Vasculature & Haemodynamics Flashcards
Name the vessels of the vascular system
- Arteries
- Arterioles: when an artery reaches a particular organ, it branches into arterioles
- Capillaries
- Venules
- Veins
Summarise the characteristics of arteries
Arteries
-
Low resistance vessels conducting blood to the various organs with little loss in pressure.
* walls have large radius - Act as pressure reservoirs for maintaining blood flow between ventricular contractions.
- Are able to withstand changes in pressure between systolic and diastolic
- Thick, highly elastic walls - 40% elastic tissue (largest component)
- Number: several hundred in the body
Summarise the characteristics of arterioles
Arterioles:
-
Major sites of resistance to blood flow.
* walls have small radius - Responsible for the pattern of blood flow distribution:
- By changing diameter of arterioles, the delivery of blood is influenced
- Highly muscular, well-innervated walls
- Walls 60% smooth muscle (largest component)
- Arteriolar walls display vascular tone
- Participate in the regulation of arterial blood pressure.
- MAP drops from 93 mmHg to 37 mmHg from beginning to end of arterioles
- We also lose the fluctuation between systolic and diastolic pressure in arterioles: Arteriolar resistance converts pulsatile arterial pressure into non-fluctuating capillary pressure
- Number: half a million in the body
Summarise the characteristics of capillaries
Capillaries:
- Site of exchange between blood and tissues.
- Wall of capillary is made by a single layer of endothelial cells
- Thin wall
- Capillary wall 95% endothelium, 5% basal lamina (connective tissue)
- Pressure
* Capillary blood pressure is non-fluctuating - Number: ten billion in the body
* large total cross sectional area, so despite the small radius of walls, overall capillaries are not the main site of resistance to blood flow
Summarise the characteristics of veins
Veins:
- Low resistance vessels for blood to flow back to the heart
- Acts as a volume reservoir for blood:
- through venal constriction flow of blood back to the heart is directed
- Capacity for blood is adjusted to facilitate flow.
- Thin walls, highly distensible, large radius
- Blood pressure is very low by the time blood circulation reaches veins (close to 0) so thin walls suffice
- 60% of venus walls connective tissue
- 0% elastic tissue
- Number: several hundred in body
Describe the layers in vessel walls
All vessel walls (apart from capillaries) comprise 3 layers:
- Intimal layer: endothelial cells
- Capillaries only have an endothelial layer
- Medial layer: smooth muscle cells
- Smooth muscle cells arranged in circular radial fashion
- Arterioles change diameter through contraction or relaxation of smooth muscle
- Adventitial layer: connective tissue
- Outer layer
- Arteries’ elastin found here
- Collagen found here: gives vessel wall strength
What determines the function of a vessel?
Composition of vessel wall determines the functional characteristics of that vessel
What drives blood flow in the vascular system?
What is the pressure exerted by a fluid termed?
Throughout the vascular system blood flow (F) is always from a region of higher pressure to one of lower pressure.
- Pressure generated by heart contractions is the driving force
- The difference in pressure (ΔP called pressure gradient) drives the blood flow
The pressure exerted by a fluid is often termed the hydrostatic pressure
What is resistance to blood flow?
What are the factors affecting resistance?
Resistance is a measure of how difficult it is for blood to flow between two points at any given pressure difference.
In other words, a measure of the friction impeding flow.
3 Factors affecting resistance:
R = ηL/r^4
1) viscosity of the blood, η: RBC, WBC friction with vessel wall
* Viscosity changes depending on subject’s hydration status (inversely) - so need an increase in pressure difference to maintain same F
2) vessel length, L
* Least important out of the 3 since vessel length remains constant
3) vessel radius, r: inversely related to R
- The major determinant of resistance to flow is the vessel’s radius (r)
- A slight change in radius brings about a notable change in flow: F ∝r^4
- In narrower vesselthe same volume of blood is spread over a much larger surface area – so comes into contact with the vessel wall much more (this increases friction)
- So smaller the radius the greater the resistance to blood flow
What is the formula for blood flow, F?
What are the units?
Blood Flow
F = ΔP/R
Where:
F = flow rate of blood through a vessel (l/min)
ΔP = pressure gradient (mmHg)
R = resistance of blood vessels (mmHg·min/l)
How is pressure gradient calculated?
How does it influence F?
Pressure gradient is calculated by the difference between the pressure at two points in a vessel
Flow rate depends on the pressure gradient (difference) NOT the absolute pressure
- If pressure gradient in a vessel is twice the pressure gradient in another vessel, the flow rate is twice as well (regardless of the absolute values of pressure)
What is the pattern of resistance across a circular tube in
cross section and
3-D?
In cross section, resistance has a parabolic distribution of sleeds of flow across a circular tube
In 3-D this is similar to a set of concentric cylinders
What is the relationship between radius and Resistance/blood Flow?
If we double the radius we get 16 times more blood flow
F ∝ r^4
R ∝ 1/ r^4
What does Poiseuille’s equation measure?
What is the most notable determinant?
Poiseuille’s equation brings together all the factors to measure blood flow rate
F = ΔP/R
F = πΔP/8R
Insert R = ηL/r^4
F = πΔPr^4/8ηL
Notable determinant: A two-fold change in radius will produce a 16-fold change in flow
- A slight change in radius brings about a notable change in flow
Why does arterial pressure fluctuate between 80-120 mm Hg? Why does it not drop to 0?
Arterial pressure fluctuates in relation to ventricular systole and diastole
- Diastolic ARTERIAL pressure DOES NOT drop all the way to 0 even though the heart relaxes and stops pumping blood to arteries because of elastic recoil of expanded arterial walls during diastole
Label the diagram and mention key characteristics of blood pressure throughout the systemic circulation
- Left ventricular pressure doesfluctuate between 0 and 120
- When we hear the term blood pressure – usually refers to arterial pressure (120/80)
- Diastolic pressure does not drop to 0 in arteries bc of elastic recoil
- Arteries offer very little resistance to flow so this 120-80 difference in arterial pressure is the same through all the large arteries of the body
- Key characteristic of arterioles is that they offer a resistance to blood flow
- So mean arterial pressure drops from 93mm Hg in arteries to 37mm Hg as blood flows from start to end of arterioles
- We also lose the fluctuation between systolic and diastolic pressure in arterioles: Arteriolar resistance converts pulsatile arterial pressure into non-fluctuating capillary pressure
- Note: by the time blood has reached the veins from venules, pressure is very low, so veins thin-walled
