Venous circulation and posture Flashcards
1
Q
pressure
A
pressure is the force (N) applied perpendicularly to a surface per unit area (m^2) over which that force is distributed
1Pa = 1N/m^2
2
Q
same force one a different area gives
A
different pressure
3
Q
how are blood pressure values expressed
A
relative to atmospheric pressure
4
Q
what is atmospheric pressure
A
760mmHg
5
Q
what is blood pressure
A
100mgHg
6
Q
what vessels have large diamaters
A
- aorta
- elastic arteries
- muscular arteries
- vena cava
7
Q
what vessels have small diameters
A
- arterioles
- capillaries
- venules
- veins
8
Q
what vessels have small total cross sectional area
A
- elastic arteries
- muscular arteries
- vena cava
(few of them)
9
Q
what vessels have large total cross sectional area
A
- arterioles
-capillaries
-venules
-veins
(lots of them)
10
Q
which vessels blood pressure greatest
A
- elastic arteries
- muscular arteries
- arterioles
11
Q
which vessels in blood pressure lowest
A
- capillaries
- venules
- veins
12
Q
where is blood velocity greatest
A
- elastic arteries
- muscular arteries
- vena cava
13
Q
where is blood velocity low
A
- arterioles
- capillaries
- venules
- veins
14
Q
what determines blood vessel compliance
A
how easy it is to increase they cross sectional area
15
Q
which blood vessels are highly compliant
A
veins
they contain more than 60% total blood volume and are capacitance vessels
- they will decrease their capacity through vascular tone
16
Q
what are capacitance vessels
A
veins - they have the ability to store charge
and so can decrease their capacity through vascular tone because
17
Q
where do veins carry blood
A
back to the heart.
all valves in veins allow blood flow only toward to heart
18
Q
what blood vessels carry blood to the heart
A
veins
19
Q
resting muscle blood flow
A
only a few capillaries open at any one time
20
Q
exercising muscle blood flow
A
vasodilation - more capillaries open - higher blood velocity - double the fall in oxygen saturation Capillary recruitment contributes to reduces the diffusion distance so that the muscle can consume more than 40times the oxygen during exercise
21
Q
blood flow redistribution
A
CO increases with exercise intensity an blood flow is redistributed to provide a large proprtionof blood to skeletal muscle and some to skin for thermoregulation
22
Q
what must be maintained during blood reflow redistribution
A
Blood pressure
23
Q
hydrostatic pressure
A
drives flirtation out of the capillary, when pressure is greater in the tissue than in surrounding tissues
24
Q
osmotic pressure
A
drives absorption into tissues from surrounding tisses
25
how is blood volume increased after large blood loss
Osmotic pressure will drive absorption into capillaries from surrounding tissue
26
how much plasma enters capillaries daily
4000L
27
net daily filtration at capillaries
8l
28
where does blood filtration occur
capillaries
29
what happens to 8L of net filtration
returned to the vascular compartment through lymphatic system
30
how much blood enters capillaries per day
7200L
31
what happens to plasma volume during exercise
whole body plasma can decrease by 16-20%
- active muscles gain fluid during excercise
- muscles swell by 20% within 15 minutes of exercise
32
orthostasis
standing p
33
standard blood pressure
120/80 mmHG
34
standard blood pressure measured at leg
220/160 mmHG
35
mercury compared to water
mercury is 13.6 times denser than water
36
why is BP greater at leg than arm when standing
weight of the column of blood increases arterial blood pressure
37
filtration during orthostasis
40 mites of standing can induce a reduction in plasma volume by 6-12 %
- increased blood pressure
- causes increased filtration
= feet swelling if standing or sitting for prolonged period of time
38
how to reduce swelling in feet when sitting for prolonged time
compression socks, which reduce the pressure gradient between capillaries and surrounding tissues
39
myogenic regulation
local compensatory response to changes in blood pressure
40
how does a rise in BP effect myogenic regulation
contraction of vessels because there is an excessive oxygen delivery from raised BP
41
how does a fall in BP effect myogenic regulation
dilation of vessels because there is limited supply of oxygen from fall in BP
42
what ensures the brain recieves the correct amount of oxygen
passive redistribution
| ensures the brain does not receive excessive or insufficient oxygen, despite changes in BP
43
BP studies on rabbits, cats and dogs
```
nerve excitation causes:
- rapid increase in BP
- rapid increase in blood volume
when excitation stops
- BP returns to pre sitmaultion value
- leg volume returns to pre stimulation value and transiently goes greater than pre value
```
44
does myogenic regulation of BP require CNS
