6.9 - Cardiorespiratory Mechanics

Question 1

What happens during the rapid ejection phase of the cardiac cycle?

Answer 1

• 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

Question 2

What are the volume and pressure changes during inspiration and expiration?

Answer 2

• 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

Question 3

Describe how alveolar pressure changes during inspiration and expiration?

Answer 3

Alveolar pressure is same as the flow rate and pressure on the main graph (pressure and volume change)

Question 4

What are the limitations with snorkelling at great depths?

Answer 4

• 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

Question 5

What is Poiseuille’s Law?

Answer 5

Resistance = (8nl) / (pi x r^4)

Question 6

How does resistance change with size and number of airways?

Answer 6

• 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

Question 7

How does resistance and conductivity of lungs change with lung volume?

Answer 7

• 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

Question 8

How does total surface area of vessels change as we go from arteries –> arterioles –> capillaries –> venules –> veins?

Answer 8

Total surface area of vessels highest at capillaries (peaks) then decreases symmetrically

Question 9

How does mean pressure change as we go from arteries –> arterioles –> capillaries –> venules –> veins?

Answer 9

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)

Question 10

How does proportion of systemic blood volume change as we go from arteries –> arterioles –> capillaries –> venules –> veins?

Answer 10

Starts off low, decreases slightly until capillaries, then increases at venules and highest at veins (then plateaus)

Question 11

What does the smooth muscle in the walls of small arteries and arterioles help them control?

Answer 11

Helps regulate their diameters and the resistance to blood flow

Question 12

How and why do veins and venules have such a large proportion of blood volume?

Answer 12

• 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

Question 13

How does pressure change across circulation?

Answer 13

• 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

Question 14

Why does pressure fall across the circulation?

Answer 14

Due to viscous (frictional) pressure losses

Question 15

Why is pulmonary circulation at so much lower pressure than systemic?

Answer 15

Lungs are close to the heart, so the heart does not need to push hard to pump blood to the lungs

Question 16

What is the equation for blood pressure and what are three assumptions made?

Answer 16

• 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

Question 17

How is regulation of blood flow achieved physiologically?

Answer 17

Achieved by variation in resistance in the vessels while blood pressure remains relatively constant

Question 18

What three variables does resistance of a tube to flow depend on? (Poiseuille’s equation)

Answer 18

1. fluid viscosity (n)
2. length of tube (l)
3. inner radius of tube (r)

Question 19

Halving the radius decreases the flow 16 times - what does this mean for blood vessels?

Answer 19

Relatively small changes in vascular tone (vasoconstriction/dilation) can produce large changes in flow

Question 20

What is the distribution of blood between organs both at rest and during exercise?

Answer 20

• 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

Question 21

What is laminar blood flow like?

Answer 21

• 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

Question 22

What is turbulent blood flow like?

Answer 22

• 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

Question 23

How is turbulent flow used to measure blood pressure?

Answer 23

• 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

Question 24

What are the equations for pulse pressure and mean arterial pressure?

Answer 24

• 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

Question 25

How could airway transmural pressure changing through inspiration and expiration cause our airways to collapse?

Answer 25

• 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

Question 26

What does our body have to prevent the collapse of airways?

Answer 26

Our large extrapulmonary airways are supported with cartilage

Question 27

What is compliance?

Answer 27

• 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

Question 28

What is elastance?

Answer 28

• 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

Question 29

Why do ventricular and aortic pressures differ?

Answer 29

• 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

Question 30

How does arterial compliance lead to continuous blood flow rather than pulsatile flow of heartbeats?

Answer 30

• 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

Question 31

If arterial compliance decreases (e.g. as arteries become stiffer with age) how would blood pressure change?

Answer 31

• 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)

Question 32

What are the two pumps of facilitated venous return?

Answer 32

• 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

Question 33

How is varicosity caused?

Answer 33

Incompetent valves cause dilated superficial veins in the leg (varicose veins)

Question 34

How is oedema caused in feet?

Answer 34

Prolonged elevation of venous pressure (even with intact compensatory mechanisms) causes oedema in feet

Question 35

How are aortic aneurysms caused?

Answer 35

• 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

Question 36

What is the difference between arterial and venous compliance?

Answer 36

• 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

Question 37

How can venous compliance be changed?

Answer 37

• 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

Question 38

How does ventilation change as you go down the length of the lung?

Answer 38

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

Question 39

How does perfusion change as you go down the length of the lung?

Answer 39

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

Question 40

How does alveolar pressure (PA), arterial pressure (Pa) and venous pressure (Pv) change as you go down the lung?

Answer 40

• zone 1 (high): PA > Pa > Pv
• zone 2 (medium): Pa > PA > Pv
• zone 3 (low): Pa > Pv > PA

Question 41

Describe the graph of perfusion and ventilation change as you go from the base to the apex?

Answer 41

• 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