Co-ordinated CV responses 1

Question 1

What are the mean pressure in the arteries of the head and feet when supine?

Answer 1

They are similar and slightly lower than MAP due to a slight loss of pressure from vascular resistance.

Around 90mmHg

Question 2

What are the mean pressure in the veins of the head and feet when supine?

Answer 2

They are similar and slightly lower than MAP due to a slight loss of pressure from vascular resistance.

Around 10-15mmHg

Question 3

What do the mean pressures in the feet rise to when going from supine to standing?

What about the arterial and venous pressures respectively?

Answer 3

They increase to around a mean of 90mmHg

The arterial increases to around 190mmHg

The venous increases to around 100mmHg

Question 4

What do the mean arterial and venous pressures in the feet decrease to when going from supine to standing?

Answer 4

The arterial decreases to around 60mmHg

The venous decreases to around 0mmHg (outside of cranium)

Question 5

In the feet when standing, what happens to the:

1) flow
2) hydrostatic pressures and oncotic pressures

Answer 5

1) flow remains the same as both the arterial and venous pressures increase by the same ratio meaning the gradient across the capillary bed is unchanged
2) the hydrostatic pressure increases and the oncotic pressure remains the same, which causes increased filtration and therefore oedema

Question 6

What orthodontic effect occurs with cardiac output when we stand?

What does this do to our blood pressure?

Answer 6

As the veins are distensible, when we stand our vein valves shut and for a short time the blood pools in the feet (300-500ml) decreasing venous return.

The C.O. therefore exceeds venous return therefore the frank-starling law will kick in and cardiac output also decreases meaning blood pressure will overall decrease.

Question 7

What central reflex takes place to overcome orthostasis when we stand?

Answer 7

The baroreceptor reflex is triggered which activates the SNS and inhibits the PNS, causing increases in HR and peripheral vasoconstriction which increases BP but can not fully restore it.

Question 8

Which receptors are activates during orthostasis to trigger the baroreceptor reflex?

Answer 8

The baroreceptors (respond to stretch)

The cardiopulmonary receptors (respond to volume)

Question 9

What local effects takes place to overcome orthostasis when we stand?

Answer 9

The veno-arteriolar axon reflex

Microcirculatory vein distension caused by the increase in pressure cause arteriolar vasoconstriction via local sympathetic axons to reduce flow and therefore pressure and oedema.

Question 10

What takes place in the veins to overcome orthostasis when we stand?

Answer 10

The skeletal muscle pump works to lower venous pressure and increase venous return.

Question 11

What stops veins inside the cranium from collapsing during orthostasis?

Answer 11

CSF is displaced downwards causing a negative intercranial pressure. This allows the veins to remain open but gives them a negative pressure.

Question 12

How much does cerebral blood flow decrease by during orthostasis?

Answer 12

Around 20%

Question 13

Why does cerebral blood flow fall when we stand up?

Answer 13

This is because the pressure gradient across the arterial-venous system decreases due to the arterial pressure decreasing more than the venous.

Arterial goes from 90mmHg to 60mmHg

Venous goes from around 10mmHg to 0mmHg

Question 14

What happens if prolong standing occurs and postural hypertension occurs?

Answer 14

A vasovagal attack happens.

Vaso refers to vasodilation

Vaal refers to vagally stimulated bradycardia

Question 15

What equation is used to calculate oxygen consumption?

Answer 15

O2 consumption = CO x (arterial - venous O2 content)

Question 16

What happens (net) to the following during exercise?

1) CO
2) SV
3) HR
4) BP
5) TPR
6) O2 consumption

Answer 16

1) CO = increases
2) SV = increases
3) HR = increases
4) BP = increases
5) TPR = decreases!!
6) O2 consumption = increases, arterial = same, venous = lowers