acceleration

acceleration

rate of change in velocity

directions of acceleration (list 6)

• +ve g (Gz) → towards feet

• -ve g (-Gz) → towards head

• Eyeballs in (Gx) → towards back

• Eyeballs out (-Gx) → towards front

• Lateral g (Gy) → towards left

• Lateral g (-Gy) → towards right

G-force

• measurement of acceleration used in aerospace

• expressed as a multiple of earth’s normal gravity

linear acceleration

change in straight line velocity

radial acceleration

change in velocity from tight turns

name the components

G = acceleration

v = velocity

g = gravitational acceleration

r = radius of turn

the impact of acceleration on blood flow

acceleration can create pressure differences from one part of your body to the next and move blood

if the pressure inside our blood vessels is negative enough (due to acceleration), they will:

collapse

fluid column (explain)

• Height of a fluid column is used to measure pressure

• Pressure is higher at the bottom of the column

• The pressure depends on height of the column and density of fluid

• AND acceleration from gravity

when you're standing, the pressure in the veins and arteries of your feet is (?) than of you’re head, due to the amount of blood from above pushing down 

higher

transmural pressure

change in pressure across the wall

driving pressure

pressure difference between the ends

describe how these two systems differ in terms of flow and pressure

• in B there is no flow acting upon the system

• in C there is flow acting upon the system

• therefore the pressure at the bottom is higher in C than in B

• however, the pressure difference between inlet and outlet is 100mmHg in C (despite pressure peaking at 130mmHg)

• this is because the driving pressure remains the same (transmural pressure has increased)

describe what is happening in this system in terms of pressure

• above the heart the pressure drops

• very low transmural pressure can make veins and arteries collapse

explain the following diagram

• blood flowing down into the capillaries of the legs and feet cause pooling

• increased transmural pressure → outward pressure on capillary walls increase → increases filtration

• causes leakage of plasma into the interstitial tissue → swelling of ankles and feet

• veins fill with blood and expand

• expansion of the veins decreases venous pressure and venous return (also due to reduced blood volume from leakage)

what happens when the pressure in the capillaries increase (colloidal osmotic pressure)

• If pressure in the capillaries increase

• The pressure at the arterial and venous end of the capillaries increases --> higher than the colloidal osmotic pressure

• Colloidal osmotic pressure pulls fluid back into the capillaries

 

• Blood will leave the capillaries and will not be pulled back in as it normally would

• Usually, at the arterial end the blood pressure is higher than at the venous end; colloidal osmotic pressure is lower at the arterial end and higher at the venous end

• Plasma that was pushed out into the extracellular fluid is drawn back in at the venous end (due to higher conc of plasma proteins in the blood vessel)

 

• When you stand up there's increased transmural pressure (from sudden increase in blood volume) and that result both in the veins expanding and filling with blood

• Increases filtration --> fluid leaves the capillaries and enters interstitial fluid

• Reduces blood volume in the veins

• Reduces venous return

• Causes blood pressure to fall

• Lowers cardiac output

baroreceptor response to standing up (explain)

1. standing up causes an increase in transmural pressure in the capillaries of the legs and feet

2. decreases venous return which decreases blood pressure

3. this change is detected by baroreceptors which will relay this message to control centres in the brain

4. sympathetic activity increases → increases heart rate, vaso-tone and veno-tone

5. this will increase blood pressure

orthostatic response to standing

1. Blood volume in legs and feet increase

2. Central blood volume decreases (as well as central venous pressure)

3. Stroke volume decrease

4. Cardiac output decrease

5. In response venous tone increases (increasing venoconstriction of veins)

6. Heart rate increases

7. Increase total peripheral resistance

8. Redirects blood toward the brain

what is meant by an increase in venous tone?

• Increases constriction of veins

• Helps to counteract the fall in cardiac output and stroke volume

• Helps reduce pooling of blood in the legs

what is total peripheral resistance?

blood pressure in peripheral blood circulation

orthostatic hypotension

• Fainting after standing up

• Mean arterial pressure abnormally falls on standing

• More common in patients with resting hypotension

• More common in hot weather due to skin vasodilation

• Reflex will override the baroceptor reflex

resting hypotension

fainting when sitting or lying down

doing tight turns at high velocity will greatly (?) the G-force

increase

negative G

• Centrifuging blood towards head

• Broken capillaries in eyes --> 'redout'

positive G

• Centrifuging blood towards feet

• Loss of blood flow to eyes --> 'blackout'

acceleration due to gravity is how many G’s?

1 G

during a 4 G turn, the pilot experiences a force (?) times gravity

4

negative G’s move blood to your:

head

you lose blood flow to the eyes before you lose blood flow to the brain which is why:

blackout occurs before loss of consciousness

what happens during acceleration down towards your feet?

• drop in blood pressure in vessels above the heart → collapse

• venous pooling occurs → also contributes to drop in pressure in vessels above the heart

• external pressure pushes arteries to close

jerk

rate of change in acceleration

a jerk of (?) G/s causes bone fractures

500

is positive G or negative G more dangerous?

negative G (towards head)

blackout occurs at about (?) G

4.8

loss of consciousness occurs by (?) G

5.4

G-suits

• Allow for high positive G maneuvers

• Inflate to compress lower body to stop blood flow to lower body → prevents venous pooling

• Increases total peripheral resistance and arterial pressure --> reduces blood flow to the lower body in the first place

summary

1. Driving pressure (pressure difference between inlet and outlet)

• Transmural pressure (pressure across blood wall)

 

2. Gravity produces hydrostatic pressures which will move blood

• When you stand up, blood moves towards your legs

• Venous pooling

• Veins expand

• Plasma leakage --> interstitial fluid

• Reduced venous return

• Reduced blood pressure

 

3. Baroreceptor reflex

• Normal response

• Failure of the reflex leads to orthostatic syncope

 

4. Aircrafts produce high G turns

• For prolonged periods of time

• Shorter periods of time --> rollercoaster

 

5. Positive G (blood flow towards feet) collapses cerebral arteries

• Causes blackout (loss of blood flow to eyes)

• Loss of consciousness

 

6. Safest way to take high Gs is towards your back

• Prevents blackout