Microanatomy 1 Cardiovascular Flashcards
what are the three levels of the majority of blood vessels minus the capillaries
tunica intima
tunica media
tunica adventitia
what is the function of the tunica initma
- Regulates the transport of substances from the lumen into vessel wall
- Controls clotting as it secrets soluble factors
- Enables migration of white blood cells through expression of cell adhesion molecules
what is the structure of the tunica intima
- It is a layer of loose connective tissue
- Most exterior it as an internal elastic lamina which separates it from the tunica media
- Internal lamina is a well-defined fenestrated sheet of elastin
- Fenestrations allow diffusion to happen
what is the structure of the tunica media
- Circularly arranged smooth muscle which contracts regulating blood flow
- Supporting extracellular matrix with collagen fibres and elastic fibres
- Elastic fibres make up the external elastic lamina separating the tunica media from the tunica adventitia
what is the structure of the tunica adventitia
- Loose connective tissue
- Larger vessels of the vasa vasorum – these are the nutrient arteries and vein providing a blood supply to the outer walls of the larger blood vessels
what are the layers of the heart wall
endocardium
myocardium
epicardium
what is the structure of the endocardium
- innermost layer
- it has supporting highly elastic fibrocollagenous tissue
what is the function of the endocardium
- Allows stretch and prevents damage to the lining epithelium during heart movement
what is the structure of the myocardium
- Middle layer
- Made up of cardiac myocytes
- Intercalated junctions
- Supported by fibrocollagenous tissue with extensive vascularisation
what is the structure of the epicardium
- Outer layer
- Fibrocollagenous tissue
- Large amounts of adipose tissue that carries coronary arteries and autonomic nerves supplying the heart with an outer mesothelium (visceral pericardium)
what does ischaemia result from
- Results from an imbalance between supply of oxygen and other nutrients and their demand therefore there is a reduction in supply or an increased demand
what type of reduction in supply can cause ischaemia
- Mechanical obstruction of blood flow (for example thrombus or arteritis of coronary arteries)
- Decreased flow of oxygenated blood for example by anaemia or hypotension
what type of increase in demand can cause ischemaeia
- Increased cardiac output for example thyrotoxicosis
- Cardiac hypertrophy
what Clincal syndromes associated with ischaemic heart disease
- Angina – chest pain at times of increased myocardial demand
- Myocardial infarction – heart attack, tissue necrosis due to loss of blood supply
- Sudden cardiac death – due to development of cardiac arrhythmia
ischeamic heart disease is the….
single leading cause of death, usually due to the atherosclerosis of the coronary arteries with superimposed thrombus formation
how is a Myocardial infract repaired
- After an MI necrotic cardiac muscle is replaced by a fibrous scar tissue
- Between 24-72 hours an acute inflammatory response is stimulated with infiltration of neutrophils to the site
- Organisation of the site occurs with granulation tissue comprised of cellular and vascularised loose connective tissue that replaces the myocardial tissue
- Dense collagenous scar is formed over a period of weeks to months
- Compensatory hypertrophy of surviving cardiac muscle may occur
what are the two types of arteries
elastic
muscular
describe the structure of elastic arteries
- dominant amount of elastin and little amount of smooth muscle in the tunica media
- Undergo expansion with each systole of the heart, on relaxation of the heart the elastic recoil helps propel the blood through the vessels maintain blood pressure
what are the elastic arteries
- Include the aorta, brachiocephalic, pulmonary, common carotid and other major arteries
what is the structure of the tunica media in elastic arteries
- Consists of concentrically arranged fenestrated elastic sheets which merge with the internal and external elastic laminae (aorta has 50-75 sheets)
- Smooth muscles cell and collagen are between the fenestrated elastic sheets, the smooth muscle cells synthesise both elastin and collagen fibres
what is the structure of the tunica adventitia in elastic arteries
– loose connective tissue carries the vasa vasorum of the large vessels
what does orcein stain do
stains the fenestrated elastic sheets are stained brown and collagen stained yellow
describe the structure of muscular arteries
- diameter decreases as we follow them more peripherally
- total cross section area increases this leads to a fall in blood pressure - this even outs the differences in systolic and diastolic pressure
