Lecture 12 Venous blood flow and the heart Flashcards
Arterioles
Decide the direction of blood flow at parallel part of systemic circuit
Blood back from left side of heart to right side of heart
To become reoxygenated
veins
Left side of heart
In charge of Cardiac output
The amount of flow of blood out
Right side of heart
Venous return
The amount of blood flow back in
Where is most blood found?
Systemic veins
what is the total blood volume in 70kg human?
5L
Distribution of blood in systems
Small amounts in
Pulmonary circuit 9%
Heart 7%
Systemic arterial system 13%
Systemic capillaries 7%
Distribution of blood in systems
Majority of blood in
Systemic venous system 64%
Why are there large amounts of blood in the veins?
Need extra blood to survive
meet the demand of organs (oxygen and nutrient)
If we get an injury lose some blood, and only just have enough the injury will be life threatening
Places where you can’t store extra blood
incl why
Heart (Packed has no space)
Pulmonary circuit (short busy to get blood to lungs to become oxygenated)
Arteries (MAP will increase)
Capillaries (tiny no room)
Blood volume and blood pressure distribution at rest
Arteries
High pressure
Low volume
Blood volume and blood pressure distribution at rest
Veins
Low pressure
High volume
How do veins store more blood at lower pressure?
Thin walls
More compliance
Structure
Artery
incl media, structure
Thick Tunica media
Rigid (more muscle)
Veins
incl media, structure, muscle
Thin Tunica media
Less muscle
More compliance
Flexible accommodate more blood
Compliance
The extent to which a vessel allows deformation in response to an applied force to accommodate more blood.
More = flexible
Less = rigid
Compliance equation
ΔV / ΔP
Artery compliance
Shape maintained as pressure increases,
small change in volume
Less compliant
Vein Compliance
At low pressure
- Collapse (drained blood)
Increase pressure (Put blood in vein)
Volume increases
Very compliant
Why is Most blood found in small veins and venules?
Compliant
More flexible to accommodate extra blood
Able to store blood in that area
Veins have
survival value
Survival value
eg Veins have a Pool of blood that you can use when needed
eg puncture small artery
Cut / puncture small artery
what happens, what senses injury, what occurs, what goes back up
Loss of arterial blood
Lose blood pressure
Brain senses the injury
Venoconstriction
Veins squeezes on the blood that it’s storing (at higher pressure to the heart)
Under neural control medium and larger veins replaces the blood to the artery
MAP goes back up
Vasodilation
Luminal radius of arterioles increase
Vasoconstriction
Luminal radius of arterioles decrease
MAP constant
Total blood flow increased
TPR decrease
High vascular compliance means that
blood tends to accumulate (pool) in veins
Supine (laying down)
venous volume, gravity
Venous volume uniform (equal) from head to toe
Blood evenly distributed not working against gravity
Upright position from supine (no moving muscles)
venous volume below & above heart, Pooling, Arteries, Veins, gravity
Venous volume below heart increases
Venous volume above heart decreases
Extreme venous pooling in legs and feet
Arteries remain same shape (high pressure low compliance)
Veins (high compliance low pressure) gravity pulls blood downward causes pooling
Veins at bottom collecting too much blood expanded and not getting the blood back to top.
What counteract venous pooling?
Venous valves
Tone of surrounding tissue (skeletal muscle)
To prevent pooling in veins
Venous valves
Tone of surrounding tissue
No valves in veins causes
Pooling due to gravity
Venous valves
(unidirectional flow) in medium veins towards heart
- Even distribution in each section
- Prevents backflow
Tone of surrounding tissue
Skeletal muscle supports veins to prevent too much blood collecting at bottom
Puts pressure on the vein to keep its shape and prevent pooling
Poor muscle tone
Elderly Prone to fainting Veins not supported Pooling Doesn’t get to heart Poor circulation Blood doesn’t get to brain
What increases venous return to the heart?
Skeletal muscle contractions
Skeletal muscle contraction
Squeeze veins and pushes blood upwards through valves, valves prevent going downwards
Increase blood flow in veins back towards the heart
Diaphragm
Large muscle
Rises and falls as we breathe
Intercostal muscles
Between ribs
Constrict and expand as we breathe
Supports veins
More that we breathe the more the diaphragm goes down and the more the intercostal muscles expand and contract and the
veins as we breathe are being squeezed on and is pushing blood up towards the heart
Exercising
Using muscles
Squeezing on veins
Breathing faster (diaphragm up and down)
Intercostal muscles in and out
Squeezing on veins in chest
Increases the amount of blood returning to the veins
Increased venous return means
increased stroke volume
Starling’s Law
The more stretched
muscle fibres are before a
contraction, the stronger
the contraction will be.
The more blood returned to the heart the more comes out for each beat
Pendent on the strength of the beat
Cellular mechanism of cardiac contraction
Actin and myosin filaments
Myosin heads grab on to actin and pull when calcium is released
Contracted sarcomere
Myosin heads grab on to actin and pull when calcium is released
To the last head last place where it can pull (defines strength of contraction, and amount of blood pushed out)
Maximum contraction
Space between the actin filament and last myosin head
What happens when we return more blood to the heart through the veins?
Heart expands to accommodate extra blood
Walls of expands with it
Sarcomeres stretched out
More space for myosin and actin to interact
Maximum contraction increases
Pushes more blood out of the system
Starling
More blood returned to the heart through the veins the more volume of blood pack into the heart before the contraction. Causes the walls to expand and gives a stronger contraction
More ventricular volume before the contraction (end of diastole)
The larger the contraction,
The larger the stroke volume (volume of blood that comes out of the heart with each heartbeat)
More blood return
More blood packed in
Bigger heart
Stronger contraction
More stroke volume
Supine
Veins aren’t working against gravity
Getting really good return to the heart through the veins
Tilted upwards
Not using muscles
Nothing to push the blood to the heart
Extra pooling due to gravity
Less return
Reduce return of blood
Smaller contraction
Diastolic volume of blood (before contraction) smaller
Smaller volume of blood before contraction
Smaller stroke volume during contraction
Starling’s law of the heart
the more the walls of the heart stretch prior to contraction,
the stronger a contraction and greater stroke volume
venous return directly affects
the stroke volume and performance of the heart.
