Blood Pressure and Flow

Question 1

What is meant by the terms compliance and resistance?

Answer 1

Compliance is the ability of an organ to distend in response to applied pressure
Resistance is the ability to oppose a change

Question 2

What are the properties of different sized arteries and how does this relate to their structure?

Answer 2

Large elastic arteries act as conduits/dampening vessels. Smaller arteries and arterioles have extensive smooth muscles to regulate resistance. Capillary walls are extremely thin for gas exchange. Veins are compliant and act as a reservoir for blood volume

Question 3

Describe the changes in pressure throughout the circulation

Answer 3

Aorta and arteries have the highest pressure - about 90mmHg in healthy resting individuals. As the blood travels through the circulation, pressure decreases due to viscous (frictional) losses. BP doesn’t fall much between the aorta and the large distributing arteries. When the blood reaches small arteries and arterioles, there’s a 50-70% drop in BP. In capillaries, the mean pressure can be 25-30 mmHg, and this falls further as the blood goes back into veins and to the heart

Question 4

Describe the pressure distribution in the circulation

Answer 4

The difference in pressure at two points is what drives blood flow, not absolute pressure. The pulse pressure (magnitude in oscillation of pressure) is dampened in smaller arteries and arterioles. The major site of resistance (greatest drop in blood pressure) is in small muscular arteries and arterioles. The pulmonary circulation shows similar pressure distribution but operates at lower pressures

Question 5

Why does blood flow?

Answer 5

The heart (acting as a pump) established a pressure in the aorta. This drives a steady flow Q through the circulation. Along the circulation, there are differences in pressure due to the viscous loss of pressure. This difference in pressure drives flow, at the end of circulation, the system empties into the RA (at almost atmospheric pressure). Stephen Hales demonstrated this with tubes in a horse

Question 6

Describe what is meant by laminar flow

Answer 6

Laminar flow means that each particle in the fluid follows a straight line with no interference. The velocity of the fluid is constant at any point. Blood usually exhibits this kind of flow.

Question 7

Describe what is meant by turbulent flow

Answer 7

Turbulent flow means that the velocity is not constant at any point - there are whirlpool regions in the fluid. They generally occur at branching points of blood vessels or as a result of vascular occlusions.

Question 8

What two forces influence endothelial cells lining the blood vessels?

Answer 8

Cylindrical strain - vessel wall distention due to an outwards (transmural) pressure
shear stress - adhesive force between vessel walls and the blood

Question 9

Describe what is meant by shear stress and shear rate

Answer 9

Shear stress is what usually damages blood vessels. If the fluid is flowing in concentric rings, the velocity of the fluid layers increases as the distance from the walls increases. Shear rate is the rate at which the fluid is worked during flow - rate at which fluid layers move past eachother.
Shear rate x viscosity = shear stress

Question 10

What are the main roles of the blood vessels?

Answer 10

Capillaries for exchange, veins and venules as blood reservoirs. Biggest players in exchange are also venules/veins and also arterioles. Biggest players in reservoir function are also aorta, pulmonary vessels, arterioles, and heart

Question 11

How does increasing smooth muscle contraction affect the veins/venules?

Answer 11

Venous compliance is 10-20x arterial compliance - it deforms easier under the same pressure and volume conditions. Increasing venous smooth muscle contraction increases constriction (vascular tone) and decreases compliance and volume of blood contained by the vessel

Question 12

What is vascular resistance?

Answer 12

The resistance that needs to be overcome in order to maintain blood flow

Question 13

What is resistance dependent on?

Answer 13

Fluid viscosity (n with long leg) eta - stays constant in most conditions
Length of tube (L) - Blood vessels don't change length
Inner radius of the tube (Variable, main determinant of vascular resistance)

Question 14

What is Poiseuille’s equation?

Answer 14

8 x L x eta / pi r^4. This means that small changes in vascular tone result in marked changes to resistance, and hence flow

Question 15

How is flow regulated physiologically?

Answer 15

By varying resistance by vascular tone and also cardiac output, while BP remains relatively constant. Mean BP = cardiac output x resistance. Cardiac output = stroke volume x HR

Question 16

How do smooth muscle fibres respond to tension in the vessel wall?