no, it is a local mechanism and can be performed in muscle post mortem therefore CNS not needed
45
increased BP on post mortem blood vessel
- increased BP causes transient increase in diameter
- maintained high BP and diameter will contract for during of increased prssure
this keeps flow relatively un altered
46
what is the skeletal muscle pump
local compensatory response to increased blood pressure
- contracting calf muscles help blood flow back to the heart
- lowers venous foot pressure by 20-30mmHg
The calf muscle acts as pump for deep leg veins
- when calf muscle relaxed, blood flows and toward feet but stopped by valves
when calf contracts, blood is forced back towards heart and pushes valves open to do so
47
how does postural change affect blood volume
- big changes in veins and arterioles
- standing causes blood pressure to rise in arterioles and veins from pooling of blood at feet
- pooling of blood in lower extremities results in orthostatic pressure changes
BUT the pressure gradient between arterioles and veins remains the same = 86mmHg
- blood flow is not affected by gravity
48
what is the pressure gradient between arterioles and veins
86mmHg
49
```
pressure when standing:
right atrium
aorta
veins at feet
arterioles at feet
```
```
pressure when standing:
right atrium 4mmHg
aorta 100mgHf
veins at feet 100mgHg
arterioles at feet 186mmHg
```
50
```
pressure when supine:
right atrium
aorta
veins at feet
arterioles at feet
```
```
pressure when supine:
right atrium 4mmHg
aorta 100mmHg
veins at feet 10mmHg
arterioles at feet 96mmHg
```
51
cardio vascular response to orthostasis
- increase in HR
- decrease in central venous pressure
- decrease in cardiac end diastolic volume
- decrease in stroke volume and cardiac output
- decrease in pulse pressure
- increase in total peripheral resistance
52
central venous pressure relates to what volume
intra-thoracic blood volume
53
intra-thoracic blood volume relates to what pressure
central venous pressure
54
what reflex sensors detect BP changes
aortic arch and carotid sinus baroreceptors discharge with each beat of the heart and sense changes in BP
55
reflex response to fall in BP
- aortic arch and carotid sinus baroreceptors have a reduced rate of AP caused by fall in BP
- this induces increase in sympathetic discharge
- and decrease in parasympathetic discharge
=
- increased HR
- arteriolar vasoconstriction in skeletal muscle and kidney
- venous vasoconstriction
56
how do parasympathetic and sympathetic discharges change in response to drop in BP
- sympathetic increase
| - parasympathetic decrease
57
autonomous nervous system response to orthostasis
- mean arterial BP transiently decreases but restored by reflexes short term
= transient hypotension which reduces cerebral perfusion causing dizziness
- vagus nerve mediates lowering of HR via stimulation of SA node
= reduction of parasympathetic vagal outflow to reduce HR
- increased outflow of sympathetic nerves increase HR and contractility and determine vasoconstriction
- coordianted increase in HR, TPR and MABP limit the fall of cardiac output to ~20%
= limited variability of cerebral perfusion during postural changes
58
how does reduced parasympathetic vagal outflow effect HR
increases HR. occurs in response to standing to prevent fainting
59
how does increased sympathetic nerve output effect CV
increases HR, contractility and determines vasoconstriction to increase TPR
60
how is fall in cardiac output limited during orthostasis
coordinated increase in HR, TPR and MABP
61
how much does CO fall during orthostasis
~20%
62
what is the overall effect of local and reflex responses to postural changes
limits variability of cerebral perfusion
63
difference in ventricles and arterioles of foetus and adult giraffe
both are hypertrophied in adult compared to foetus
64
what is fainting
sudden, transient loss of consciousness
| observed when cerebral blood flow decreases to ~less than half normal value bc of decreased arterial pressure
65
what causes decrease in cerebral blood flow
decreased arterial pressure
66
what causes fainting
- blood volume redistribution during orthostasis
- blood volume reduction, hypovolaemia
- psychological stress
- combination of the above
67
what are the initial CV effects of stress
induces alarm response:
- tachycardia
- muscle vasodialtion
- cutaneous vasoconstriction
- hyperventilation
68
what follows the alarm response to stress
```
increased vagal outflow which causes:
- bradycardia
-drop in peripheral resistance
- drop in arterial BP
- drop in cerebral perfusion
= loss of consciousness
= vaso-vagal episode
```
69
what restores consciousness
supine position - hence naturally occurs when you faint
70
vaso-vagal episode
temporary fall in BP related to fainting
