cardiovascular system - blood vessels Flashcards
arteries
transport blood AWAY from the HEART, therefore under high pressure
veins
transport blood TO the HEART
capillaries
exchange transported material with interstitial fluid (ISF) - between vessels and cells
blood vol. distribution
-arteries: 13%
-arterioles + capillaries: 7%
-heart: 7%
-pulmonary circulation: 9%
-veins, venules + venous sinuses: 64%
blood vessel walls
-adventitia
-media
-intima
tunica adventitia structure
-connective tissue
-fibroblasts and fibrous elements
tunica media structure
-circumferentially arranged smooth muscle cells
-elastic fibres
-collagen fibres
tunica intima structure
-endothelial cells
-sub-endothelium (collagen + smooth muscle)
-elastic fibres
arteries
-elastic
-muscular
-terminal
-arterioles
elastic arteries
-largest arteries (closest to heart)
-tunica media contains MORE elastic tissue and little smooth muscle
-absorb pressure wave generated by heart as it beats, converting pulsative flow into smoother continuous flow
muscular arteries
-contain smooth muscle but less elastic fibres
end(or terminal)-arteries (resistance vessels)
-diameter 100-150µm
-sole source of blood to a tissue
-when an end-artery is blocked the tissues it supplies die as there is no alternative blood supply
arterioles (resistance vessels)
-diameter 10-100µm
-tunica media consists almost entirely of smooth muscle
-control release of blood to capillaries
blood pressure (120-20 (mm Hg))
aorta + other elastic arteries > muscular arteries > arterioles > capillaries > venules > veins > venae cavae
anastomotic arteries
-form a link between the main arteries supplying an area
-if one artery supplying an area is blocked these arteries provide a collateral (alternate) circulation
capillaries
-single layer of endothelial cells sat on very thin basement membrane
-basement membrane permeable to water and other SMALL molecules
-diameter varies from 3-4µm to about 170µm (e.g. liver)
types of capillary
-continuous
-fenestrated
-discontinuous (sinusoidal)
continuous capillaries
-diffusion distance around 300µm
-complete endothelial cell lining with tight junctions between them (incomplete so leave intercellular clefts)
-solute exchange can occur through intercellular clefts or caveolae-mediated transport
-occurs in fat, muscle + the nervous system (almost all vascularised tissues)
fenestrated capillaries
-fenestrae (pores) have a diameter of 70-100 nm to allow larger molecules through
-occurs in S.intestine, kidney + endocrine glands (where there is extensive molecular exchange with the blood)
discontinuous capillaries
-capillaries with incomplete basement membranes and larger lumens than usual
-intercellular gaps and large fenestrations
-typically diameters of about 30-40µm
-occurs in liver, spleen + bone marrow
importance of discontinuous capillaries
-blood flows through them MORE SLOWLY under LESS pressure
-can come directly into contact with cells OUTSIDE the sinusoid wall
-allows much faster exchange of substances between blood and tissues
capillary bed
site of exchange of substances between blood and tissue fluid
why is blood pressure low in the venous system ?
BP drops significantly in the capillary beds, so blood passing into venous system does so under VERY low pressure
veins
-same three layers of tissue as arteries
-thinner walls than arteries, but muscular, so can act as a reservoir
-contain valves
venules
-capillaries converge to form tiny venules
-diameter 50-200 µm
-endothelial layer surrounded by longitudinal smooth muscle
-occasional fibroblasts (pericytes surrounding endothelium)
valves
-prevent backflow of blood to ensure one-way flow of blood back to the heart
-formed by a fold of tunica intima
-strengthened by connective tissue
-cusps are semilunar with their concavity towards the heart
veins are called capacitance vessels…
due to their stretchy nature they have the capacity to hold a large proportion of the body’s blood (approx. 2/3 of body’s blood)
importance of capacitance vessels
the vascular system is able to absorb sudden changes in blood vol. to an extent
e.g. in haemorrhage, veins can constrict to help prevent the sudden fall in blood pressure
hydrostatic pressure
PRESSURE in the circulatory system exerted by the VOL. OF BLOOD when it is CONFINED in a blood vessel
osmotic pressure
PRESSURE exerted by the FLOW OF LIQUID through a semi-permeable membrane separating two solutions with DIFFERENT conc. of solute
hydrostatic pressure > osmotic pressure
= fluid leaks out
osmotic pressure > hydrostatic pressure
= fluid seeps in
Darcy’s law
Q = (-k/µL) x △P
Q= flow
k= permeability
A= cross-sectional area
µ= viscosity
P= pressure
L= length
Q = (-k/µL) x △P
thus:
-no pressure difference = no flow
-flow occurs from high pressure to low pressure
-flow is proportional to pressure difference
-only valid for laminar flow
Darcy’s law and its relation to circulation
cardiac output = arterial pressure / total peripheral resistance
types of flow through blood vessels
-laminar
-turbulent
-single-file
laminar flow
-RBC concentrate in middle as flow fastest in the centre of blood vessel (less friction)
-flow slowest near the vessel wall (more friction)
-thickness of marginal plasma layer is 2-4µm so flow profile not exactly parabolic
where does laminar flow occur ?
-arteries, arterioles, venules and veins
-flow in aorta is PULSATILE (blood flow almost zero during diastole)
turbulent flow
-if pressure difference is above a critical point, transition to turbulent flow occurs
-flow now increases only as √pressure difference
-blood flows in ALL directions
where does turbulent flow occur ?
-occurs in the heart ventricles
-and behind atherosclerotic lesions
single-file flow
-diameter of most capillaries = 5-6µm, less than width of RBC (8µm)
-friction between RBC and vessel wall minimised by glycocalyx/thin film of plasma
-efficiency of flow dependent on RBC’s ability to change shape (impaired in some conditions)
-leukocytes stiffer than erythrocytes
where does single-file flow occur ?
-capillaries
resistance to flow is dependent on vessel diameter, which is described by…
Poiseuille’s law
Poiseuille’s law
Q= △Pπr^4 / 8µL
- Q= flow
- △P= pressure gradient along vessel
- r= radius of vessel
- µ= viscosity of fluid
- L= length of vessel
resistance of blood vessels in SERIES (e.g. arterioles)…
R = R1 + R2
resistance of blood vessels in PARALLEL (e.g. in capillaries)…
1/R = 1/R1 + 1/R2
