Microanatomy - cardiovascular system

Question 1

Give the basic structure of vessels

Answer 1

Same basic structure consisting of three layers:
Tunica intima - innermost layer, comprised of endothelium
Tunica media - middle layer
Tunica adventitia - outermost layer

Question 2

Describe the structure and function of the tunica intima of vessels

Answer 2

Innermost layer

Comprised of endothelium:
Regulates transport of substances from the lumen into the vessel wall
Controls local blood clotting via secretion of soluble factors
Enables migration of white blood cells via expression of cell adhesion molecules

Consists of a layer of connective tissue

Most exteriorly, contains an internal elastic lamina:
Separates the intima from the media
This is a well defined fenestrated sheet of elastin which allows for diffusion of substances through the sheet

Question 3

Describe the structure and function of the tunica media

Answer 3

Comprised of circularly arranged smooth muscle - contracts to regulate blood flow
Contains supporting ECM with collagen and elastic fibres - the elastic fibres make up the external elastic lamina and this separates the media from the adventitia

Question 4

Describe the structure and function of the tunica adventitia

Answer 4

Comprised of loose connective tissue

Contains the larger vessels of the vaso vasorum

Question 5

What are the ‘vaso-vasorum’?

Answer 5

These are nutrient arteries and veins that provide a blood supply to the outer walls of the larger blood vessels

Question 6

Describe the three layers of the heart in detail

Answer 6

Three layers, like vessels

1. Endocardium
   Inner endothelium lining of heart
   Supporting elastic fibrocollagenous tissue - allows stretch and prevents damage to lining epithelium during heart movement
2. Myocardium
   Comprised of cardiac myocytes interlinked with intercalated junctions
   Supported by fibrocollagenous tissue with extensive vascularisation
3. Epicardium
   Outer fibrocollagenous tissue
   Large amounts of adipose tissue
   Carries the coronary arteries and autonomic nerves supplying the heart with the outer mesothelium (visceral pericardium)

Question 7

Describe the structure of cardiac myocytes

Answer 7

Short, branching cells
Round, central nuclei
Connected by intercalated disks with mechanical and electrical gap junctions
Stains dark pink with H and E
Has the features of striated muscle (similar to cardiac muscle)

Question 8

Describe heart disease and how it can occur

Answer 8

Occurs from an imbalance between the supply of O2 and other nutrients and their demand

Reduced supply may be caused by mechanical obstruction of blood flow e.g. thormbus of coronary arteries or by a decreased flow of of oxygenated blood e.g. hypotension

Increased demand can be caused by increased cardiac output or cardiac hypertrophy

Question 9

What are the clinical symptoms associated with heart disease?

Answer 9

Angina - chest pains at times of increased myocardial demand
Myocardial infarction - ‘heart attack’ with tissue necrosis due to loss of blood supply
Sudden cardiac death - due to development of cardiac arrhythmia

In the UK, iscaemic heart disease is the single leading case of death and is usually due to atherisclerosis of the coronary arteries

Question 10

What is meant by the ‘arterial system’?

Answer 10

This system comprises of the main vessels that lead away from the heart and distribute blood to the rest of the body
The vessels can be elastic or muscular and the transition between these two types is not abrupt

Question 11

Describe ‘elastic arteries’

Answer 11

Found just downstream of the heart
Acts to prevent large fluctuations in blood pressure occurring from the heartbeat
Expands with systole of the heart and elastic recoils on diastole of the heart - helps to propel blood through the vessels and maintains blood pressure

Question 12

Give the structure of elastic arteries

Answer 12

Tunica intima - is the same as the general tunica intima

Tunica media - has predominance of elastin and relatively little smooth muscle
Consists of high fenestrated elastic sheets which merge with the internal and external elastic laminae
There are smooth muscle cells and collagen sandwiched between the fenestrated elastic sheets
Smooth muscle cells synthesis the elastin and collagen fibres

Tunica adventitia - consists of loose connective tissue which carries the vaso vasorum of the large vessels

Question 13

State common elastic arteries

Answer 13

Aorta, brachiocephalic, pulmonary, common carotid and other major arteries

Question 14

Describe muscular arteries and state which arteries tend to be muscular

Answer 14

Medium to small sized
Conform most closely to the basic structural plan
Characterised by a predominance of smooth muscle in the tunica media
Forms the main distributing branches of the arterial tree e.g. femoral, radial, coronary, cerebral arteries

Question 15

Give the structure of muscular arteries

Answer 15

Tunica intima - reduced connective tissue layer compared to elastic arteries

Tunica media - contains 10-40 layers of circularly arranged smooth muscle and some fine elastic fibres

Tunica adventitia - relatively thick and contains collagen and thicker elastic fibres - these fibres merge with surrounding tissue

