Haemodynamics: Arterioles and Veins Flashcards
How does TPR affect BP and flow?
What controls TPR?
- An increase in resistance means that pressure needs to be increased to maintain the same blood flow:
BP = CO x TPR - Poiseuille’s Law, Myogenic response, Blood viscosity
How does vasodilation and vasoconstriction affect TPR and blood flow at the same CO?
LOOK AT DIAGRAM!
- Vasodilation = ↓TPR, so BP decreases to maintain blood flow.
Vasoconstriction = ↑TPR, so BP increases to maintain blood flow.
What is Poiseuille’s Law?
What is the radius to the power of in the equation? What does this suggest?
- Describes the factors that affect TPR, which include Vessel radius, Blood viscosity, and Vessel length.
- 4, suggests that small changes in radius will hugely affect blood flow. LOOK AT PICTURE!
Why do arterioles determine TPR and not capillaries?
Why is there less resistance in capillaries?
- • For the radius, capillaries have no sympathetic innervation or smooth muscle, so their radius can’t be changed
- They have a smaller pressure drop than arterioles - less resistance
- They have a shorter length, individually, than arterioles
- The bolus flow (plasma trapped between RBCs) reducing blood viscosity = low resistance
What are the types of control mechanisms for arteriole radius?
Give an example of an intrinsic control mechanism?
- Intrinsic or Extrinsic
- Bayliss Myogenic Response
How does the Myogenic response work?
Why is this response a protective mechanism?
LOOK AT GRAPH!
- ↑Distension of vessel → Constriction
↓Distension of vessel → Dilation - It maintains a local blood flow during changes in local BP.
If BP drops, there’ll still be a good blood flow, and if BP is high, there’ll be less flow to AVOID DAMAGE!
What is blood viscosity?
How does it regulate blood flow?
What does it depend on?
- A measure of the internal friction that opposes the separation of the lamina
- ↑Viscosity = ↓Blood flow - more resistance
- Blood velocity, Vessel diameter, Haematocrit (RBC volume : Total blood volume)
Describe the function of veins
Describe the structure of veins
How is the volume of venous blood changed?
- Low pressure blood reservoir (60%)
- Thin-walled, collapsible, compliant, and contractile - have smooth muscle with sympathetic innervation to control its radius
- Contraction expels blood into central veins = ↑CVP/EDV/venous return = ↑SV (via Starling’s)
What is the affect of venous pressure on volume during relaxed state and upon sympathetic stimulation?
LOOK AT GRAPH!
Relaxed:
↑Pressure in venous system = ↑Vein radius - lots more blood can be stored
Sympathetic stimulation:
Causes VENOconstriction = blood pushed towards heart = ↑CVP/EDV/venous return = ↑SV (via Starling’s)
What are the 3 ways that blood is returned to the heart?
Why can standing still for a long time cause someone to faint?
- Pressure gradient:
Pressure in the RA is much lower than that in the veins, so blood moves towards it.
- Pressure gradient:
- Thoracic pump:
Inhalation = ↑Thoracic volume = ↑Abdominal pressure, which forces blood upwards towards heart. - Skeletal muscle pump:
Contraction of leg muscles forces blood towards heart. Retrograde flow is prevented by the valves in veins.
- Due to Gravity, Heat-induced vasodilation, LACK OF MUSCLE USE!
How does Bernoulli’s Law show how blood flow can occur with poor pressure gradients?
Flow isn’t only determined by pressure. Bernoulli’s Law states that the energy of flow is determined by pressure, kinetics, AND potential energies.
When standing, compared to the feet, kinetic energy is greater at the heart due to ejection, as well as potential energy due to it being higher up:
This kinetic and potential energy overcome the pressure gradient to maintain flow.
