CVS Histology of the cardiovascular system

Question 1

What factors affect the exchange of substances between capillaries and surrounding tissues?

Answer 1

Area available for exchange: determined by the capillary density (highest in most metabolically active tissues)

Diffusion resistance: determined by the nature of the barrier, the nature of the molecules and the path length

Concentration gradient: most important variable is the flow of blood through the capillary. If blood is not supplied at an appropriate rate the gradients will dissipate

Question 2

What is the normal cardiac output for the average male?

Answer 2

5L/min at rest, increasing to 25L/min during exercise

Blood flow increases to the heart and muscle during exercise, and decreases to the gut.

Question 3

What are the major components of the circulatory system?

Answer 3

The pump: heart
Distribution vessels: arteries

Flow control: resistance vessels (the arterioles) and pre-capillary sphincters
Cardiac output needs to be distributed appropriately by restricting flow to the areas that are easy to perfuse so blood is driven to harder areas.

Capacitance: veins
Capacitance is the ability to cope with changes to cardiac output and is a store of blood that can be called upon when there is a temporary imbalance between the amount of blood returning to the heart and the amount required to pump out.

Question 4

How is the blood distributed between the major parts of the circulation?

Answer 4

~10% arteries + arterioles
~5% capillaries
~15% heart + lungs
~70% veins

Question 5

How are the different types of blood vessel named?

Answer 5

Arteries:
Elastic conducting arteries - expand with each beat
Muscular distributing arteries - branch into arterioles which regulate the amount of blood reaching the tissue

Capillaries:
Capillary walls can be continuous or fenestrated. Both types can be surrounded by pericytes, contractile cells that wrap around the capillary.

Veins:
Thinner walls and wider lumen than arteries.

Question 6

Describe the structure of elastic arteries

Answer 6

Elastic arteries:
Tunica intima has endothelial cells, narrow CT and discontinuous internal elastic lamina.

Tunica media has 40-70 FENESTRATED ELASTIC MEMBRANES with smooth muscle and collagen between the lamellae.

Tunica adventitia has a layer of fibroelastic CT containing vast vasorum, lymphatic vessels and nerve fibres.

Question 7

Describe the structure of muscular arteries

Answer 7

Muscular arteries:
Tunica intima has endothelium, sub-endothelial layer and which elastic lamina.

Tunica media has 40 LAYERS OF SMOOTH MUSCLE CELLS connected by gap junctions and elastic lamina.

Tunica adventitia has a thin layer of fibroelastic CT with vasa vasorum, lymphatic vessels and nerve fibres

There are unmyelinated sympathetic nerve fibres to stimulate vasoconstriction. NA is released at the nerve endings and diffuses through fenestration’s into the tunica media to depolarise the smooth muscle

Question 8

Describe the structure of arterioles

Answer 8

As arteries get smaller the number of smooth muscle layers in the tunica media decreases
Arterioles have 1-3 layers of smooth muscle in their tunica media
In small arterioles the tunica media is composed of a single smooth muscle cell encircling the endothelial cells

Only larger arterioles have thin internal elastic lamina

Question 9

Describe the structure of metarterioles

Answer 9

Metarterioles are arteries that supply blood to capillary beds.
They differ from arterioles because the smooth muscle layer is not continuous, instead the muscle cells encircle the endothelium of a capillary arising from the metarteriole (called the PRECAPILLARY SPHINCTER).

Question 10

Describe the structure of capillaries

Answer 10

Capillaries are made of a single layer of endothelium with a basement membrane.

Continuous capillaries: pericytes form a network on the outer surface of the endothelium, and are capable of dividing into muscle cells, fibroblasts, and wound healing

Fenestrated capillaries: pericytes are present but with pores/fenestrations in the endothelium

Question 11

Describe the structure of venules

Answer 11

Postcapillary venules: more permeable than capillaries, but the wall is similar (endothelial lining with pericytes).

Venules: a tunica media begins to appear as smooth muscle associates with the endothelium.

Question 12

Describe the structure of veins

Answer 12

Veins:
Larger diameter than any accompanying artery and a thinner wall with more CT and fewer elastic and muscle fibres. Well developed adventitia except the superficial veins of the legs which have a defined muscular wall.

Large veins:
e.g. vena cava, pulmonary, portal, renal, jugular, iliac
Well developed smooth muscle in the tunica adventitia (longitudinal) with circularly arranged muscle in the media.