Peripheral resistance and blood pressure/flow

Question 1

Poiseuille’s Law (1846

Answer 1

describes the factors that determine the flow through a tube in terms of pressure, flow and resistance
Flow (Q) is proportional to the pressure gradient (ΔP ) and inversely proportional to the resistance (R)

Flow (Q) is directly proportional to the pressure gradient (P ) vessel and the radius4 (r4) of the tube and inversely proportional to the length (L) of the vessel and to the viscosity (η) of the fluid.
Same law can be used to describe flow in a blood flow

Question 2

Flow is inversely proportional to the length and viscosity

Answer 2

Length (L)
Longer the tube = more resistance = less flow
remains constant and therefore does NOT control blood flow

Viscosity (η)
Higher the viscosity (i.e. treacle) = more resistance = less flow
Viscosity is related to the hematocrit (red blood cells/erythrocytes)
Dehydration
High altitude
Erythropoietin (EPO) – stimulate production RBC
Remains constant under physiological conditions and therefore does NOT control blood flow

Question 3

Radius of the vessel is the most important factor in determining resistance (flow)

Answer 3

Radius (r)
Caused by the friction of the fluid against the vessel wall
The effect of changes in the radius is raised to the power of 4
reducing radius by two fold from 2 to 1 reduces flow by sixteen fold (i.e. resistance increased sixteen fold)
Possible to regulate the radius of
blood vessels using smooth muscle

Question 4

Measuring Blood Pressure

Answer 4

direct measurement
-cannulate artery measure pressure with transducer
-not very accurate
-not very convenient

Question 5

What causes the pulse?

Answer 5

Pulse = vibration of the arteries caused by the ejection of the blood from the heart
(left ventricle) into the systemic circulation

Question 6

What is the pulse pressure and mean arterial pressure?

Answer 6

Pulse Pressure = difference between systolic and diastolic pressure (120 – 80 = 40 mmHg)
Mean arterial pressure (MAP) is the average pressure over the cycle
- since diastole is ~ twice as long as the systole
= diastolic pressure (DP) + 1/3 pulse pressure (PP)
= 80 + 1/3 (40) = 93mmHg
= commonly used measurement

Question 7

Blood pressure is influenced

Answer 7

by height , posture

P = h  g
P = pressure (mmHg)
h = height
(rho) = density
g = gravity

Heart = ‘zero pressure point’
Standing
Increased pressure of mean arterial blood pressure in legs (from 90 to 195 mmHg) leads to pooling of blood in the veins
feet swelling on flights
fainting in stationary soldiers
However, since arterial and venous blood are at the same height
Blood gradient P = constant = blood flow continues

Standing
Initial reduction in blood pressure to head can make you feel faint

Question 8

Hypertension is associated with increased mortality

Answer 8

myocardial infarction
(rupture of the coronary
blood vessels in the heart)
Stroke
(rupture of the blood
vessels in the heart)
Heart failure
Chronic kidney failure

Hypertension is treated by reducing cardiac output, blood volume and peripheral resistance

Question 9

Peripheral resistance is predominantly located in the

Answer 9

arteriole

Question 10

Blood pressure can be determined using the

Answer 10

Korotkoff sounds

Question 11

Peripheral resistance is predominantly determined by the

Answer 11

radius of the blood vessels