Blood Vessels (M1 13/11/17) Flashcards Preview

Case 3: Hypertension > Blood Vessels (M1 13/11/17) > Flashcards

Flashcards in Blood Vessels (M1 13/11/17) Deck (18)
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These are the LO's dodo brain

LO: How does the composition of a blood vessel wall relate to its function? 

LO: Can you describe the resistance to flow, the changes in pressure and the distribution of blood volume in different parts of the circulation?

LO: What are the effects of local and external factors on arterioles and tissue fluid formation?




What are the equations for blood flow?

  • Blood flow is proportional to the pressure gradient. But blood flow is also affected by resistance. So blood flow is the pressure gradient (diff in pressure between the aorta and vena cava) divided the resistance (R = 8Lη/ πr4)
  • So overall (after cancelling out) blood flow is Pressue Gradient x Radius to the Power Four.


What is the equation for calculating resistance in a tube?

Resistance equals 8 times length of the blood vessel times the viscosity of blood/ 3.14 times radius to the power 4 (R = 8Lη/ πr4)

Since the 3.14, 8, length and viscosity do not change, then the most important factor in determining resistance is radius of the blood vessel. If we remove the 'constants' the R is 1/ radius to the power four.

If we substitute this into the flow equation, flow is equal to pressure gradient divided by one over radius to the power four. A neater way of writing this is flow equals pressure gradient times radius to the power four.

If radius (r) doubles then:

Resistance  (R) decreases 16 fold  and

Flow  (Q) increases 16 fold



What is the effect of radius on resistance to blood flow?

The narrower the vessel, the faster the velocity of flow, but not the rate of flow. Think of it as straws. What really makes blood flow faster is a straw with a large radius, and one that is short.



  • Form chief resistance to flow in systemic circulation

  • Pressure drop across the arterioles is larger than across any other class of vessel

  • Control Total Peripheral Resistance 

  • Major role is to match local blood flow to local need. Vasodilate = wider straw = greater flow

  • Surrounded by smooth muscle, contracts, resistance increases, blood flow decreases.



What is the myogenic response?

When pressure increases, small blood vessels like capillaries have a safety mechanism to prevent damage and keep flow constant with constant pressure. The smooth muscle contracts, this slows blood flow, greater resistance.


What is metabolic control of tissue blood flow?

  • Aids blood flow when excercising.
  • Metabolites produced when excercising, see below, cause vasodilation, increasing blood flow to muscles.
  • –­ high CO2

    –­ high H+ (decreased pH) - lactate

    –­ high K+

    –­ high osmolality

    –­ high temperature

    – low oxygen


What are autonomic vasomotor nerves?

Vasomotor refers to actions upon a blood vessel which alter its diameter. More specifically, it can refer to vasodilator action and vasoconstrictor action.

Sympathetic fibres:  Most blood vessels are stimulated by sympathetic nerve fibres.  Noradrenaline binds to alpha-one-adrenergic-receptors on vascular smooth muscle, causes vasoconstriction.

  • If noradrenaline and alpha receptor, will get increase in calcium
  • If have noradrenaline and beta receptor then cyclic AMP is the secondary messenger.

Sympathetic vasodilator fibres: 

Sympathetic cholinergic (use ACh) e.g. found in arterioles of skeletal muscle (alerting response), external genitalia and sweat glands

Parasympathetic vasodilator fibres​:

Parasympathetic cholinergic. Lead to NO production  e.g. found in the salivary glands


What is blood flow proportional to?

Blood flow is proportional to the pressure gradient.


What is the pressure gradient in the systemic circulation caused by?

In the case of the systemic circulation, the pressure gradient is the difference in pressure between the vena cava and the aorta.


What is blood flow affected by?

But flow is also affected by resistance.


Where is blood?

  • Heart 8-10 %
  • Lungs 10-12 %
  • Systemic veins 60-70 %


What is 'tone' in blood vessels?

  • Vascular tone refers to the degree of constriction experienced by a blood vessel relative to its maximally dilated state.
  • Vascular tone is determined by many different competing vasoconstrictor and vasodilator influences acting on the blood vessel. 
  • The primary function of extrinsic factors is to regulate arterial blood pressure by altering systemic vascular resistance, whereas intrinsic mechanisms are important for local blood flow regulation within an organ. Vascular tone at any given time is determined by the balance of competing vasoconstrictor and vasodilator influences.


What local factors modulate vascular tone?

  1. Myogenic response-vasoconstriction
  2. Metabolites – vasodilation


What external factors modulate vascular tone?

  1. Nerves  (Autonomic nervous system)
  2. Hormones: Circulating & Local (NO)


Why are capillaries ideal for efficient exchange of metabolites & gases and important sites for fluid transfer?

  • Narrow, short and thin with large SA for  exchange

  • Velocity of blood (mm/sec) is low because of large SA. This gives 1-2 sec for rbc in vessel, slow so exchange can happen.



Why do capillaries have low resistance despite having a small radius (thin straw)

  • Length is short (250-750 mm)
  • Vessels are arranged in parallel


Describe veins and venules

  • Major role is to act as a blood reservoir: at rest up to 70% of blood in the veins. They are a reservoir because they easily stretch without a great increase in pressure, have high compliance.
  • Veins can constrict  due to ANS - sympathetic supply, ( alpha-adrenergic receptors). This pushes blood back to heart (helps venous return)
  • This is also aided by one way valves. They are important in posture, exercise and haemorrhage.