4: The CV System II Flashcards
arterial system
high pressure system delivering blood away from heart
arterioles = smallest branches of arteries that lead to capillary beds
arterioles useful for blood pressure regulation as they can change drastically in their size
capillary bed
smallest blood vessels with very thin walls (one cell thick for diffusion)
location of exchange between blood and interstitial fluid
thoroughfare channels within capillary bed provides more rapid exchange between arteriole and venous system
precapillary sphincters are muscular structures at opening of capillaries to control rate of blood flow into capillary (stops high pressure)
arteriovenous anastomosis are low resistance channels to bypass the capillary bed, in skin for rapid control of blood flow during temp regulation
venous system
venules are smallest branches of veins that collect blood from capillaries to return to heart
not under high pressure so blood moves slower
nearly 2/3 of blood at any given moment at rest sits in systemic venous system
blood vessel structure
blood vessel walls have three layers: tunica intima, tunica media, tunica externa
common between veins and arteries but have different characteristics
tunica intima
inner layer
encompasses an endothelial lining and connective tissue layer
in arteries it has an internal elastic membrane providing passive elasticity
endothelium
damaged endothelium = more likely to get clots
endothelium provides non-stick surface
chambers of heart have endothelium constant with major blood vessels
releases vasoactive substances which affect vascular tone, blood pressure and blood flow
maintains vascular homeostasis
vasodilators: nitric oxide
vasoconstrictors: endothelin
tunica media
middle layer
contains concentric sheets of smooth muscle in loose connective tissue, accompanied by elastic fibres in arteries and collagen in veins
in arteries, between the tunica media and tunica externa there is an external elastic membrane
tunica externa
outer layer
anchors vessel to adjacent tissues
contains collagen fibres, elastic fibres and in veins smooth muscle cells
vasa vasorum are small arteries and veins in walls of large arteries and veins
types of artery
elastic artery = elastic fibres, e.g. aorta, only artery with internal elastic membrane
muscular artery = thick tunica media, less elastic fibres, femoral artery is example, can change greatly in diameter
arterioles = only made of tunica media, endothelium and basement membrane
capillaries
endothelial tube into thin basement membrane
no tunica media or externa
diameter similar to that of red blood cell
different capillaries for different roles: continuous, fenestrated, sinusoid
continuous capillaries
found in all tissues except epithelia and cartilage
have complete endothelial lining
permit diffusion of water, small solutes and lipid soluble materials
specialised continuous capillaries in CNS and thymus that have very restricted permeability
fenestrated capillaries
have pores in endothelial lining
permit rapid exchange of water and large solutes
found in endocrine glands, kidneys, intestinal tract, choroid plexus
sinusoid capillaries
have gaps between adjacent endothelial cells
permit free exchange of water and large plasma proteins
found in liver, spleen, bone marrow (RBCs move into CV system), endocrine organs
phagocytic cells monitor blood at sinusoids (engulf foreign cells)
venous system
veins collect blood from capillaries and return it to heart
compared to arteries, veins having large diameter, thins walls, low BP
large veins = thick tunica externa
medium veins = less smooth muscle
venules = very thin walls, only has endothelium and tunica externa
venous valves
made of folds of tunica intima
prevent backflow of blood
compression of veins from skeletal muscle pushes blood back to heart (skeletal muscle pump)
when walls of veins near valve weaken, varicose veins may result
veins
smooth muscle present in vein wall is appropriate for allowing autonomic control over blood flow and pressure
the smooth muscle in veins can vasoconstrict in order to help with propulsion of blood back to heart
also assisted by the skeletal muscle pump: muscles contract around veins compressing the veins to move blood
blood flow
total capillary blood flow = cardiac output = stroke volume x bpm (L/min)
is determined by pressure and resistance in CV
pressure
generated by the heart to overcome resistance
absolute pressure is less important than pressure gradient
pressure gradient = the difference in pressure from one end of a vessel to the other
flow (F) is proportional to the pressure gradient divided by resistance
circulatory pressure must overcome total peripheral resistance (resistance of entire CV system)
change in pressure across the systemic circuit is about 85mmHg (change from aorta to capillary bed)
total peripheral resistance affected by: vascular resistance, blood viscosity, turbulence
vascular resistance is due to friction between blood and vessel walls, depends on vessel length and diameter
adult vessel length is constant, vessel diameter varies by dilation
normal pressure = 120/80 (systolic/diastolic)
hypertension = abnormally high BP (above 140/90)
hypotension = abnormally low
responses to exercise
increased HR = increased cardiac output
increase in systolic B
redistribution of blood flow
- blood flows to tissue in proportion to their metabolic demands
- major proportion of exercise cardiac output diverts to active muscles
- increase from 7ml per 100g of muscle at rest to 75ml during exercise
regulation of HR
at rest - parasympathetic nervous system dominates via vagus nerve, slows HR by inhibiting SA and AV node
sympathetic nervous system increases HR by stimulating SA and AV node via cardiac output accelerator nerves
low resting HR due to parasympathetic tone
increase in HR once exercise starts - initial increase due to parasympathetic withdrawal (up to approx. 100bpm), later increase to SNS stimulation
changes in stroke volume
increased force of contraction = increased stroke volume
two main ways:
1. increase SNS activation - effects circulating adrenaline and noradrenaline, direct stimulation of heart muscle
2. increased end diastolic volume (the amount of blood in ventricles before it contracts), leading to increased stretch of sarcomeres and increased force of contraction
training may improve left ventricle compliance
for end diastolic volume - the frank-starling mechanism states that “the force of contraction is proportional to fibre length”
changes in cardiac output
cardiac output increases due to increased HR
linear increase in HR to max
increased SV but plateaus at 40% of VO2 max
no plateau in highly trained subjects
cardiac output can increase to about 35 L/min in highly trained endurance athletes, about 20-25L/min in untrained
high in males - higher SV, lower HR
