Lecture 12: Venous blood flow Flashcards
Why cant the blood go anywhere?
Its a closed system
When does blood loss become critical
Around 25% loss
Where is the reserve blood stored?
In the venous system
How much is store
Around 2/3s
Amount of blood in arteries
1/3 or 13% because P is very high
Why dont arteries get bigger
Because they are very thick to maintain pressure
Why are veins compliant
the extent to which a vessel allows
deformation in response to an applied force
Compliance formula
∆V/∆P
Features of veins
Like balloon
- High compliance due to thin tunica media, more elasticity and can hold reserve blood even at low P
Survival value of veins
- Puncture in arteries cause drop in BP and BV
- The HR ^
There is venoconstriction to push the reserve blood to arteries
How is the blood distributed in supine position
uniformly distributed from head to toe
What happens in the upright position?
In the upright position, venous
volume below the heart
increases; whereas venous
volume above the heart
decreases.
- Venous pooling as veins expand and very hard to get blood to move against gravity
What do valves do?
Prevent gravitational of blood
- Breaks P
- unidirectional
How do muscles splint veins
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
How do muscles help during exercise
Exercise needs more CO –> muscles contract to squeeze the veins, pushes the blood back to the heart
