Lecture 22 : Blood Vessels - Part 1 Flashcards
What factors affect blood flow?
Pressure gradient and resistance
F=ΔP/R
What is the poiseuille relationship?
For laminar flow of fluid in a rigid tube, the volume flow is proportional to:
- ΔP between ends of tube (P2-P1)
- radius⁴ (r⁴ )
is inversely proportional to:
- length of tube (l)
- viscosity of liquid (η)
What is the equation for flow?
F = ∆P/R
∆P = P1 - P2
R = 8ηL/πr4
F = (P2-P1)πr4
How does a laminar vs. turbulent flow affect flow?
Turbulent flow decreases flow
How do series and parallel resistance affect flow?
- Series - Higher resistance; Rt= R1 + R2 + R3
- Parallel - Lower resistance; Rt= 1/R1 + 1/R2 + 1/R3
Describe the structural differences between an artery and a vein:
Artery
- Smaller lumen, thicker wall
- Many layers of smooth muscle, elastic and connective tissue
Vein
- Large lumen, thinner wall
- Few layers of smooth muscle, elastic and connective tissue
Same diameter
What are the functions of the aorta and large arteries?
- Reduce fluctuations in flow and pressure generated by intermittent ejection of stroke volume - highly elastic walls
- During systole energy is stored in the vessel wall as the elastic is stretched - energy released during diastole
- Prevents large swings in arterial BP between heart beats and results in a more or less steady blood flow into the smaller arteries
What is pulse pressure?
Difference between systolic pressure and diastolic pressure
What 3 factors determine the magnitude of pulse pressure?
- Stroke volume
- Speed of ejection of the stroke volume
- Arterial compliance - less stretchy = less pressure
What is ateriosclerosis?
A decrease in arterial compliance (stiffness)
- Higher systolic pressure & lower diastolic
How is mean blood pressure calculated?
- Diastolic pressure + 1/3[pulse pressure]
-> Mean BP = DBP + (SBP-DP/3)
What is TPR?
Total resistance to flow calculated from the resistance of all blood vessels in the body
– Takes into account series and parallel flow
How is arterial pressure calculated?
MABP = CO x TPR (Total Peripheral Resistance)
What structures are important in adjusting TPR?
Arterioles - this maintains MABP
How can arterial pressure be adjusted?
By altering CO and/or TPR
What is regional blood flow?
Arterial pressure/regional resistance
How is regional blood flow regulated?
By vessel radius so that the regional blood flow and mean arterial pressure are ideal
What are the intrinsic blood flow mechanisms and what do they control?
- Myogenic response - resistance vessels
- Paracrine factors (metabolities, autocoids) - resistance vessel, venule permeability, venule/vein (capacitance)
- Physical factors (temp, pressure) - resistance vessels, venule/vein (capacitance)
What are the extrinsic blood flow mechanisms?
- Vasodilator nerves - resistance vessels
- Sympathetic vasoconstrictor nerves - resistance vessels, venule/vein (capacitance)
- Endocrine factors (adrenaline, vasopressin) - resistance vessels, venule/vein (capacitance)
Which is the most important extrinsic control?
Sympathetic vasoconstrictor fibres
What is the control hierarchy of vascular regulation?
- Myogenic regulation
- Endothelial secretions; vasoactive metabolites; autacoids
- Extrinsic factors
Why are extrinsic controls needed for blood vessels?
- To regulate overall TPR, hence control blood pressure
- To allow brain to alter blood flow selectively to individual organs (e. g. vasodilatation of secreting salivary gland)
What are sympathetic vasoconstrictor nerves controlled by?
The brainstem
Where are vasoconstrictor nerves found?
- Innervate most arterioles & veins
- Terminate at edge of tunica media, in strings of synaptic varicosities
Describe the mechanism of vasoconstrictor nerves:
- Varicosities release dense-cored vesicles, containing noradrenaline & ATP
- Activates α-adrenoceptors on vascular myocyte
- Causes vasoconstriction
- Fibres are tonically active (1 impulse/sec)
- Fall in activity or α-blockers (phentolamine), causes vasodilatation
What is the result of reduced sympathetic activity on vasoconstrictor nerves?
- Vasodilatation
- Increase in blood pressure → activates baroreflex →
inhibits sympathetic activity → reduced blood pressure
What is the result of increased sympathetic activity on vasoconstrictor nerves?
- Vasoconstriction
- Tissue blood flow is reduced → contraction of local resistance vessels → TPR increase → increase blood pressure
