Cardiorespiratory Mechanics

Question 1

Please note bank this one

Answer 1

Okay

Harroop slide 516

Question 2

What drives ventilation?

Answer 2

Pressure

Inspiration: atmospheric pressure > intralveolar pressure > intrapleural pressure

Expiration: intraalveolar pressure > atmospheric pressure

Question 3

What is poiseuilles law?

Answer 3

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

N = eta whic represents viscosity

Question 4

How does the radius (of an airway) affect resistance?

Answer 4

Resistance is inversely proportional to (radius)^4

However the lower the flow the less resistance

Question 5

How does resistance change with the airway generations?

Answer 5

For the first 4 or so generations, as generation increases (and radius decreases) resistance increases

However beyond that as generation increasss (radius decreases) air flow through the tubes also greatly decreases. This means that resistance also greatly decreases

Question 6

What is conductance?

Answer 6

Ability for airways to conduct and allow air to pass through

Conductance increases with increasing lung volume

As conductance increase, resistance decreases (airway radius increases during inspiration)

Question 7

How do surface are, pressure, and proportion of blood volume change across the dofferent types of blood vessels?

Answer 7

Arteries: small SA, highest mean pressure, lowest proportion of volume

Arterioles: small SA, high pressure, small proportion of volume

Capillaries: highest SA, low mean pressure, low ish proportion of blood

Venules: small SA, low mean pressure, high proportion of blood volume

Veins: small SA, lowest mean pressure, highest proportion of blood

Veins and venules are highly compliant and act as a reservoir for blood volume

Small arteries and arterioles have extensive smooth muscle in their walls to regulate their diameters and resistance to blood flow

Question 8

How does pressure differ across the circulation?

Answer 8

Pressure falls across the circulation due to viscous (frictional) pressure losses

Small arteries and arterioles have the highest resistance to flow

Blood pressure decreases as you get further away from the heart (arteries->arterioles->capillaries->venules->veins)

Pressure in the left ventricle is much higher than that in the right

Question 9

What are the haemodynamic determinants of blood pressure? (Equation)

Answer 9

Blood pressure (MAP) = cardiac output (Q) x resistance (PVR)

This relation is an approximation because it assumes:

1) steady flow (doesn’t occur due to pumping of heart)
2) rigid vessels
3) right atrial pressure is negligible

Physiologically the regulation of blood flow is achieved by variation in resistance in the vessels while blood pressure remains relatively constant

Question 10

What three variable does resistance to blood flow depend on?

Answer 10

Viscosity (in vivo remains pretty constant)

Length (fixed)

Radius (main determinant)

R=8Ln/pi r^4 (poiseulles)

Halving radius increases the radius x16 (decreases flow x16)

Question 11

What is the approximate blood distribution to different organs?

Answer 11

At rest cardiac output is 5L/min

During exercise output is 20L/min

Digestive: R-1L E-0.75L

Heart: R-0.25L E- 1.25L

Kidneys: R-1L E-1L

Brain: R-0.75L E-0.75L

Skin: R-0.25L E-0.25L

Bone: R-0.15L E-0.25L

Skeletal muscle: R-0.75L E-16L

Question 12

What is laminar flow?

Answer 12

Straight/smooth

Velocity of the fluid is constant at any one paint and flows in layers

Blood flows fastest closest to the centre of the lumen

Question 13

What is turbulent flow?

Answer 13

Blood flows erratically, forming eddys and is prone to pooling

Associated with pathophysiological changes to the endothelial lining of blood vessels

Question 14

How is blood pressure measured? Also equations?

Answer 14

Blood pressure is usually measured in the upper arm as it is level with the heart

Slow deflation of the cuff causes turbulent flow which can be heard with a stethoscope (korotkoff sound). This sound will be at the systolic pressure

Further reduce pressure in cuff until you get laminar flow so will no longer really hear the sounds, they will be quieter. This is diastolic BP

Should be around 120/80

Pulse pressure is the difference between these two values

Pulse pressure = SBP - DBP

MAP (mean arterial pressure) = DBP + 1/3 PP

Question 15

What happens in collapsible tube flow?

Answer 15

Patent means open, allowing air to flow through

All the way through inspiration and expiration the airway is open

(Transmural pressure is inside - outside)

In forced expiration: total lung capacity->residual volume. There is a significant increase in intrapleural pressure

The airway has cartilaginous rings to stop it collapsing

But if pleural pressure > airway pressure (during forced expiration), without cartilage rings the airway would collapse

Question 16

What are compliance and elastnace?

Answer 16

Compliance:

The tendency to distort under pressure

Delta V / delta P

High in condoms, low in balloon

Elastance:

The tendency to recoil to its original volume

Delta P / delta V

Low in condom, high in ballon

They are kinda opposites

Question 17

What do ventricular and aortic pressures differ?

Answer 17

Once the aortic valve closes ventricular pressure falls quickly but aortic pressure falls slowly

This is due to the elasticity of the aorta and the large arteries which act to buffer the change in pressure, so the aorta contracts a little

The elasticity of a vessel is related to its compliance

Question 18

What is arterial compliance?

Answer 18

During ejection, blood enters the aorta and other downstream elastic arteries faster than it leaves them

When the 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

If arterial compliance decreases (eg arteries become stiffer with age) systolic pressure would increase and diastolic pressure would decrease. So pluse pressure increases

Question 19

What is facilitated venous return?

Answer 19

The skeletal pump: the contraction of the muscle facilitates the movement of blood through veins to the heart. This decreases venous capacitance (contraction and valves allow the blood to flow unilaterally)

Respiratory pump: intrathoracic pressure decreases during inspiration through increased volume. The reduction in pressure provides a pressure potential allowing blood to return to the heart through the vena cava more easily. Venous return increases. Helped by the diaphragm

Incompetent valves and elevation of venous pressure can cause varicose veins and oedema in feet

Question 20

What is compliance?

Answer 20

The relationship between transmural pressure and the vessel volume is called the compliance and depends on elasticity

Veins are around 10 times more compliant than arteries

Increased smooth muscle decreases compliance

Question 21

What is ventilation and perfusion like across the lung?

Answer 21

Ventilation:

At the top Pressure in the plural cavity is more negative, this gives a greater transmural pressure gradient. This means the alveoli are larger and therefore less compliant. LESS VENTILATION

At the bottom the pressure of the pleural cavity is less negative (due to gravity), so there is a smaller transmural pressure gradient. The alveoli are smaller so more compliant. MORE VENTILATION

Perfusion:

At the top there is intravascular pressure (gravity effect) so there is less recruitment, greater resistance and a lower flow rate

At the bottom there is higher intravascular pressure is higher. This means there is more recruitment, less resistance and a higher flow rate. Greater perfusion y

So the top of the lung has minimal ventilation and perfusion

The sweet spot in the lung is around the middle to bottom