6.9 - Cardiorespiratory Mechanics Flashcards
(41 cards)
1
Q
What happens during the rapid ejection phase of the cardiac cycle?
A
- opening of aortic and pulmonary valves mark the start of this phase
- as ventricles contract, pressure within them exceeds pressure in aorta and pulmonary arteries
- SL valves open, blood pumped out and volumes of ventricles decrease
- no heart sounds for this phase
2
Q
What are the volume and pressure changes during inspiration and expiration?
A
- diaphragm contracts and is pulled down, reducing thoracic pressure below 0mmHg (ambient pressure) and creating a pressure gradient
- air flows down the pressure gradient (increases volume) - this increases pressure in lungs until it is 0 again (same as outside)
- inspiratory effort is removed and lungs are squeezed which increases pressure in lungs until pressure gradient between inside and outside is created in which air moves out of the lungs, decreasing volume in lungs
- pressure: normal, low, normal, high, normal
- volume: increases, peaks, decreases
3
Q
Describe how alveolar pressure changes during inspiration and expiration?
A
Alveolar pressure is same as the flow rate and pressure on the main graph (pressure and volume change)
4
Q
What are the limitations with snorkelling at great depths?
A
- the amount of dead space increases (the distance between alveoli and outside air) which we need to move oxygen through
- at 0.3m with 2.2cm diameter snorkel, the dead space is 1.21 x pi x 30cm = 114mL, which is 1/4 of resting tidal volume
- at 100m this increases to 38L of dead space which is 7x total lung capacity
5
Q
What is Poiseuille’s Law?
A
Resistance = (8nl) / (pi x r^4)
6
Q
How does resistance change with size and number of airways?
A
- resistance is inversely proportional to the fourth power of the radius of airways - as airways get smaller, resistance increases
- changes after the 4th generation - the amount and velocity of air going through the smaller pipes further down is much less than higher up
- higher up airways also have different structural support - cartilaginous discs which limit ability to dilate
- resistance does not continue to increase as airways get smaller as airways are not rigid pipes - they dilate as lung volume increases
7
Q
How does resistance and conductivity of lungs change with lung volume?
A
- as lung volume increases (i.e. as we breathe in) our airways dilate which decreases resistance - because airways are not rigid pipes
- the conductivity of the airways increases with increasing volume
8
Q
How does total surface area of vessels change as we go from arteries –> arterioles –> capillaries –> venules –> veins?
A
Total surface area of vessels highest at capillaries (peaks) then decreases symmetrically
9
Q
How does mean pressure change as we go from arteries –> arterioles –> capillaries –> venules –> veins?
A
Starts of high and gradually decreases, decreases more rapidly at capillaries, then rate of decrease decreases after at venules (looks like Z-shaped titration graph)
10
Q
How does proportion of systemic blood volume change as we go from arteries –> arterioles –> capillaries –> venules –> veins?
A
Starts off low, decreases slightly until capillaries, then increases at venules and highest at veins (then plateaus)
11
Q
What does the smooth muscle in the walls of small arteries and arterioles help them control?
A
Helps regulate their diameters and the resistance to blood flow
12
Q
How and why do veins and venules have such a large proportion of blood volume?
A
- blood at high pressure gets pushed from arteries + arterioles through capillaries into venules + veins where it slows down
- veins + venules are highly compliant and act as a reservoir for blood volume
13
Q
How does pressure change across circulation?
A
- systemic circulation - high (zigzag) at arteries, decreases at arterioles then gets low at capillaries and continues to decrease until veins
- starts to increase slightly at right ventricle
- pulmonary circulation: slightly higher (zigzag) at arteries, decreases slightly through arterioles –> capillaries and basically flat until veins
- steep increase left ventricle
- cycle repeats
14
Q
Why does pressure fall across the circulation?
A
Due to viscous (frictional) pressure losses
15
Q
Why is pulmonary circulation at so much lower pressure than systemic?
