Acceleration, Deceleration and microgravity

Question 1

What mechanisms prevent venous pooling under normal circumstances given existence of gravity?

Answer 1

• Baroreceptors (Low BP = increase HR + CO

- Measure stretch, function of pressure, function of MAP, function of CO

Question 2

Where are baroreceptors? What types?

Answer 2

Carotid sinus: Measures changes out between 60-180mmHg

Aortic arch: less sensitive, higher threshold

Question 3

Mode of Action Baroreceptors:

Answer 3

Blood pressure decrease -> Baroreceptors innervating Medulla oblongata (Decrease vagal tone, increase sympathetic innervation) -> Increased TPR and Increased CO (Decrease parasymp only affects CO) -> increase blood pressure

Question 4

Physiology of blackout

Answer 4

Blood away from head = loss of vision. Decreased BF to head -> eyes outside of braincase = decreased BP -> critical closing pressure of eye BVs

Question 5

Physiology of Red

Answer 5

Blood pooling in head.

Question 6

Longest stay in space?

Answer 6

438 days - Valeri Polyakov

Question 7

Equation to determine difference in pressure pressure in column of fluid

Answer 7

Where,
• P is expressed as cm H2O; h is the distance(cm) measured in the direction of the acceleration force,
•r is the density of the fluid(g/cm3); C is the acceleration force acting upon the column(a multiple of the
normal gravitational force).

Question 8

Grey out symptoms

Answer 8

Loss of ability to distinguish colour

Question 9

Grey out physiology

Answer 9

Lack of BF to eye, specifically fovea containing majority of cones

Question 10

Blackout symptoms

Answer 10

Loss of vision but not consciousness

Question 11

Blackout physiology

Answer 11

Lack BF to head and eyes, critical closing pressure reached

Question 12

Describe Critical closing pressure (Law?)

Answer 12

Laclasses Law - Tension in wall is a function of the pressure and resistance to Blood flow.

Question 13

Laclasses Law

Answer 13

T = pr. 
T = tension in blood vessel wall, p = pressure across wall, r = radius of blood vessel

Question 14

How is the brain vascular bed better preserved than eyes in regards to perfusion during headwise acceleration?

Answer 14

The brain is surrounded by an indistensible skull and its centre is filled with CSF.
In the brain as in the eye, the arteriovenous difference in pressure is preserved because the venous pressure falls as much as the arterial pressure.
If brain perfusion does decrease, autoregulation of vascular tone occurs (hyperaemia).

Question 15

What is an immediate consequence of headwise acceleration and why?

Answer 15

Decreased CO. Organs move away from head = decreased intrathoracic presssure. pooling of blood = decreased venous return.

Question 16

What changes occur to the air blood gradients in headwise acceleration?

Answer 16

Apex =less perfusion

Middle and Base have more

Question 17

What changes occur to the air blood gradients in feetwise acceleration?

Answer 17

Improves ventilation-perfusion matching, more perfusion at apex, better air flow at base

Question 18

Prevention of Decreased CO through valsalva manoeuvres, shouting and positive breathing, how dis work?

Answer 18

• Increase pressure of the thoracic cavity
• Uses muscles of expiration in order to make it harder for blood to leave the head.
• Valsalva: Exhalation against a closed airway

Question 19

Why are subjects of study in weightlessness tilted at 6 degrees with their head down

Answer 19

• To ensure that the carotid sinus is at the same level as the aortic arch
• Basically puts the pressure difference to zero to better imitate a microgravity environment

Question 20

Weightlessness: What is the underlying physiology that stimulates diuresis?

Answer 20

Blood w/out gravity increase in head, thorax neck = Increased pressure to low-pressure baroreceptors in left atrium and venous structures.-> Stimulates ANP -> ANP opposed RAAS. (Inhibits NA+ Reabsorption and increases glomerular filtration.

Question 21

What is initial effect of weightlessness on CO

Answer 21

Increases due to increased Venous return

Question 22

How does CVS adjust to microgravity

Answer 22

Decrease in SNS and increased PNS. Work performed by heart decreases.

Question 23

What occurs after >50 days of microgravity? (CVS)?

Answer 23

Degree of ventricular atrophy

Question 24

What neural inputs cant be replicated in waterbed studies which affect astronauts in space

Answer 24

• Input from skin, muscle and joint receptors → reflecting postural forces
• Input from otoliths → reflecting orientation of head in gravitational field

Question 25

What happens to muscles in microgravity environment?

Answer 25

Atrophy (can be determined by urea in pee). Counteracted by 2hrs of exercise/day

Question 26

What occurs to an individuals bones in microgravity environment?

Answer 26

Demineralisation and loss of Ca+

Question 27

Countermeasures to prevent bone loss in microgravity?

Answer 27

Synthetic calcitonin (Oesteoporosis medication). Resistance training, Increased calcium and vit D exposure.

Question 28

What are some risks asssociated with the prevention of bone loss?

Answer 28

Kidney stones, cardiac arrythmia.

Question 29

What is necessary when ‘returning home’ after microgravity?

Answer 29

• drink copiously, their kidneys retaining Na+ in order to reload blood volume
• circulating reticulocytes increases as previously low haemopoietic activity of bone marrow corrected