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
What happens if arterial compliance decreases (e.g. arteries become stiffer with age)?
Systole increases
Diastole decrease
Pulse pressure increases
What are 2 factors that facilitate venous return?
Skeletal muscle pump- muscles compress against veins causing blood to be pushed pack to heart (can’t go bad down due to valves)
Respiratory pump- diaphragm pulls down and decreases interthoracic pressure, along with reducing pressure in veins causing blood to flow up
What is compliance and how do you measure it?
The tendency to distort under pressure
Compliance = change in volume/ change in pressure
What is elastance and how do you calculate it?
The tendency to recoil to its original volume
Elastance = change in pressure/ change in volume
What is varicosity?
Incompetent valves cause dilated superficial veins in the leg (varicose veins)
What causes oedema?
Prolonged elevation of venous pressure causes oedema in feet
What is aneurysmal disease?
Overtime vessel walls can weaken causing balloon- like distension
Its a pathological example of Law of LaPlace
Vascular aneurysms increase radius of the vessel- this means for the same internal pressure, inwards force exerted by muscular walls must also increase
However if muscle fibres have weakened, force needed cannot be produced so aneurysm will continue to expand until it ruptures
How does venous and arterial compliance compare?
Relationship between transmural pressure and vessel volume is the compliance and it depends on elasticity
Arteries are less complaint than veins at low pressure
Relatively small changes in venous pressure distend veins and increase volume of blood in them
How does ventilation change across the lung?
Gravity causes base of lung to become compressed due to natural stretchiness of lung tissue
This means alveoli at bottom are smaller in size so more compliant- the means theres more ventilation
What is ventilation and perfusion like at the apices (top) of the lung?
Theres lower intravascular pressure (gravity effect) Less recruitment Greater resistance Lower flow rate Large alveoli and less compliant Less ventilation
What is ventilation and perfusion like at the base of the lung?
Higher intravascular pressure More recruitment Less resistance Higher flow rate Smaller and more compliant alveoli More ventilation
What is ventilation perfusion matching?
V/Q matching
When ventilation doesn’t match perfusion we call this V/Q mismatching
It occurs in conditions like COPD or pulmonary fibrosis where blood is sent to alveoli that aren’t effectively undergoing gas exchange due to air trapping or lack of inflation
Ratio of amount of blood to amount of O2 that can be exchanged is mismatched- can cause hypoxemia with or without hypercapnia (elevated CO2)
