Vascular control

Question 1

Define vascular control

Answer 1

The body’s ability to selectively change blood flow to organs and tissues according to their metabolic needs.

Question 2

What drives fluid flow in a tube?

Answer 2

Pressure differences at the two ends of the tube.

Flow = P1 - P2 = dP

Question 3

What factors resist fluid flow?

Answer 3

fluid viscosity
length of the tube
radius of the tube

Question 4

What is the Poiseuille equation?

Answer 4

It is used to approximate blood flow through a vessel.

F = dP/R = (dP * r^4)/(L * V)

Where:
dP = P1 - P2
r = radius 
L = length 
V = viscosity

Question 5

Assuming that blood viscosity and vessel length do not change under normal, healthy conditions, what factor has the biggest influence on blood flow?

Answer 5

Vessel diameter

Question 6

What drives pulmonary blood flow?

Answer 6

The pressure difference in the pulmonary artery (aka pulmonary trunk) and the left atrium.

Question 7

What drives systemic blood flow?

Answer 7

The pressure difference between the aorta and and in the vena cava.

Question 8

What is systole?

Answer 8

The contraction of the left ventricle (heartbeat) which sends blood out into the body.

Question 9

What is diastole?

Answer 9

The relaxation of the heart in between beats when it refills with blood.

Question 10

What keeps diastolic blood pressure from dropping to 0?

Answer 10

The elasticity of blood vessels (especially large arteries) acts as a pressure reservoir which forces blood to flow between heart beats.

Question 11

What are the four kinds of arterial blood pressures?

Answer 11

Pulse pressure
Systolic pressure
Diastolic pressure
Mean arterial pressure (MAP)

Question 12

How are pulse pressure and MAP calculated?

Answer 12

Pulse pressure = systolic - diastolic

MAP = Diastolic + (pulse pressure/3)

Question 13

What are normal arterial blood pressure values?

Answer 13

Systolic = 120
Diastolic = 80
Pulse pressure = 30
MAP = 93

Question 14

What is the significance of MAP?

Answer 14

Since the blood pressure in the vena cava is essentially 0:
dP = P-aorta - P-VC = MAP - 0 = MAP

MAP represents the pressure that drives the system.

Question 15

What is the equation for systemic blood flow?

Answer 15

Systemic Blood Flow = (MAP)/(Peripheral resistance)

Question 16

What is the site of greatest vascular resistance?

Answer 16

Arteriole beds

Capillary beds have lower resistance because there are more capillaries running in parallel. Think of parallel resistors.

Question 17

What is cardiac output?

Answer 17

Cardiac output = Flow = Stroke volume * Heart rate

Question 18

Define stroke volume. What factors influence it?

Answer 18

Stroke volume = volume of blood pumped per heartbeat.

It is influenced by ventricular contractility and venous return.

Question 19

Define ventricular contractility.

Answer 19

Ventricular contractility is the strength with which a ventricle contracts during systole. It influences stroke volume.

Extra info:
It is affected by:
1. Ventricular muscle length which is determined by the amount of blood in the ventricle prior to contraction. More blood = more stretch = stronger contraction
2. Ventricular myocontractility is the ability of a cardiac myocyte to contract at any given length.

Question 20

Define venous return.

Answer 20

Venous return is the rate of blood flow from the “venous reservoir” back to the heart. It influences stroke volume. It is influenced by the skeletal muscle pump, the respiratory pump, and sympathetic innervation of veins.

Question 21

What is the skeletal muscle pump?

Answer 21

Muscle contractions, especially in the legs and abdomen, squeeze the veins and can help get blood back to the heart. This does not work if a person is standing or sitting still.

Question 22

What is the respiratory pump?

Answer 22

During inspiration, the movement of the diaphragm decreases pressure in the thorax. This causes the vena cava to swell, increasing blood flow into the vena cava. Inspiration also increases the pressure within the abdomen, forcing blood out of abdominal veins and into the vena cava.

Question 23

How does sympathetic innervation contribute to venous return?

Answer 23

Sympathetic stimulation can increase vasoconstriction, “squeezing” blood out of veins and into the heart.

Question 24

How do we define resistance to flow under normal, healthy conditions?

Answer 24

Since viscosity and vessel length are regarded as stable factors in a normal healthy person, the only factor that is considered is vessel radius. Thus:

Resistance = 1/(r^4)

Question 25

What effect does Ca2+ have on vascular smooth muscle?

Answer 25

It leads to muscle contraction which leads to vasoconstriction.

Question 26

What effect does cAMP have on vascular smooth muscle?

Answer 26

It leads to muscle relaxation which causes vasodilation.

Question 27

Define vascular tone.

Answer 27

The degree to which a blood vessel is constricted relative to its maximally dilated state. Under resting conditions, it differs in varying organs.

Question 28

What does high vascular tone mean? In what organs is vascular tone high?

Answer 28

High vascular tone means that a vessel can dilate considerably.
Vascular tone is high in organs that have large variations in oxygen demand - myocardium, skeletal muscles, skin, etc.

Question 29

What does low vascular tone mean? In what organs is vascular tone low?

Answer 29

Low vascular tone means that a vessel cannot dilate very much.
Vascular tone is low in organs that have a fairly constant oxygen demand such as the brain or kidneys.

Question 30

What extrinsic factors affect vascular tone?

Answer 30

Hormones and sympathetic innervation

Question 31

What hormones cause vasodilation?

Answer 31

Epinephrine at LOW TO MODERATE levels

Atrial natriuretic peptide.

Question 32

What hormones cause vasoconstriction?

Answer 32

Epinephrine at HIGH levels
Vasopressin
Angiotensin II