- elastic decreases in the arteries
- it is medium to small sized
- has a lot of smooth muscle in the tunica media
- forms the main distributing branches of the arterial tree
what are the examples of muscular arteries
- Femoral, radial, coronary, cerebral arteries
describe the structure of the tunica intima in muscular arteries
- the connective tissue layer is reduced compared to elastic arteries
describe the structure of tunica media in muscular arteries
- Contains 10-40 layers of circularly arranged smooth muscle with some fine elastic fibres
describe the tunica adventitia in muscular arteries
- Thick for the size of the vessel with collagen and thicker elastic fibres merging with the surrounding tissues therefore it has darker labelling when stained with H&E
what is the definition of arteriosclerosis
progressive arterial changes characterised by the thickening and loss of elasticity of the artieral walls
what is the difference between arteriosclerosis and atherosclerosis
atherosclerosis is a variant of arteriosclerosis
what part of the blood vessel layer does an arteriosclerosis effect
- tunica intima
- tunica media - muscle cells and elastic fibres may gradually be replaced by collagen fibres, calcium salts and may also be deposited
how is arteriosclerosis caused
- exaggerated in hypertension
- muscle cells and elastic fibres in the tunica media, and these are gradually replaced by collagen fibres, calcium salts and may be deposited
- loss of elasticity causes increased peripheral resistance and arterial blood pressure with increased risk of rupture
- this narrowing of the vessel results in ischaemia
what part of the blood vessel layer does an atherosclerosis effect
tunica intima
- characterised by the formation of yellowish fatty plaques (atheromas) in large and medium size arteries which evolve through distinct stages
atheroma leads to …
ischaemia - this is due to the narrowing of the lumen vessel
- Endothelial ulceration resulting in thrombus formation, subsequent arterial blockage leads to MI
- Aneurysm formation duet to weakening off the vessel wall
what is atherogenesis
it is the response to injury hypothesis and aims to conceptualise how atheromatous plaques arise, mature and change
describe how atherogenesis works
- initiated by endothelial damage which is induced by factors such as hypertension, hyperlipidaemia or smoking
- the endothelial permeability alters and macromolecules such as low density lipoprotein and cell adhesion factors which result in monocytes and T lymphocyte migration
- the endothelial dysfunction results in the intimal accumulation of lipid laden macrophages and T lymphocytes
- growth factors stimulate the migration of smooth muscle cells from tunica media and these proliferate becoming foam cells that engulf the lipid this results in a raised pale cellular lesion that is called a fatty streak
- this turns into an atheromatous plaque, this results in a development of a fibre cap between the endothelium and accumulating constituents of a lipid rich core
- fibrous cap is made up of the smooth muscle cells and extracellular matrix synsthesed by these cells comprised of dense collagen, elastic fibres and proteoglycans
- beneath and to the side of the fibrous cap and surrounding a lipid rich necrotic core of foam cells extracellular lipid and otters debris are inflammatory cells, more smooth muscle and peripheral neovascularisation
what are plaques
- Plaques are dynamic variable exhibiting cell proliferation, death and degeneration, extracellular matrix remodelling, intra-plaque thrombus formation and organisation and calcification
what are the cells on the surface on the atheroma
endothelial cells
what are the two cell types that accumulate lipid atheroma
muscle cells
macrophages
what size diameter is the arteries becoming the arterioles
0,1mm diameter blood vessels are called arterioles
what is a precapillary sphichter
- at some sites where there are capillaries there are bands of smooth muscle that circles the vessel
- these contract and determine the extent of the capillary bed which participates in transcapillary exchange
what do thoroughfare channels (AV shunts) do
- they connect arterioles directly to venules,
- the relaxation of the arteriole smooth muscle in such vessels cause blood to be shunted away from the capillary bed
what are capillaries
- thin walled vessels that allow exchange of substances between blood and tissues
what is the luminal diameter of capillaries
4-10um
describe the structure of capillaries
- only have a tunica initma therefore they only have a single layer of endothelial cells
