Cardiorespiratory Mechanics Flashcards
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Harroop slide 516
What drives ventilation?
Pressure
Inspiration: atmospheric pressure > intralveolar pressure > intrapleural pressure
Expiration: intraalveolar pressure > atmospheric pressure
What is poiseuilles law?
Resistance = (8nl)/(pi r^4)
N = eta whic represents viscosity
How does the radius (of an airway) affect resistance?
Resistance is inversely proportional to (radius)^4
However the lower the flow the less resistance
How does resistance change with the airway generations?
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
What is conductance?
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)
How do surface are, pressure, and proportion of blood volume change across the dofferent types of blood vessels?
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
How does pressure differ across the circulation?
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
What are the haemodynamic determinants of blood pressure? (Equation)
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
What three variable does resistance to blood flow depend on?
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)
What is the approximate blood distribution to different organs?
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
What is laminar flow?
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
What is turbulent flow?
Blood flows erratically, forming eddys and is prone to pooling
Associated with pathophysiological changes to the endothelial lining of blood vessels
How is blood pressure measured? Also equations?
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
What happens in collapsible tube flow?
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
What are compliance and elastnace?
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
What do ventricular and aortic pressures differ?
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
What is arterial compliance?
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
What is facilitated venous return?
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
What is compliance?
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
What is ventilation and perfusion like across the lung?
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
