HUBS1404 Chap.2 Haemodynamics, Perfusion and Blood Pressure
Haemodynamics, Perfusion and Blood Pressure
Begin every flow claim with two points
Blood flow needs a pressure difference between an upstream and downstream location. Quoting pressure at one point is not enough because two equally high pressures provide no net driving gradient. In a simplified comparison, flow rises with pressure difference and falls with resistance. If both quantities change, neither can be interpreted alone.
Writing the relationship before substituting numbers prevents the familiar mistake of choosing a higher-flow answer merely because the upstream pressure is higher.
Resistance has several determinants
Length, viscosity and lumen radius all contribute to resistance, but their physiological roles differ. Vessel length is usually stable during an acute response. Viscosity can vary with blood composition.
Smooth muscle can change arteriolar radius rapidly, allowing local tissues and autonomic control to redistribute flow. Because resistance is highly sensitive to radius, a modest constriction can markedly reduce perfusion under an unchanged gradient.
Dilation has the opposite effect, although actual flow still depends on the pressure available across the bed.
Different vessel classes solve different tasks
Elastic arteries accept pulsatile ejection and help maintain movement between beats. Muscular arteries distribute output, arterioles provide adjustable resistance, capillaries create a thin exchange interface and veins hold a large compliant volume.
The many parallel capillaries have a great total cross-sectional area, so average blood velocity can be low even while total flow through the organ continues.
Pressure, velocity, resistance and cross-sectional area are therefore related but cannot be substituted for one another.
Exchange requires a directional force balance
Capillary hydrostatic forces tend to move fluid according to pressure differences, while osmotic forces related to retained solutes influence movement in the opposite direction.
Permeability and surface properties determine how effectively those forces act. A correct description must name whether fluid is moving from plasma to interstitial space or returning toward plasma.
Venous valves, skeletal muscle activity, respiratory pressure changes and sympathetic venoconstriction then support the return of existing blood volume rather than generating new volume.
Keep local and systemic regulation distinct
A single tissue can dilate its arterioles to support metabolic demand while another bed constricts.
Whole-body arterial pressure depends on cardiac output and the combined resistance of many parallel regions. Rapid neural responses alter heart and vessel activity, while slower control of fluid volume changes the amount available to circulate.
A local resistance rise does not by itself prove that mean arterial pressure rose, and an elevated systemic pressure does not guarantee that a narrowed downstream tissue is well perfused. State the scale before selecting the consequence.
What this chapter covers
- 01
Pressure gradients and flow
- 02
Radius and vascular resistance
- 03
Vessel classes and velocity
- 04
Capillary exchange forces
- 05
Cardiac output and pressure control
Worked example · free
Competing pressure and resistance changes
- 1Initial flow is 20 divided by 4, which equals 5 units.
- 1Final flow is 24 divided by 6, which equals 4 units.
- 1The absolute change is negative 1 unit.
- 1The relative change is negative 1 divided by 5, or a 20% fall.
Key terms
- Perfusion
- Perfusion is blood flow delivered through the vessels of a tissue.
- Resistance
- Resistance is opposition to flow produced by vessel and fluid properties.
- Mean arterial pressure
- Mean arterial pressure is the time-averaged arterial pressure driving systemic perfusion.
Haemodynamics, Perfusion and Blood Pressure FAQ
Does higher pressure always mean higher flow?
No. Flow depends on pressure difference relative to resistance. If resistance rises by a greater proportion, flow can fall even while pressure difference rises. Both terms must be compared together.
Why are arterioles important?
Their smooth muscle can change lumen radius rapidly, producing large resistance changes that redistribute flow and contribute to systemic vascular resistance. This makes arterioles powerful control sites.
How can I avoid a scale error in haemodynamics?
Write whether the claim concerns one vessel, one organ bed or the whole systemic circulation. A local resistance change can alter regional perfusion without determining mean arterial pressure, which also depends on cardiac output and the combined resistance of parallel beds.
Why is capillary velocity relatively low?
The capillary network has a very large total cross-sectional area. Distributing flow across that area lowers average velocity and supports exchange. Individual narrow capillaries do not determine the network total by themselves.
Exam move
Write pressure difference over resistance for every flow question, then identify whether the question concerns one bed or the whole circulation. Build a table with vessel class, dominant function, relative pressure, total cross-sectional area and typical velocity, but explain each entry instead of memorising its rank.
Practise comparisons in which pressure and resistance move in the same direction, because those reveal whether you are actually using the quotient. For capillary questions, draw plasma and interstitial fluid as separate boxes and place hydrostatic and osmotic arrows before naming filtration or return.
Add venous valves, muscle compression and respiratory changes to a different diagram so they are not confused with capillary exchange. Finish each scenario by writing LOCAL or SYSTEMIC above the answer. A regional arteriole can change its tissue perfusion without establishing the direction of mean arterial pressure, and whole-body pressure can rise while a narrowed downstream region remains poorly supplied.
Check units, numerator and denominator before calculating a percentage change.
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