Cardio 5 – The Microcirculation

Anil Chopra

1.  Describe the branching structure of the microvasculature. List the three types of capillary and order them in terms of their permeability to water and small lipophobic solutes.

(1)  Arterioles

v  Main regulators of blood flow rate

o  Volume of blood passing per unit of time.

o  Proportional to pressure gradient

o  Inversely proportional to resistance

v  FLOW = CHANGE IN PRESSURE ÷ RESISTANCE

v  Pressure gradient ΔP à the difference in pressure between the beginning and end of a vessel. Higher pressure gradient = faster flow rate.

v  Resistance R – hindrance to blood flow

o  Increase in resistance decreases flow rate

o  Depends on blood viscosity

o  Depends on vessel length – longer vessels = greater resistance.

o  Depends on vessel radius – smaller radius = greater resistance.

v  Resistance = 1/radius4

v  Arterioles are major resistance vessels

o  Blood enters arterioles at 93mmHg

o  Blood leaves arterioles at 37mmHg

v  They have smooth muscle which can be innervated with sympathetic stimulation à contraction = vasoconstriction

Relaxation = vasodilation

(They are normally part constricted)

v  This can change by local (intrinsic) control i.e. to get more or less blood flowing to a particular organ depending on the organ’s metabolic activity.

o  Can include stretch response – increased arterial stretching results in myogenic constriction.

o  Temperature change – colder temperatures cause vasoconstriction.

o  O2 tension – decrease in O2 concentration causes vasodilation of that organ’s vessels.

v  Can also regulate arterial blood pressure by extrinsic mechanisms:

o  Neural control by medulla

o  Hormonal control by adrenaline, vasopressin and angiotensin II all causing vasoconstriction.

(2)  Terminal Arterioles

v  Blood entering them then passes through the precapillary sphincter.

(3)  Capillaries

v  Deliver the metabolic substrates to the tissue by diffusion. About 7μm in diameter and allow red blood cells to squeeze through in single file. No cell is more than 0.01cm away from a capillary.

v  3 types of structure

o  continuous – narrow filled pores at junction

o  Fenestrated – endothelium has small gaps called fenestrae which allow fluid exchange.

o  Discontinuous – have intracellular gaps in the capillary which can allow proteins and red blood cells through e.g. bone marrow.

(4)  Pericytic postcapillary venules

v  Have no smooth muscle, their walls have pericytes.

(5)  Venules

v  30-50μm in diameter, have smooth muscle.

2.  Describe the factors controlling capillary blood flow. Explain the functional importance of capillary density.

Capillary blood flow controlled by precapillary sphincters:

Ø  Only 10% of precapillary sphincters are open at rest.

Ø  During exercise precapillary sphincters relax.

Capillary Density: how many capillaries per g of tissue. The more metabolically active a tissue is, the denser the capillary network e.g.

skeletal muscle 100cm2/g

Myocardium and brain 500cm2/g

Lung 3500cm2/g

Increasing capillary density increases the area available for gas exchange.

3.  Identify the different mechanisms via which solute is transported between blood and tissue (depending on size and lipid solubility). Explain how the starling forces influence fluid transfer across the capillary wall.

  • Lipid soluble substances readily pass through endothelial cells.
  • Small water soluble substances (ions and glucose) can diffuse through the gaps in the endothelium.
  • Large water soluble substances (proteins) are transported via vesicles.

2 forces acting at capillary wall (STARLING FORCES)

·  Hydrostatic Pressure: this is the high pressure generated by the heart acting against the low interstitial fluid pressure, this is highest at the arterial end of the capillary network.

·  Osmotic pressure: this is the high water potential of the interstitial fluid and the low water potential of the blood. This causes an inward net movement at the venous end of the capillary network.

4.  Describe the origin of lymph fluid. Describe the branching structure of the lymphatic system. Understand how clinical oedema arises.

v  Lymph vessels permeate every tissue in the body. Contain one-way valves which open to allow fluid to drain into the vessels.

v  Lymph nodes are lined with macrophages and lymphocytes which phagocytose debris and microbes.

v  Their pumping action is induced only by the movement of skeletal muscle.

v  3L/day returned to the circulation (120ml/h)

v  Vessels are blind ended.

v  If the rate of tissue fluid production exceeds the ability of the lymph vessels to drain the fluid, oedema results…

(1)  if capillary pressure increases because of blockage of venous drainage.

(2)  if blood oncotic pressure decreased due to kidney disease, starvation.

(3)  if interstitial fluid oncotic pressure increases because local infection increases the capillary permeability to proteins.