- intimal pericytes if present form a discontinuous layer external to the endothelium
- large diameter capillaries are known as sinusoidal capillaries
what are pericytes
Pericytes are contractile cells, in addition following injury these cells can proliferate becoming mesenchymal stem cells that supply new endothelial cells, fibroblasts and myofibroblasts
where are sinusoids present
liver, spleen, lymph nodes, bone marrow and endocrine glands
what are sinusoids
- large diameter capillaries
what are the three types of capillaries
- Continuous
- Fenestrated
- Discontinuous
how does transport occur across the capillary wall
- Diffusion of lipid soluble substances, gases, ions and low molecular weight metabolites occur through the membrane
- Water and small molecules move through clefts between endothelial cells
- If the capillary is fenestrated macromolecules smaller than the size of plasma proteins can pass through the endothelium
- Pinocytotic vesicles and caveolae transporter larger proteins and some lipids via transcytosis
describe continuous capillaries
- Most common type of capillary found in skin, muscle, fat, connective, tissue and brain
- Characterised by an uninterrupted endothelium which sits on a continuous basement membrane
- In the brain continuous capillaries form the blood brain barrier and are impermeable
describe fenestrated capillaries
- Found at sites of extensive molecular exchange with the blood such as the kidney, intestine and endocrine glands
- Characterised by an uninterrupted endothelium possessing circular pores or fenestrae, pores allow passage of small macromolecules through the endothelium
- Have a thin proteinaceous membrane spanning the fenestration – this is not found in the kidney
describe discontinuous capillaries
- Only found in the liver, spleen, and bone marrow where blood components interact directly with cells in the organ
- Endothelium does not form a continuous lining e.g. there are gaps between adjacent endothelial cells
- The basement is incomplete or absent, endothelial cells are supported by a fine network of reticulin fibres
describe the venous system
- low pressure system which returns blood to the heart
- it uses pressure gradients
what are the function of veins
- Act as blood reservoirs
- Facilitate leukocyte migration
- Important roles in inflammatory responses
- Returns blood to the heart
what is the structure of veins in comparison to arterioles
- Veins and venules have larger lumina and thinner walls
- Same 3 structural layers but it is more difficult to distinguish the layers since there is less muscle and elastin present
how do you distinguish venules from capillaries
20-30 um in diameter - post capillary venues
describe the structure of venules
- begin by having only pericytes surrounding their endothelium,
- as they become larger, smooth muscle cells appear in the wall
– Muscular venules (50-100um) have a continuous smooth muscle layer forming the tunica media and identifiable fibrocollagenous adventitia
whats the difference between the venules and arterioles
- Venules have larger Lumina and thinner walls than arterioles
what is the function of venules
- leaky
- postcapilary venules play an important part in fluid and cellular exchange and are the major site of leukocyte migration into tissues, intracellular junctions of venue endothelium are sensitive to inflammatory mediators, these substances increase the permeability of the vessels to proteins and thereby contribute to tissue swelling and comes with inflammation
describe the tunica initma of the vein
- comprised of endothelium and a connective tissue layer with elastic fibres
describe the tunica media of the vein
- In vessels returning blood to the heart by gravity from the head, the smooth muscle is sparse e.g. cerebral meningeal and retinal veins
- Thicker circiular muscular walls are found in vessels in the periphery such as the leg where they return blood against gravity and must stand hydrostatic pressure, saphenous vein extends from the ankle to the groin and is an example of a muscular vein with these stresses
describe the tunica adventitia of the vein
- Thick layer of fibrocollagenous connective tissues with a prominent vasa vasorum in the medium to large veins
where are valves found
- found in medium sized veins that are greater than 2mm in diameter and carry blood against gravity
describe the structure of the valves
- semilunar projections of tunica initma into the lumen of the vessel
- they are flaps or pockets composed of fibro-elastic tissue covered by the endothelium
- Back pressure and flow of blood is prevented by the filling of the pockets with blood thereby closing the valve
