Hemodynamics Principles Flashcards
If the cross-sectional area of a blood vessel decreases, how must the velocity of the blood change to maintain the same flow?
The velocity must increase.
Flow = Area x Velocity
What is Fick’s law of diffusion?
Flow = D x [ ] gradient x A
where
D = diffusion coefficient
[ ] gradient = (C out - C in)/distance
A = Area
What is the relationship between resistance to flow and viscosity of a fluid?
Resistance to flow is directly proportional to the viscosity of a fluid.
What is the relationship between resistance to flow and the length of the blood vessel (tube)?
Resistance to flow is directly proportional to the length of the blood vessel (tube).
What is the relationship between the resistance to flow and the radius of the blood vessel?
Resistance to flow is inversely proportional to the 4th power of the radius (i.e. the greater the radius, the lesser the resistance to flow).
Define hemodynamics
The branch of physiology studying the forces involved in the circulation of blood.
Define diffusion
Spontaneous movement of particles caused by random thermal motion.
When gas moves across the capillary beds, either in the lungs or in the systemic tissues, it moves by diffusion. What type of movement describes the movement of air through the airways when it is inhaled, or the movement of blood from the heart across the great vessels?
Convective/Bulk flow
What is the relationship between the rate of diffusion and distance?
Inversely proportional
The greater the distance over which diffusion occurs, the slower the rate of diffusion.
Diffusion is most effective over short distances!
What is the relationship between the rate of diffusion and area?
Directly proportional
The greater the area over which particles can diffuse, the greater the rate of diffusion.
What is the diffusion coefficient?
Calculated value value specific to substance diffusing and the substance through which it is diffusing
Which diffuses better in water?
a) O2
b) CO2
b) CO2 diffuses around 20x better in water than O2 (diffusion coefficient)
In a standard individual, what is the total blood volume value?
~75mL/kg i.e. 5L
To avoid congestion in the circulation system, the flow must be…
the same in each part of the “series” (i.e. pulmonary flow must be equal to systemic flow).
How come flow to individual organ systems can change based on demand, without causing congestion?
Because the branches of circulation to the organs are organized “in parallel”
Changes in flow to the organ systems are controlled by…
resistance vessels, i.e. small arteries and arterioles
What is the advantage of having an in-series arrangement of the pulmonary and systemic circuits?
Ensures constant total blood flow across any given area of the circuit.
What is the advantage of having a parallel arrangement of the systemic organs and branches of the CV system?
Slower transit time through capillaries, allowing sufficient time and large total surface area for diffusion (Fick’s law).
How does the high total cross-sectional area of the branching of the CV system affect the total resistance of the system (how hard does the heart have to work)?
Lots of branching = lower total resistance
I.E. the heart does not have to pump blood as hard.
Define pressure
Pressure = Force/Area
What is a normal blood pressure for the standard individual?
120/80 mmHg
How does resistance influence the pressure gradient needed to drive blood through the CV system?
The greater the resistance, the greater the pressure gradient needed to maintain the adequate (constant) blood flow
What is the resistance like in large arteries vs smaller vessels?
There is little resistance in large arteries (due to their large diameter) compared to smaller vessels (smaller diameter = greater resistance).
Define hydrostatic pressure
The pressure exerted by a fluid at equilibrium at any given point within the fluid, due to the force of gravity.
As you move downward in a column of fluid, the hydrostatic pressure…
increases
Define perfusion pressure
Pressure gradient that drives blood flow through an organ or tissue.
Perfusion pressure = Arterial pressure - Venous pressure
However, since venous pressure is so small relative to the arterial pressure, perfusion pressure can simply be approximated to the arterial pressure.
Define laminar flow
Movement of fluid in parallel, concentric layers. The resistance of each layer decreases as you move inward.
Laminar flow is synonymous to…
parabolous flow
In laminar flow, where is the resistance the lowest? Where is it the highest? What creates resistance in laminar flow?
Resistance is lowest at the centre of the flow.
Resistance is highest near the walls of the vessel (friction).
Resistance is caused by the viscosity of the fluid (each concentric layer exercise friction on adjacent layers).
What is the relationship between blood flow, perfusion pressure and resistance?
Blood flow = Perfusion pressure/Resistance
Blood flow is …. to resistance.
Blood flow is INVERSELY PROPORTIONAL to resistance (the greater the resistance, the lesser the flow).
In the CV system, how is the resistance controlled? What parameters can’t be controlled?
We cannot change the vessel length nor viscosity of the blood.
Therefore, to create changes in resistance, the CV system alters the diameter of blood vessels (smooth muscle contraction).
What types of substances lead to vasodilation
Metabolites (waste products) - produced in high quantities during periods of intense exercise (thereby encouraging greater blood flow to muscles).
Other than local metabolites, what three other factors contribute to resistance control in blood vessels?
- Parasympathetic and sympathetic innervation
- Endothelial factors (i.e. chemicals produced by the endothelium)
- Hormones (epinephrine, norepinephrine)
What are 3 ways in which you can increase the mean arterial pressure (blood pressure)?
Increase stroke volume
Increase heart rate
Increase systemic vascular resistance
We can increase a patient’s heart rate by…
administering epinephrine
We can increase a patient’s stroke volume by…
administering fluids (to a hypovolemic patient)
We can increase a patient’s vascular resistance by…
administering vasopressins
When a patient has sepsis, their blood pressure drops significantly (septic shock). How would you fix this patient’s blood pressure?
- Administer fluids (increase stroke volume)
- Administer epinephrine (increase heart rate)
- Administer vasopressins (increase vascular resistance)
Knowing that the systemic pressure is greater than the pulmonary pressure, but systemic and pulmonary cardiac output is the same, what can you say about the systemic and pulmonary resistance?
Systemic resistance must be greater than pulmonary resistance to create a greater pressure in the systemic system while maintaining the same cardiac output.
Describe how the anatomy of arteries vs veins influences their compliance.
Veins:
* wide lumen, thin wall, few layers of smooth muscle
* highly compliant (low resistance)
Arteries:
* small lumen, thick wall, many layers of smooth muscle
* poor compliance (high resistance)
Differentiate the following:
* Blood pressure
* Transmural pressure
* Perfusion pressure
Blood pressure: The force exerted by circulating blood on the walls of blood vessels, typically measured in arteries, and expressed as systolic/diastolic values (e.g., 120/80 mmHg).
Transmural pressure: The pressure difference across the wall of a blood vessel, calculated as the internal pressure minus the external pressure. It influences vessel shape and blood flow.
Perfusion pressure: The pressure gradient that drives blood flow through tissues, calculated as the difference between arterial and venous pressures. It ensures adequate tissue oxygenation and nutrient supply.
Define blood vessel compliance (formula)
Compliance = ΔV/ΔP
Ability of a blood vessel to expand (change its volume) in response to a given pressure change.
Since arteries have low compliance, even high increases in pressure will yield…
little changes in volume of the arteries
Since veins have high compliance, even small increases in pressure will yield…
large changes in volume of the veins
Compare the compliance of the heart chambers.
Right heart (atrium and ventricle): highly compliant
Left heart (atrium and ventricle): low compliance
