6) Control of blood flow

Question 1

What is conductance?

Answer 1

• It is how well a vessel conducts flow.

- It is the opposite of resistance and is calculated by 1/ Total Peripheral Resistance

Question 2

What is the equation for Darcy’s law using conductance rather than TPR?

Answer 2

• Flow = Pressure difference x Conductance

Question 3

How does vasoconstriction and vasodilation affect blood flow and blood pressure?

Answer 3

• During vasoconstriction the total peripheral resistance increases. As a result the blood pressure upstream (behind the vasoconstriction site) is higher and there is a decreased flow down stream.
• During vasodilation the total peripheral resistance decreases. As a result the blood pressure upstream (behind the vasodilation site) is lower and there is an increased flow down stream.

Question 4

What is hypertension?

Answer 4

• The over constriction of arterioles. It leads to higher arterial BP but less capillary flow (called under perfusion)

Question 5

Explain the changes in blood flow in response to changes in needs.

Answer 5

• When sedentary (resting) superior mesenteric arteries are dilated to improve blood flow to the intestines (for improved digestion) and common iliac arteries (in the leg) are constricted reducing blood flow to the legs.
• When exercising superior mesenteric arteries are constricted to decrease blood flow to the intestines and common iliac arteries are dilated to increase blood flow to the legs

Question 6

What is Poiseuille’s law?

Answer 6

• It describes parameters that govern Total Peripheral Resistance (TPR)
• TPR is controlled by radius, pressure difference across vessels, length of blood vessels and blood viscosity
• If we increase blood viscosity and vessel length then resistance increases
• If we increase the radius of blood vessel we decrease resistance

Question 7

Which factor has the largest effect on blood flow and how is this change brought about?

Answer 7

• A small change in the radius has a massive change on blood flow.
• Changes in the radius of blood vessels is brought about through sympathetic activity of vasoconstriction and vasodilation

Question 8

In which blood vessel do we find the largest pressure drop?

Answer 8

• Arterioles

Question 9

How is the radius of the arterioles controlled?

Answer 9

• Through sympathetic nerves providing constant tone of dilation vs constriction

Question 10

Which vessel is TPR not controlled by?

Answer 10

• Capillaries.
• There is low pressure drop due to less resistance to blood flow in capillaries
• Radius remains unaltered as there are no smooth muscles and no sympathetic innervation
• Individual capillaries remain very short
• Less resistance in capillaries due to bolus flow which reduces viscosity

Question 11

How is local blood flow to individual tissues/organs controlled?

Answer 11

• It is controlled by changes in radius of arterioles supplying the tissue/organ.

Question 12

What are the two control mechanisms of arteriole radius?

Answer 12

• Intrinsic factors: Factors found within the organ or tissue (allows response to local factors)
• Extrinsic factors: Factors found outside the organ or tissue (nervous or hormonal control of blood vessels)

Question 13

What is the Bayliss myogenic response?

Answer 13

• Increasing distension (swelling due to increased pressure) of vessels makes them constrict and decreasing distension within vessels makes them dilate.
• This is because increasing distension means the vessels are more stretched causing ion channels to open, which then depolarize, leading to smooth muscle contraction
• Due to the linear relationship between pressure and blood flow a large difference in pressure would cause a large difference in blood flow.
• The Bayliss myogenic response maintains blood flow at the same level despite changing arterial pressures.

Question 14

What is viscosity?

Answer 14

• A measure of the internal friction between the fluid and the inside of the walls of the vessel

Question 15

What affects blood viscosity?

Answer 15

• Haematocrit (percentage of blood that is made of RBCs): Increased Haematocrit increases viscosity. This increases TPR and BP and decreases flow. Decreasing Haematocrit decreases viscosity. This reduces TRP and BP and increases flow and heart rate
• Tube diameter: Blood viscosity is reduced in narrow tubes as cells move to the centre of the tube to reduce friction. We find decreased resistance and increased flow in microvessels
• Red cell deformability: Normally cells are able to deform in order to flow efficiently through vessels. However in some cases (e.g. sickle cell) the RBCs loose their deformability which increases viscosity and reduces flow.
• Velocity of blood: Slow venous flow (e.g. in immobile legs) increases viscosity due to partial clotting

Question 16

Why are veins blood reservoirs?

Answer 16

• They contain the majority of blood in the body.
• This is because they contain a thinner layer of smooth muscles (compared to arteries) which means they are more compliant (can stretch better) and contract less.

Question 17

How does a contraction of smooth muscles in the veins increase stroke volume?

Answer 17

• Contraction of smooth muscles in the veins expels more blood form the veins into the heart.
• This causes the heart to stretch more than usual and, due to Starling’s law, it will have a higher force of contraction
• This means more blood is expelled out of the heart leading to a higher stroke volume

Question 18

What causes veins to increase preload?

Answer 18

• Normally venous pressure is higher at the lower parts of our bodies. This pressure allows the veins to return blood to the heart.
• However stimulation of sympathetic nerves causes venoconstriction and shifts the blood centrally.
• This increases venous pressure, central venous pressure (CVP) and end-diastolic pressure.
• Increased CVP increases preload and so increases stroke volume (Starling’s law)

Question 19

How does Bernoulli’s theory explain arterial blood flow?

Answer 19

• When standing there is a pressure gradient against the flow from the heart to the feet.
• This is because pressure is higher in the feet and lower in the heart.
• However blood is ejected from the heart with greater kinetic energy than that at the feet as it has more velocity.
• Furthermore it also has greater potential energy than the feet as it has a greater height
• The greater potential and kinetic energies overcome the pressure gradient and maintains blood flow to the feet.
• Blood flow to the feet can be easily compromised

Question 20

Explain the different methods of venous return.

Answer 20

• Cardiac output: The circulation is a closed system so the heart pushes blood through the cardiovascular system through the arterial side of the capillary bed into the venule side in the direction of the heart.
• Breathing: When inhaling the intra-abdominal pressure increases as the diaphragm moves downwards. This causes venous valves (in the pelvic veins) to close and the blood moves up the thorax. When exhaling the intra-abdominal pressure decreases and the pelvic veins and inferior vena cava refills.
• Muscle pump: There is a deep venous system embedded in muscles. Every muscle contraction squeezes the veins which pushes a column of blood towards the heart. When the muscles relax the valves close and prevent the retrograde (back) flow of blood.
• Venous tone: Blood in the veins exert pressure on the walls of the veins which generates tension and maintains pressure. Furthermore, sympathetic signals can cause vasoconstriction and can mobilise blood back to the heart.