Question 16

State and describe two common diseases of the arteries that can occur

Answer 16

Asteriosclerosis
General term for progressive arterial changes
Characterised by the thickening and loss of elasticity of arterial walls
Exaggerated in hypertension
May affect the tunica media/intima - in the media, elastic fibres may be gradually replaced with collagen fibres and calcium salts may be deposited
Can cause a narrowing of the vessels

Athersclerosis
This is the most common pathological variant of the above
Disease of the tunica intima
Characterised by the formation of yellowish, fatty plaques (atheromas) in large and medium sized arteries

Question 17

State what is meant by ‘arterial anastomoses’

Answer 17

When two or more arteries supply the same region

Provide a collateral circulation

Question 18

Describe ‘arterioles’

Answer 18

These are the smallest of the arteries and retain general features of muscular arteries
The last part of these feed into capillaries and are known as ‘terminal arterioles’
At some terminal arterioles, smooth muscle circulates the vessels and forms a PRECAPILLARY SPHINCTER - the state of contraction of these sphincters determines the extent of the capillary bed
These are connected directly to venules via THOROUGHFARE CHANNELS

Question 19

Describe capillaries and briefly describe their structure

Answer 19

Thin walled vessels
Allow exchange of substances between blood and tissues
Consist of single layer of endothelial cells - may contain pericytes
No tunica media and little or no tunica adventitia

Question 20

What are pericytes?

Answer 20

Contractile cells
These can proliferate to form mesenchymal stem cells
These supply new endothelial cells, fibroblasts and myofibroblasts

Question 21

What are sinusoids?

Answer 21

Large diameter capillaries

These are found in the liver, spleen lymph nodes, bone marrow and endocrine glands

Question 22

What are the three main types of capillaries?

Answer 22

Continuous
Fenestrated
Discontinuous

Question 23

Describe continuous capillaries and where they are found

Answer 23

Most common type
Characterised by uninterrupted endothelium sitting on continuous basement membrane

Found in skin, muscle, fat, connective tissue and the brain
In the brain, these form the BBB and are relatively impermeable

Question 24

Describe fenestrated capillaries and where they are found

Answer 24

Characterised by uninterrupted endothelium with circular pores/fenestrae - these allow passage of small macromolecules through endothelium
Commonly have a ‘diaphragm’ - thin, proteinaceous membrane that spans the fenestration (not found in kidney)

Found at sites of extensive molecular exchange with blood e.g. the kidney in renal glomerulus, intestine and endocrine glands

Question 25

Describe discontinuous capillaries and where they are found

Answer 25

The endothelium does not form a continuous lining - there are gaps between the adjacent endothelial cells
Basement membrane is incomplete or absent and the endothelial cells are supported by a fine network of reticulin fibres

Only found in the liver, spleen and bone marrow where blood components interact directly with cells in the organ

Question 26

Describe what is meant by the ‘venous system’

Answer 26

Low pressure system returning blood to the heart

Passive process involving movement of blood along pressure gradients

Question 27

Briefly describe veins compared to arteries

Answer 27

Veins and venules have a larger lumina and thicker walls compared to the corresponding arterial vessels

It is more difficult to distinguish between the three layers as there is much less muscle and elastin present

Question 28

Give the functions of veins, other than transporting blood back to the heart

Answer 28

Act as blood reservoirs
Facilitate leucocyte migration
Important role in inflammatory responses

Question 29

Describe venules and their structure

What are muscular venules?

Answer 29

Originally formed at the junction of a number of capillaries
Distinguished by size and are called postcapillary venules
Initially, they only have pericytes surrounding their endothelium and as they become larger, they have smooth muscle cells in their wall

Have a continuous smooth muscle layer forming the tunica media and an identifiable fibrocollagenous adventitia

Question 30

Describe the role of venules

Answer 30

Venules are generally leaky and so play an important role in fluid and cellular exchange
They are a major site of leucocyte migration into tissues

Question 31

Describe the structure of veins

Answer 31

Tunica intima - composed of endothelium and a connective tissue layer with elastic fibres

Tunica media is variable - in vessels where blood is returning to heart by gravity i.e. from head, smooth muscle content is sparse (meningeal, retinal, cerebral veins) - in areas such as leg, there are thicker, muscular walls to go against gravity and withstand hydrostatic pressure

Tunica adventitia - has a relatively thick layer of fibrocollagenous connective tissue and a prominent vasa vasorum in the medium to large veins

Question 32

Describe the valves that are present in veins

Answer 32

These are found in medium sized veins that carry blood against gravity
Semi-lunar projections of tunica intima into the lumen of the vessel
There are flaps/pockets (usually 2 per valve) composed of fibro-elastic tissue covered by endothelium
These prevent back pressure and back flow of blood by the closing of the valve when the pockets are full