Flashcards in Session 5 Deck (27):
What is velocity in relation to blood vessels?
Rate of movement of blood passing along a vessel
What is laminar flow?
Flow where there is a gradient of velocity from the middle to the edge of the vessel: velocity is highest in the centre and fluid is stationary at the edge
What is turbulent flow?
Layers of fluid move past eachother too fast so the velocity gradient breaks down and fluid tumbles over causing resistance to increase
What is viscosity?
The extent to which fluid layers resist sliding over each other
What effect does diameter of a vessel have on flow rate?
Increasing diameter increases velocity so flow rate increases: mean velocity is proportional to cross sectional area
What effect does viscosity have on resistance?
As viscosity increases resistance increases
What effect does radius have on resistance?
As radius increases resistance decreases
What is the effect of combining flow resistances in series?
The resistances of the individual tubes summate
What is the effect of combining flow resistances in parallel?
The reciprocal of the total resistance is equal to the sum of the reciprocal of the resistances of the individual tubes
What is the pattern of flow resistance in the systemic circulation?
- Arteries are low resistance
- Arterioles are high resistance
- Venules and veins are low resistance
What is the pattern of pressure through the systemic circulation?
- Pressure drop in the arteries is small
- Pressure drop in the arterioles is large due to the high resistance
- Pressure drop in the venules and veins is small
What is the relationship between arteriolar resistance and arterial pressure at a constant flow?
As arteriolar resistance increases, arterial pressure also increases
What are the advantages of having distensible blood vessels?
Transmural pressure across the vessel walls allow a them to stretch so resistance falls and flow increases, making it easier for blood to flow through them
How does vessel distensibility produce capacitance?
As vessels widen more blood flows in and out and vessels can store more blood; capacitance. Veins have the highest capacitance as they are most distensible
What factors affect systolic pressure?
- How hard the heart pumps
- Total peripheral resistance
- Compliance of arteries
What factors affect diastolic pressure?
- Systolic pressure
- Total peripheral resistance
What is the pulse pressure and what is its typical value?
- The difference between systolic and diastolic pressure
- Typically 40mmHg
How is average pressure calculated?
Diastolic pressure plus 1/3rd pulse pressure
What is total peripheral resistance?
The sum of the resistance of all the peripheral vasculature in the systemic circulation
What property of arteries acts to reduce arterial pressure fluctuations between systole and diastole?
Elasticity: walls can distend in systole so pressure doesn't rise too much, and recoil in diastole to allow flow to continue until the next period of systole
What factors affect vasomotor tone?
- Sympathetic innervation of alpha1 adrenergic receptors produces vasomotor tone
- Tone is antagonised by vasodilator factors
- Actual tone is determined by the balance between each factor
How do metabolites modify vasomotor activity and why?
- Metabolically active tissues produce metabolites that act as vasodilator a (e.g. Hydrogen, potassium and adenosine)
- Metabolites act to relax smooth muscle in the vessels so resistance lowers and blood flow increases
- Increased blood flow provides metabolically active tissues with more blood to remove toxic metabolites
What is reactive hyperaemia?
- If a tissue has had blood supply cut off then restored a large amount of blood will enter the tissue
- Tissue will continue producing metabolites to dilate vessels so maximum blood flows to the vessels when perfusion is restored
How can vasodilation be auto-regulated?
- Increased metabolite concentration causes vasodilation
- Increased vasodilation increases blood flow so metabolites are removed at a faster rate
- When the tissue metabolism decreases less metabolites are produced so vessels become less dilated
What is central venous pressure?
The pressure in the great vessels supplying the heart
What is venous return?
The rate of blood flow back to the heart, it acts to limit cardiac output