Venous circulation and posture

Question 1

Why are veins called capacitance vessels?

Answer 1

Store up tp 60% of blood. Have a high capacticance - can fill and distend.

Question 2

Explain the anatomy of large veins.

Answer 2

Thin walls, large lumen, valves (endothelium infoldings), sympathetically innervated smooth muscle.
Characterised by size and location - not function.

Question 3

Compliance =

Answer 3

Distensibility x vessel volume.

Question 4

Can veins fill and empty with very little changes in internal pressure?

Answer 4

Yes.

Question 5

What is venous capacitance in the presence of sympathetic outflow?

Answer 5

Venoconstriction. 3/4 of the volume could be displaced to the heart.

Question 6

What is delayed complience?

Answer 6

When volumes of blood are added to/subtracted from a blood vessel…initial rise/fall in pressure followed by a compensation.

Question 7

What is venous return aided by?

Answer 7

1) Skeletal muscle pump
2) Thoracic muscle pump - decrease intrathoracic pressure
3) Fall in atrial pressure during systole.

Question 8

Upright posture?

Answer 8

Odedma od feet/ankles - lympatics are overloaded…fainting.

Question 9

Lying down?

Answer 9

Good! Mean arterial pressure is approx 100mmhg in all artieries. Venous pressures are all low.

Question 10

What is the conversion factor for the ‘weight’ of the coloum of blood?

Answer 10

0.78

Question 11

What is the effect of the ‘weight’ of the column of blood?

Answer 11

1) Adds to pressure generated by heart in vessels below the heart.
2) Subtracts from pressure generated by heart in vessels above it.
3) Effect is approx 0.78mmHG/cm
4) If heart is approx 115cm above feet - static pressure in feet is approx 90mmHg lighter than heart.

Question 12

What stops the feet from swelling?

Answer 12

Skeletal muscle pumps, and arteriolar constrictions reduces foot blood flow.

Caused by

1) reflec sympathetic vasoconstriction via baroreceptors
2) Local sympathetic axon reflexes
3) Autoregulation

Question 13

Varicose veins?

Answer 13

Incompetent valves impair skeletal muscle pumping - foot and ankle oedema.

Question 14

Baroreceptor reflex?

Answer 14

Carotid sinu and aortic arch….increase firing, increase sympathetic drive to increase BP to normal.

Question 15

What happens when you stand still?

Answer 15

300-500ml of blood accumulates in veins of legs

• decrease return and decrease CVP
• decrease CO
• decrease BP usually transient due to reflex response.

Question 16

Explain feedforward response to orthostatic challenge.

Answer 16

Anticipated fall in CO that will occur on standing - otilith organis - increase symp discharge - increase tpr, increase BP.

Question 17

What happens in the 60 seconds following orthostasis.

Answer 17

Hr increase by 15-20 bpm.
SV and pulse pressure remain depressed.
CO falls (~20)
TPR increase  by 30-40%
MAP restored - settles 4-10mmHG above resting,

Question 18

What happens in the 30 minutes following orthostasis?

Answer 18

Increase capillary filtration - net fluid loss
Decrease SV - tachycardia
RAAS is activated…decrease Na and H20 excretion…vasoconstriction from AngII and vasopressin.

Question 19

Prolonged standing - postural hypotension. What happens?

Answer 19

Progressive venous pooling (no muscle pump)
Progressive fall in SV and pulse pressure
Progressive rise in TPR and HR…

…eventually MAP decreases- especially if warm/dehydrated.

Sudden fall in both TRP and HR, steep fall in BP and BF = SYNCOPE (faint)…vasovagal syncope….makes your horizontal and venous return is restored.