright position?
Below the heart it increases and above the heart it decreases (due to gravity)
Where is there extreme venous pooling?
In the legs and feet
What counteracts venous pooling?
Valves and tone of surrounding tissue
What does a continuous column have?
No valves
What happens in a continuous column?
Heavy at bottom
What does a discontinuous column have?
Valves
What happens in a discontinuous column?
More even distribution of weight
What tissue particularly counteracts venous pooling?
Skeletal muscle because it can alter its tensile state
What varies among individuals?
Resting muscle tone
What does muscle tone act to do?
Stiffen the veins - makes them less compliant and prone to pooling
What do some people prone to fainting have?
Low muscle tone and excessive venous pooling
What does skeletal muscle contractions increase?
Venous return to the heart
What happens when a muscle contracts?
It pushes blood up and down
What do valves superior to the contracting muscle do?
Open, allowing blood to move to the heart
What do valves inferior to the contracting muscle do?
Are forced close, preventing back flow of blood to the capillaries
What is contracting muscle causing blood to flow to the heart called?
The skeletal muscle pump
What does increased venous return mean?
Increased stroke volume
What is starlings law of the heart?
The more stretch muscle fibres are before a contraction, the stronger the contraction will be
How does a stronger contraction occur (starlings law)?
When ventricles fill with more blood because of increased venous return, the thick and thin filaments are pulled further apart so can generate a greater maximum force of contraction
