Microanatomy - cardiovascular system Flashcards
Give the basic structure of vessels
Same basic structure consisting of three layers:
Tunica intima - innermost layer, comprised of endothelium
Tunica media - middle layer
Tunica adventitia - outermost layer
Describe the structure and function of the tunica intima of vessels
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
Describe the structure and function of the tunica media
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
Describe the structure and function of the tunica adventitia
Comprised of loose connective tissue
Contains the larger vessels of the vaso vasorum
What are the ‘vaso-vasorum’?
These are nutrient arteries and veins that provide a blood supply to the outer walls of the larger blood vessels
Describe the three layers of the heart in detail
Three layers, like vessels
- Endocardium
Inner endothelium lining of heart
Supporting elastic fibrocollagenous tissue - allows stretch and prevents damage to lining epithelium during heart movement - Myocardium
Comprised of cardiac myocytes interlinked with intercalated junctions
Supported by fibrocollagenous tissue with extensive vascularisation - 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)
Describe the structure of cardiac myocytes
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)
Describe heart disease and how it can occur
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
What are the clinical symptoms associated with heart disease?
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
What is meant by the ‘arterial system’?
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
Describe ‘elastic arteries’
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
Give the structure of elastic arteries
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
State common elastic arteries
Aorta, brachiocephalic, pulmonary, common carotid and other major arteries
Describe muscular arteries and state which arteries tend to be muscular
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
Give the structure of muscular arteries
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
State and describe two common diseases of the arteries that can occur
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
State what is meant by ‘arterial anastomoses’
When two or more arteries supply the same region
Provide a collateral circulation
Describe ‘arterioles’
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
Describe capillaries and briefly describe their structure
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
What are pericytes?
Contractile cells
These can proliferate to form mesenchymal stem cells
These supply new endothelial cells, fibroblasts and myofibroblasts
What are sinusoids?
Large diameter capillaries
These are found in the liver, spleen lymph nodes, bone marrow and endocrine glands
What are the three main types of capillaries?
Continuous
Fenestrated
Discontinuous
Describe continuous capillaries and where they are found
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
Describe fenestrated capillaries and where they are found
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
Describe discontinuous capillaries and where they are found
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
Describe what is meant by the ‘venous system’
Low pressure system returning blood to the heart
Passive process involving movement of blood along pressure gradients
Briefly describe veins compared to arteries
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
Give the functions of veins, other than transporting blood back to the heart
Act as blood reservoirs
Facilitate leucocyte migration
Important role in inflammatory responses
Describe venules and their structure
What are muscular venules?
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
Describe the role of venules
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
Describe the structure of veins
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
Describe the valves that are present in veins
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
