Cardiorespiratory Mechanics Flashcards
What is poiseuille’s law?
Resistance = 8ηl / πr^4
η= viscociy
l= length
r= radius of tube
Decreasing radius massively decreases blood flow (resistance is larger)
Resistance is inversely proportional to the fourth power of radius
Halving radius will increase resistance and decrease flow by 16 fold
How is resistance maintained?
Smooth muscle in arterial walls regulate vessel diameter to control resistance and maintain blood flow
Veins and venules are highly compliant (can distend a lot) and act as a reservoir for blood volume
Resistance doesn’t continue to increase as airways get smaller due to the structural support of higher airways- cartilaginous disks limit constriction
How does conductivity change with volume?
Conductivity of airways increases with increasing volume
How does pressure change across circulation?
Pressure falls across the circulation due to viscous pressure losses
Small arteries and arterioles have most resistance to flow
How do you calculate blood pressure?
Blood pressure (mmHg) = Cardiac output x resistance (mmHgL^-1min)
Why is the calculation for blood pressure an approximation?
It assumes
- Steady flow (doesn’t occur due to intermittent pumping of heart)
- Rigid vessels
- Right atrial pressure is negligible
Physiologically regulation of flow is achieved by variation in resistance in the vessels while BP remains constant
What are the 2 types of blood flow?
Laminar flow
Turbulent flow
What is laminar flow?
Velocity of fluid is constant at any one point and flows in layers
Blood flows fastest at centre of lumen
What is turbulent flow?
Blood flows erratically, forming Eddys and is prone to pooling
Associated with pathophysiological changes to endothelial lining of blood vessels
How is blood pressure measured?
BP usually measured on upper arm as its easily accessed and at heart-level
Slow deflation of cuff causes turbulent flow which can be heard by stethoscope
How do you calculate pulse pressure?
Pulse pressure = Systolic blood pressure - diastolic blood pressure
How do you calculate mean arterial blood pressure?
Mean arterial pressure = Diastolic blood pressure + 1/3 pulse pressure
What causes a collapsed lung?
Collapsed lung occurs when there is a negative transmural pressure and intrapleural pressure is greater than airway pressure
This is why we need our extrapulmoary airways to be supported by cartilage
What are 2 structural properties of the lung?
Elastance
Compliance
What is the importance of arterial compliance?
During ejection, blood enters aorta and other arteries faster than it leaves them
When aortic valve closes ejection ceases but due to recoil of elastic arteries, pressure falls slowly and theres diastolic flow in downstream circulation
This is the windkessel effect- compliance of aorta turns a pulsatile flow into a continuous flow