Changes in posture

Question 1

What happens when a recumbent subject assumes an upright position?

Answer 1

Blood shifts from central blood volume reservoirs and other veins to large veins in dependent limbs (BP highest in the feet and lowest in the head)

Question 2

What will happen if there are no compensatory mechanisms for changes in pressure during orthostasis?

Answer 2

Arterial blood pressure would be lower, venous return would be lower and so would cardiac output

Question 3

What is right arterial pressure constantly maintained at?

Answer 3

2mmHg

Question 4

What are the major factors that reduce pooling and maintain right arterial pressure?

Answer 4

1) Non uniform initial sistribution of blood
2) Non uniform distensibility of vessels
3) Muscle pumps
4) Autonomic reflexes

Question 5

Describe how non-uniform initial distribution of blood acts to decrease pooling

Answer 5

In the cylinder, blood was distributed evenly throughout the body so the postural change in pressure was greater (less in the head)
However, in a recumbent human, most of the blood is in the large veins located in the central blood volume (vessels near the heart) - the majority of the 500ml of blood that pools in the legs comes from intrathoracic vascular compartments

Question 6

Describe how non-uniform distensibility of vessels acts to decrease pooling

Answer 6

If vessels are less distensible = standing causes a less dramatic shift of blood to dependent vessels
Can assume lower distensibility for leg veins because small vessels are far stiffer than larger ones, such as the aorta and vena cava
Can reduce distensibility further through regulated contraction of VSM (will be discussed further)

Question 7

Describe how muscle pumps act to decrease pooling

Answer 7

When a person stands, the muscles of the legs and abdomen tighten
The presence of valves, as well as intermittent muscular movement, contributes to the flow of blood upward along the veins
Vesse;s pf the abdominal region remain nearly unaffected by orthostasis because the abdominal viscera are contained in a water-filled jacket that is maintained by the tone of the abdominal muscles

Question 8

Describe how autonomic reflexes lead to decreased pooling

Answer 8

Decreased venous return → fall in right arterial blood pressure → decrease in stroke volume → decrease in arterial pressure → detected by baroreceptors → increased sympathetic output → raised vascular tone throughout the body and heart rate/contractility → restoration of systemic mean arterial pressure
(in dependent regions of the body, the sympathetic response also increases tone of the veins, thereby decreasing their diameter)

Question 9

What are two examples of variability of the orthostatic response?

Answer 9

Postural hypotension

Temperature effects

Question 10

Discuss postural hypotension

Answer 10

Sudden orthostatic tilt can cause such a large fall in arterial pressure that an individual becomes dizzy and faints - cerebral perfusion becomes inadequate

Question 11

Describe temperature effects

Answer 11

Cool environment = arterioles in the lower extremities are constricted = decrease in stroke volume is small because increased resistance delays the transfer of blood from the thoracic pool to the legs
Warm environment = dilated = orthostasis leads to faster transfer of blood from thoracic pool to legs → before the sympathetic response can develop, the decreases in stroke volume, mean arterial pressure and pulse pressure can be large