Term 2 Lecture 7: Blood Pressure And Flow Flashcards
(see term 2 lecture 3 diagram of the blood vessels)
Different tissues and their proportions are what decide the properties of blood vessels
Arteries
Serve as rapid-transit passageways to organs and a t as pressure reservoirs
-large diameter
-elastic and fibrous tissue to withstand high pressure (required to deliver blood to arterioles)
- stiff with some flexibility
- pressure reservoirs (low resistance)
-low compliance - not v. stretchy
Arteries as pressure reservoirs
Arteries make sure that the body has continuous flow throughout the vascular tree even when the heart is at rest.
During contraction/emptying:
Blood is pumped into the arteries, amount of blood put into the arteries is greater than what flows out the other end so the artery vessel stretches
At rest/relaxed:
no blood input so to keep blood flowing the stretched wall collapses (constricts) to push blood forward
- blood flow is continuous through the capillaries to the end organs
–driving force for continuous flow during diastole is provided by elastic properties of the arterial wall - systole greater blood flow into arteries from the heart than leaves to flow into the centrioles
- elastic arteries expand to hold this excess volume storing pressure energy in the stretched walls
-diastole stretched walls passively recoil exerting pressure on blood pushing it downstream ensuring flow
Arterioles
- main resistance vessels because their radius is small enough to offer resistance to flow
-marked drop in mean arterial pressure (MAP) across the vessels (93-37mmHg) it is reduced so that the capillary beds are not damaged by force - arterioles maintain flow to end organs
- convert the pulsatile systolic to diastolic pressure in arteries to non-fluctuating pressure in the capillaries creating smooth laminar flow
- radius (resistance) adjusted independently to variably adjust CO to end organs & regulate arterial BP
During vasoconstriction there is increased contraction of circular smooth muscle in the arteriolar wall leading to increased resistance and decreased flow in the vessel
During vasodilation decreased contraction of smooth muscle in the arterial wall leads to decreased resistance and increased flow through the vessel
Smooth muscle is innervated by sympathetic and parasympathetic nervous system determining the vascular tone
Arterioles have the capacity to alter their fiameter
-little elastic connective tissue
- thick layer of smooth muscle
-sympathetic innervation
-chemical sensitive
-circulating hormone-stretch
-smooth muscle:
Contraction = vasoconstriction
Relaxation= vasodilation
Capillaries
- wall is 1 endothelium cell thick
-Exchange of material between blood and tissue cells
-branch extensively to bring blood within reach of essentially every cell
- no carrier mediated transport system (except at blood brain barrier)
- diffusion maintained by minimal distance from cells and maximum area
- thin walled with narrow diameter
3 types:
Continuous - have pores (standard)
Fenestrated - have larger pores (intestine and kidney)
Discontinuous - no close contact between cells (liver)
The 3 types of capillary
Continuous
Substances cross via:
diffusion (O2 & CO2)
pores (Na+, H+, aas, glucose)
Vesicular transport (exchangeable proteins)
Plasma membranes are too large to pass through the wall.
Fenestrated capillaries
Have larger pores to allow more fluid exchange and more CO2/O2 exchange
(In kidneys and intestines)
Discontinuous capillaries
Found in the liver they have huge holes to allow more substance exchange
Velocity (speed) of flow
Slow velocity of flow in capillaries which is ideal for CO2/O2 exchange
Flow rate is identical through all levels of the cardiovascular system and equal to cardiac output
Velocity of flow varies throughout the vascular tree and is inversely proportional to total area of all the vessels at a given level
In one minute an identical volume of blood will move a far smaller distance in the capillaries than in the arteries or veins due to lower velocity. Sometimes for this reason veins and arteries are referred to as rivers and capillaries as ‘lakes’
Blood travels through arteries and veins at the same speed
Veins
-internal diameter same as arteries ~5mm
-walls are half the thickness of the arterial walls
- smooth muscle, elastic and fibrous connective tissue
- valves present in peripheral veins but not in central veins
- one way valves in peripheral veins push blood back to the heart from the extremities particularly important in the legs when standing - varicose veins occur due to problems with these valves leading to blood pooling.
- volume reservoirs
- high compliance (very stretchy)
- small change in pressure causes large degree of stretch
- veins contain more blood than the arteries
Blood pressure - gradient - blood flow
-Blood pressure = force exerted by the blood against the vessel wall
- depends upon the volume of blood in the vessel and the compliance of the vessel
- MAP is the main driving force for blood flow
- MAP=DP+1/3(SP-DP) ~93mmHg
-AP remains closer to diastolic than systolic pressure
- blood flow through a vessel depends on the pressure gradient and vascular resistancr
Different percentages of blood go to the organs depending on what their metabolic demands are
Different at rest compared to during exercise or other cardiovascular challenge (e.g. illness or thin air)
How is the blood distributed?
-Blood is at MAP when it leaves the left ventricle and normally half that pressure on entering the right ventricle - this producing a pressure gradient
-flow rate through a vessel (volume of blood passing through per unit of time) is directly proportional to the pressure gradient (as pressure increases flow increases)
Flow = ∆P/R
∆P= pressure gradient - difference in pressure between the beginning and end of the vessel
R = resistance in the vessels, mean pressure drops as blood flows through the vessel
- contraction of heart imparts pressure to blood
Resistance in blood flow
Directly proportional to:
1)viscosity of the blood
2) vessel length
3) vessel radius
(1&2 should be constant)
R alpha nL/r⁴
(r⁴ = radius⁴)
Smooth vessels have a smooth laminar flow. Cholesterol deposits cause turbulent flow this allows cardiovascular disease to be detected by a stethoscope pressed to a limb - a lub dup sound here suggests congestion - that sound should only be in the heart (sound of turbulent flow caused by shutting valves)
Resistance in the cardiovascular system
Combined resistance of all organs including blood vessels
= total peripheral resistance (TPR)
Arterioles/small arteries are resistance vessels
60% of TPR is from arteries
Low resistance allows blood flow to kidneys and brain
Influence of TPR on MAP
(Google cardiac output at rest compared to during exercise diagram in flow per min showing change during exercise)
Changes in arteriolar resistance change MAP
F=∆P/R
F= flow in systemic/pulmonary circulation = cardiac output
∆P of entire systemic circulation= difference in pressure between beginning and end of systemic circulation
Beginning= MAP
End= central venous pressure (CVP) ~0mmHg
∆P = FxR
MAP= COxTPR
