Cardiovascular and Lymphatic Systems Answers

The cardiovascular and lymphatic systems conform to the same basic structural plan consisting of three layers {or tunicae pl. (L)- coverings or coats}:

This basic plan varies according to:

1. Tunica intima {(L)-"innermost"}consists of:

2. Tunica media {(L)-"middle"} consists of:

3. Tunica adventitia {(L)- "outermost"} consists of:


How does this basic structure vary through the vascular tree?


The heart has the same layers as the rest of the cardiovascular system. Think of it as a large blood vessel! These layers are called the endocardium, myocardium and epicardium. Which tunicae do these terms refer to?


1. Endocardium & Myocardium
2. Electron micrograph of Myocardium


Myocardial ischaemia or ischaemic heart disease

Myocardial ischaemia results from an imbalance between the supply of oxygen and other nutrients and their demand. Reduced supply may be caused by:

Demand for oxygen can increase in certain conditions due to increased cardiac output (eg. thyrotoxicosis) or myocardial hypertrophy.

Ischaemic heart disease is associated with several clinical syndromes:

In the UK, ischaemic heart disease is the largest single cause of death. Ischaemic heart disease is almost always caused by atherosclerosis of the coronery arteries usually with superimposed thrombosis (see the section Arteriosclerosis and Atherosclerosis below).

SELF STUDY:Following myocardial infarct, necrotic cardiac muscle is replaced by fibrous scar tissue. Between 24-72 hours after an infarct, an acute inflammatory response is stimulated with infiltration of neutrophils to the site. Organisation of the site occurs with granulation tissue comprised of relatively cellular and vascularised loose connective tissue replacing the myocardial tissue. A dense collagenous scar is formed over a period of weeks to months. Compensatory hypertrophy of surviving cardiac muscle may occur.

3. Repair of Myocardial infarct


Elastic arteries Structure:
Tunica intima- same as basic plan
Tunica media- main site of differences to other vessels

Tunica adventitia- loose connective tissue carries the vasa vasorum typical of large vessels

4. Aorta H & E
5.High magnification of tunica media
6. Aorta
7. Aorta - Orcien Stain

Muscular arteries

Tunica intima- connective tissue layer is reduced compared to elastic arteries.
Tunica media- contains 10-40 layers of circularly arranged smooth muscle with some elastic fibres.
Tunica adventitia- is normally thick with prominent vasa vasorum.

Remember, there is a gradual transition between elastic and muscular arteries.

Where two or more arteries supply the same region, they or their branches will join to form anastomoses (Gr. opening or outlet- a connection between two vessels) and thereby provide a collateral circulation (ie. an alterative route) which may be important clinically when there is slow blockage of one artery. End arteries are distributing arteries in organs where there is no anastomosing circulation. The central artery of the retina is one such end artery.

AB 8. Coronary artery
Low and high magnification

Arteriosclerosis and Atherosclerosis

Arteriosclerosis (Gr. sklerosis- hardness; 'hardening of the arteries'):

Atherosclerosis (Gr. athere -gruel- denoting fatty degeneration)

Atheroma predisposes to:

9. Coronary artery, Orcein stain
10. Coronary artery, Orcein stain

Arterioles, capillaries and venules



Continuous capillaries

11.Scanning electron micrograph of a continuous capillary (freeze fracture method)
  12. Electron micrographs of a continuous capillary.
b higher magnification.

Fenestrated capillaries

  13. Electron micrograph of a fenestrated capillary in the kidney glomerulus.

Discontinuous capillaries:

14. Liver
  15. Electron micrographs of a liver sinusoid
b higher magnification.


The venous system represents a low pressure system returning blood to the heart. This is for the most part a passive process involving movement of blood along pressure gradients. Veins also have other functions including acting as blood reservoirs, facilitation of leukocyte migration and important roles in inflammatory responses.
Compared to the corresponding arterial vessels, veins and venules have larger lumina and thinner walls.
Although the same three structural layers are present in these vessels, it is more difficult to distinguish the layers since there is generally less muscle and elastin present.


16. Arterioles and venules


Tunica intima- comprises endothelium and a connective tissue layer with a network of elastic fibres (not laminae)
Tunica media- variable:

Tunica adventitia- is normally a thick layer of fibrocollagenous connective tissue in medium and large veins with prominent vasa vasorum supplying blood to both the adventitia and media when extensive.

Valves- found in medium sized veins >2mm in diameter that carry blood against gravity, these are semilunar projections of tunica intima into the lumen of the vessel. These flaps or pockets (usually 2 per valve) are composed of fibro-elastic tissue covered by endothelium. Excessive back pressure and flow of blood is prevented by the filling of the pockets with blood and thereby closing the valve.

 The tunica media in a muscular vein is thin compared with that of an artery of the same size. Usually in arteries the thickness of the wall approximates the lumen diameter.

Fluid in the extracellular (interstitial) space of tissues is mainly derived from capillaries and venules. Some of this returns to the venous circulation but the rest is slowly drained by vessels of lymphatic system and is ultimately returned to the cardiovascular system. Lymph is the fluid within a lymph vessel and comprises excess tissue fluid, dissolved molecules and some cells, particularly lymphocytes.

Lymphatics are endothelial-lined tubes that begin as blind ending vessels. They:

Larger lymphatics resemble venules and veins. They have:

Peristaltic action of the muscular walls of the larger lymphatics pump the lymph into themain lymphatic vessels: the thoracic duct. This duct empties the lymph into the venous system.

The lymphatic system plays an important role in immune function. On its course to the larger lymphatic vessels and ultimately venous circulation, lymph passes through one or more lymph nodes where foreign materials (antigens) in the lymph are sampled and where antibodies and activated cells from the immune system may enter the lymph. Lymphatic vessels also may carry cancer cells from the tissues of their origin to other sites. Regional lymph nodes may trap cancer cells, which continuing to multiply, form secondary tumours (metastasis).

Note that the major difference between the venule (V)and small lymphatic vessel (L) is that the venule has red blood cells within it and the lymphatic doesn't.