Blood Flow Flashcards
What is the primary function of blood flow?
- Bulk transport
- Delivery of oxygen, nutrients and water to tissues
- Removal of metabolic waste products from tissues
How long does it take a red blood cell to circulate for a normal adults at rest?
- one minute
What does the vascular system comprise of?
- 2 closed circuits in a series
- Systemic circulation from left ventricle
- Pulmonary circulation from right ventricle
- Volume of blood ejected into each is the same
Describe the systemic circulation
- Left heart –> aorta –> distributes blood all around body
- Capillaries
- Veins –> right heart –> lungs –> oxygenated blood –> left heart
What occurs at capillaries?
- Gas exchange
- Nutrient exchange
- Movement across the capillaries
What is the general arterial structure?
- Internal lining- endothelium
- Elastic tissue- expansion and contraction of arteries
- Smooth muscle found in lining
What is the general venous structure?
- Fibrous tissue and collagen is found here
- Capacitants and elastin
Describe the structure of capillaries
- Endothelial tube with pericytes
Describe the general structure of arterioles
- Function involves directing/changing and driving blood flow
- Proportionally more smooth muscle found in the small arterioles than the arteries
What is the impact of size on tension?
- Law of Laplace
- Tension = Pressure x Radius
- Larger arteries experience more tension than the smaller arteries
What is the driving force of blood flow generated by?
Heart
What is the biphasic response in the aorta?
- Blood ejected from the heart- peaks
- During diastole- drops
What is Darcy’s law?
- Blood flow analogous to electrical current
- Q = ∆P / R
- Q = flow
- ∆P = Pressure gradient
- R= resistance
What varies flow?
- Flow varies proportionally with pressure gradient
- Inversely with resistance
What determines systolic pressure?
- Cardiac output (CO)
- Increased CO increases pressure
What happens to pressure across the vascular tree?
- Drops
- Aorta = ~100mgHg (biphasic mean)
- Capillaries= ~25mmHg
- Great veins= ~2mmHg
What is Poiseuilles Law?
- Describes resistance to flow through a cylinder
- R = nL/r⁴
Describe the relationship between resistance and radius
- Inversely promotional to the fourth power of the radius
- 1/r⁴
- This mains small changes in vessel diameter have large effects on resistance, and subsequently flow
What is the relationship between flow and radius?
Flow is proportional to the fourth power of the radius
What types of vessels will generate the largest amount of resistance?
- Smaller arteries and arterioles
- Large conductance arteries have limited capacity to vary diameter
- Unlike smaller muscular arteries and arterioles
Why do conduit arteries offer little resistance?
- Large in size
- 20% increase in diameter would be required to more than double the flow
- Aorta 1cm, needs 0.2 cm increase
- Arteriole 0.01 cm needs 0.002 cm increase for same flow change
What happens if one arteriole is dilated in a network?
- There will be a greater increase in flow because blood flows down the path of least resistance
What is the relationship between viscosity and resistance?
- Viscosity proportional to resistance
- Inversely proportional to flow
- Thicker solution = less flow
- Blood is 2.5/3x more viscous than water
Why is blood viscous?
- Haematocrit
- Proportion of blood volume occupied by red cells
- Usually expressed as percentage
- Females 37-47% and males 40-54%
- Concentration and type of plasma protein will also have an effect
What is the relationship between haematocrit and viscosity?
- Haematocrit increases, relative viscosity increases
- Capillaries tend to have a lower viscosity
What factors affect viscosity?
- Generally, does not change
- Anaemia and polycythaemia, decrease/increase viscosity respectively
- Capillaries tend to have lower haematocrit due to axial streaming- higher flow
What is meant by laminar blood flow?
- Fastest at the centre of the vessel
- As it flows through, the cells at the sides of the walls put at the wall of the artery
- Silent
What is shear stress?
- Increased flow increases shear
- Slowed and pulled
What is meant by non-laminar flow?
- Turbulence
- At high velocities, flow can become turbulent
- Or at large diameter (like bronchial breathing)
When does turbulent flow occur under normal conditions?
- Asending aorta
- Around branch points
Describe the sounds of turbulent flow
- Not silent
- Basis of Korotkoff sounds in measuring blood pressure
- Tapping, cut off supply and then allow it to flow and there will be sound at that point
- Basis of bruit in athermoa
What is the equation for flow?
(∆P x πr⁴)/ 8nL