A
Lungs are close to the heart, so the heart does not need to push hard to pump blood to the lungs
16
Q
What is the equation for blood pressure and what are three assumptions made?
A
- delta P = Q (flow rate) x TPR :
- blood pressure (MAP) = cardiac output (CO) x resistance (PVR)
- assumes steady flow (which does not occur due to the intermittent pumping of the heart)
- assumes rigid vessels
- assumes right atrial pressure is negligible
17
Q
How is regulation of blood flow achieved physiologically?
A
Achieved by variation in resistance in the vessels while blood pressure remains relatively constant
18
Q
What three variables does resistance of a tube to flow depend on? (Poiseuille’s equation)
A
- fluid viscosity (n)
- length of tube (l)
- inner radius of tube (r)
19
Q
Halving the radius decreases the flow 16 times - what does this mean for blood vessels?
A
Relatively small changes in vascular tone (vasoconstriction/dilation) can produce large changes in flow
20
Q
What is the distribution of blood between organs both at rest and during exercise?
A
- at rest: GI system 1L, heart 0.25L, kidney 1L, brain 0.75L, skin 0.25L, bone 0.15L, skeletal muscles 0.75L
- exercise: GI system 0.75L, heart 1.25L, kidney 1L, brain 0.75L, skin 0.25L, bone 0.25L, skeletal muscles 16L
21
Q
What is laminar blood flow like?
A
- velocity of the fluid is constant at any one point and flows in layers
- blood flows fastest closest to the centre of the lumen - further away from the walls, the faster the flow
22
Q
What is turbulent blood flow like?
A
- blood flows erratically, forming eddys, and is prone to pooling
- associated with pathophysiological changes to the endothelial lining of the blood vessels
- more likely to activate clotting factors or produce a thrombus
- both laminar and turbulent flow can happen in the same system
23
Q
How is turbulent flow used to measure blood pressure?
A
- BP usually measured on upper arm, as it is easily accessed and at heart-level
- slow deflation of cuff causes turbulent flow which can be heard with a stethoscope
24
Q
What are the equations for pulse pressure and mean arterial pressure?
A
- pulse pressure (PP) = systolic blood pressure - diastolic blood pressure
- MAP = DBP + 1/3PP
- e.g. for 120/80 BP, PP = 120 - 80 = 40 and MAP = 80 + 1/3(40) = 93 mmHg
25
How could airway transmural pressure changing through inspiration and expiration cause our airways to collapse?
- transmural pressure is the relative pressure between the alveoli and the intrapleural space, +ve if higher in alveoli
- patent means open airways - through inspiration our airways are open since transmural pressure is positive
- however if we do a hard expiration (contract stomach muscles, breathe out hard, hunch over etc) we increase the pressure in the airways a lot but also increase the intrapleural pressure
- if intrapleural pressure > pressure of airway at any point, the airway at that point will collapse
26
What does our body have to prevent the collapse of airways?
Our large extrapulmonary airways are supported with cartilage
27
What is compliance?
- the tendency to distort under pressure
- delta V / delta P
- e.g. with a certain amount of pressure, a condom will increase in volume a lot more than a balloon since it has greater compliance
28
What is elastance?
- the tendency to recoil to its original volume (reflects resistance to change in shape, inversely proportional to compliance)
- delta P / delta V
- e.g. if you want to make a small volume change in a balloon, it will take greater pressure than with a condom - balloon more likely to return to original volume
29
Why do ventricular and aortic pressures differ?
- once the aortic valve closes, ventricular pressure falls rapidly but aortic pressure falls slowly
- this is explained by the elasticity of the aorta and large arteries which act to 'buffer' the change in pulse pressure
- the elasticity of a vessel is related to its compliance
30
How does arterial compliance lead to continuous blood flow rather than pulsatile flow of heartbeats?