Answer 16

Increase in pressure = increased vascular tone (contraction of muscle fibres). This is most likely mediated by stretch sensitive ion channels - “myogenic theory” of vascular resistance regulation

Question 17

How does vascular injury contribute to vascular tone regulation?

Answer 17

Vessel wall damage leads to the secretion of serotonin which forms platelets which lead to vasoconstriction. Also, endothelial cells release vasorelaxants like nitric oxide and prostacyclin, and also vasoconstrictors like endothelins and thromboxane.

Question 18

How do circulating hormones affect vascular tone?

Answer 18

Kinins and atrial natriuretic peptide are relaxants, and ADH (Vasopression), noradrenaline and angiotensin II are vasoconstrictors

Question 19

Describe NS control of vascular tone

Answer 19

SNS uses noradr to increase HR and force and vasoconstrict most blood vessels - resulting in increased BP. blood away from non essential organs for fight or flight.
PSNS uses acetylcholine to decrease HR (lowering cardiac output) and hence lower BP.

Question 20

Describe how baroreceptors provide feedback

Answer 20

Baroreceptors are stretch receptors in the aortic arch and carotid sinues that send impulses to the cardiovascular centre for regulation of BP. The NS then alters the vascular tone, HR, etc. There are also low pressure baroreceptors in the walls of venae cavae and RA

Question 21

What is blood pressure? How is it measured?

Answer 21

Mean systemic bp = cardiac output x total peripheral resistance. The pressure exerted by blood on atrial walls. It’s recorded as two measurements, systolic (contraction) and diastolic (between heartbeats - no contraction). Usually 120/80 mmHg. Difference between systolic and diastolic is pulse pressure.

Question 22

How can segmental blood pressures be used as diagnostic devices?

Answer 22

Usually BP taken form left arm. Segmental BPs are taken along the extremeties and are used to evaluate occlusions or blockages in the blood vessels of limbs

Question 23

How does arterial compliance maintain steady blood flow and minimise pulse pressure over the cardiac cycle?

Answer 23

During ejection, the blood enters aorta and other elastic arteries faster than it leaves. About 40% of stroke volume is stored by elastic arteries. Closure of the aortic valve means the ejection ceases, but due to the elastic recoil of the artery, the blood pressure falls slowly and there is diastolic flow in the downstream circulation. This dampening effect is the Windkessel effect.

Question 24

How does age influence the Windkessel effect?

Answer 24

If arterial compliance decreases (as is common with age), the Windkessel effect is reduced and pulse pressure increases

Question 25

What is the cause of brain freeze?

Answer 25

“Trigeminal headache”. Something cold touching the roof of the mouth causes local vasoconstriction to minimise heat loss and then rapidly vasodilate. This is picked up by pain receptors which send the signal via the trigeminal nerve (responsible for facial pain) so it is interpreted as pain from the forehead

Question 26

How can moderate alcohol consumption lower blood pressure?

Answer 26

Alcohol inhibits the vasomotor centre in the medulla which is responsible for vasocontriction, so causing vasodilation. It also inhibits ADH (vasopressin) release, increasing urine outflow, decrease blood volume, decreasing BP

Question 27

How can greater than moderate alcohol consumption increase blood pressure?

Answer 27

Disturbance of CNS system - including increased corticotrophin releasing hormone (stress)
Baroreceptors have impaired sensitivity to BP changes
Overactivation of SNS
Heightened cortisol levels resulting in vasoconstriction
Upregulation of renin-angiotensin system (angiotensin causes vasoconstriction)
Inflammation and oxidative stress in blood vessels

Question 28

Why do we not faint when standing?

Answer 28

Activation of SNS to vasoconstrict, and also increase HR and force of contraction to maintain BP. Also myogenic vasoconstriction which is inherent in smooth muscle cells in response to transmural pressure.

Question 29

What effect does flexing leg muscles have on blood flow?

Answer 29

Contraction of the leg muscles squeezes the veins, opening the valves and helps blood to return to the heart - acts as a pump.

Question 30

What is the inspiratory pump? What effect does it have on blood flow and ?

Answer 30

A primitive gasp reflex that results in inspiration through a low level of resistance, lowering intrathoracic pressure. This draws air in, increases venous return to heart, increases stroke volume, and lowers intracranial pressure by improving cerebral perfusion due to lowered resistance to blood flow