CR2.5 Microvascular structure and function

Question 1

Describe the nutritional and non-nutritional functions of the microcirculation.

Answer 1

Nutrition of tissues:

• Delivery of oxygen and nutrients and the removal of waste products such as carbon dioxide
• Regulate blood flow and tissue perfusion thereby affecting pressure and responses to inflammation (i.e. edema –> swelling)
• Capillary density propotional to tissue metabolic activity (e.g. extremly dense in the lungs, high in liver, muscle, heart, kidney, and relatively low in adipose tissue)

Non-nutritional:

• Glomerular capillaries of kidney (cleaning the blood)
• Temperature regulation of skin (arteriovenous anastomoses)
• Cell signalling
• Host defense

Question 2

List the mediators responsible for vasoconstriction and vasodilation of arterioles.

Answer 2

Vasoconstriction:

• SNS; adrenoreceptors - alpha 1 and 2
• Circulating catecholamines - ß2
• ATII
• ADH
• enothelin-1

Vasodilation:

• local chemical factors
• NO
• inflammatory mediators
• *withdrawal of SNS stimulation

Question 3

Describe some features of metarterioles and precapillary sphincters.

Answer 3

Metarterioles are larger than a capillary and provide a short-cut through vascular bed. Discontinous VSMC layer and usually not innervated.

Precapillary are characterised by wisps of non-innervated smooth muscle which responds to local conditions. Reduction in oxygen is the major stimulus for relaxation (i.e. opening).

Question 4

Describe the differences between smooth muscle and skeletal muscle.

Question 5

Define Fick’s law.

Answer 5

Diffusion of small, water soluble solutes as well as gases (e.g. O2, CO2) across a capillary wall depends on both the permeability and the concentration gradient. Three important variables are the surface area (A), concentration difference, and thickness of barrier to diffusion.

Question 6

Define bulk flow and describe the differences between arterial and venous systems.

Answer 6

Bulk flow is the mass movement of fluids into and out of capillary beds that requires a transport mechanism far more efficient than mere diffusion –> movement of water AND solutes due to pressure gradient.

Filtration –> volumes of fluid moving from an area of higher pressure in a capillary bed to an area of lower pressure in the tissues.

Reabsorption –> movement of fluid from an area of higher pressure in the tissues to an area of lower pressure in the capillaries.

Two types of pressure drive this movement; hydrostatic pressure (Pc) and osmotic pressure (πc).

Pc is the main variable that acutely determines bulk flow (i.e. NFP).

N.B. Edema can result from congestion and congestion can be cause by increase venous pressure, obstructions within and outside veins.

Question 7

Describe the process of angiogenesis and relevant promoters and inhibitors.

Answer 7

The development of new vessels is called angiogenesis. E.g. endurance exercise increases angiogenesis to increase diffusion to meet metabolic demand. Angiogenesis depends on a balance between positive and negative regulators.

Promoters:

• VEGF
• FGFs
• Angiopoietin-1

Inhibitors:

• Endostatin
• Angiostatin
• Angiopoietin-2