- during ejection, blood enters the aorta and other downstream elastic arteries faster than it leaves them (40% of SV is stored by the elastic arteries) - the left ventricle contracts and aorta and arteries stretch
- when aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation - left ventricle relaxes and aorta and arteries recoil
- this arterial recoil is what squeezes blood on after the ventricles squeeze it to provide continuous blood flow rather than pulsatile
31
If arterial compliance decreases (e.g. as arteries become stiffer with age) how would blood pressure change?
- systolic BP will increase as ability of vessels to stretch in response to ventricular systolic pressure has decreased
- diastolic BP will decrease as ability of vessels to recoil once stretched to push blood to create 80 mmHg of diastolic pressure has decreased
- pulse pressure will go up (as systolic - diastolic)
32
What are the two pumps of facilitated venous return?
- skeletal muscle pump - in lower limbs, effect of muscle compressing veins in presence of functional vein valves means the blood cannot push back down to foot but goes up to heart
- respiratory pump - in thorax, we have IVC - when diaphragm pulls down to create -ve intrathoracic pressure, this vacuums the vein out which pulls vein apart = reduces pressure in thorax relative to abdomen which causes blood to flow up IVC back to heart
33
How is varicosity caused?
Incompetent valves cause dilated superficial veins in the leg (varicose veins)
34
How is oedema caused in feet?
Prolonged elevation of venous pressure (even with intact compensatory mechanisms) causes oedema in feet
35
How are aortic aneurysms caused?
- over time, vessel walls can weaken causing balloon-like distension
- pathological example of Law of Laplace - vascular aneurysms increase radius of vessel = for same internal pressure, the inward force exerted by the muscular wall must also increase
- however if the muscle fibres have weakened, the force needed cannot be produced and so the aneurysm will continue to expand (often until it ruptures)
- this pathology and the underlying physical forces involved also holds for the formation of diverticuli in the gut
- abdominal aortic aneurysms are most common and more common in men than women
36
What is the difference between arterial and venous compliance?
- compliance is the relationship between transmural pressure and the vessel volume, and depends on vessel elasticity
- venous compliance is 10-20x greater than arterial compliance at low pressures
37
How can venous compliance be changed?
- increasing smooth muscle contraction decreases venous volume and increases venous pressure
- most blood volume is stored in the veins
- relatively small changes in venous pressure distend veins and increase the volume of blood stored in them
38
How does ventilation change as you go down the length of the lung?
Higher up in lung:
- pleural pressure more negative (-8 cmH2O)
- greater transmural pressure gradient (0 vs -8)
- alveoli larger and less compliant
- less ventilation
Lower down in lung:
- pleural pressure less negative (-2 cmH2O)
- smaller transmural pressure gradient (0 vs -2)
- alveoli smaller and more compliant
- more ventilation
- partly due to gravity pulling tissue and extracellular fluid down to base of lung
39
How does perfusion change as you go down the length of the lung?
Higher up in lung:
- lower intravascular pressure (due to gravity)
- less recruitment
- greater resistance
- lower flow rate
Lower down in lung:
- higher intravascular pressure (due to gravity)
- more recruitment
- less resistance
- higher flow rate
- flow tends to follow path of least resistance - easier for heart to pump downhill to bottom of lung
40
How does alveolar pressure (PA), arterial pressure (Pa) and venous pressure (Pv) change as you go down the lung?
- zone 1 (high): PA > Pa > Pv
- zone 2 (medium): Pa > PA > Pv
- zone 3 (low): Pa > Pv > PA
41
Describe the graph of perfusion and ventilation change as you go from the base to the apex?
- if you divide perfusion by ventilation you get the curve
- base - more blood going past respiratory exchange surface that can participate in gas exchange - wasted perfusion
- apex - little blood going there and a little air, but air is still more than blood - wasted ventilation as you are moving gases into parts of the lung that are not getting blood supply
- we want to be where the lines cross - right amount of perfusion for amount of ventilation - different lung diseases distort this relationship
