Cardiovascular mechanics 3 Flashcards
What is circulation designed for
Circulation is designed to transport blood around the body (to deliver O2, nutrients and hormones; and to clear CO2 and metabolites) and to regulate temperature
When is diffusion effective
Diffusion is only effective over short distances so a capillary needs to be ~10m from every cell. This necessitates a highly branched structure
What is diffusion essential for
Diffusion is crucial for movement of materials through tissues
Describe the dual circulation
Two circulations, each with its own pump (left and right ventricles) which are physically coupled together (i.e. one organ, two sides)
The two circulations are systemic and pulmonary circulation
How is blood pumped through the circulation
Muscular pump (heart) contracts to increase pressure of blood which generates a pressure gradient that propels blood through a network of tubes (blood vessels).
Which blood vessel provides the short distances required for diffusion
Capillaries
Describe the events in the dual circulation
PUMP- LV- ELASTIC ARTERIES- RESISTANCE- EXCHANGE (BODY)- RESERVOIR (VEINS) - PUMP- RV- ELASTIC ARTERIES- RESISTANCE- EXCHANGE (LUNGS)- RESERVOIR (VEINS)- PUMP
What type of vessels are the veins described as and why
The venous side has a large CAPACITANCE for blood and acts as a reservoir (so contains the largest percentage of blood compared to the arterial side). o During exercise, the reservoir is utilised and the CO increases.
They are highly compliant vessels.
What are arteries and arterioles described as and why
The arteries and arterioles are described as ‘elastic’ as they can vasoconstrict/dilate to alter blood flow.
How do arteries and arterioles regulate blood flow
Small arteries and arterioles have extensive smooth muscle in their walls to regulate their diameters and the resistance to blood flow
Why do veins and venules have valves
The blood is flowing at a low pressure, and thus valves are required to ensure the uni-directional flow of blood
Which blood vessels take up the largest surface area in the cardiovascular system
Capillaries- essential for diffusion
What happens to the volume of blood in each of the vessels
Decreases from arteries to arterioles, increases from arterioles to capillaries- venules- veins
What happens to pressure in the CVS
It decreases as you go around the circuit.
Why do veins and venules store a larger amount of blood
The blood is flowing at a lower pressure
These vessels are highly compliant.
What is meant by compliant
The veins are stretchy, hence they are compliant and can store blood.
What drives the movement of blood
Pressure difference, not absolute pressure
How do we calculate resistance in a fluid circuit (Darcy’s Law)
▪ Resistance is the resistance of all peripheral vessels. 𝑀𝐵𝑃=𝐶𝑎𝑟𝑑𝑖𝑎𝑐 𝑂𝑢𝑡𝑝𝑢𝑡 𝑥 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
Why is Darcy’s Law only an approximation
steady flow (which does not occur due to the intermittent pumping of the heart),
rigid vessels
right atrial pressure is negligible- otherwise it can influence back pressure
How is the regulation of flow achieved physiologically
Physiologically, regulation of flow is achieved by variation in resistance in the vessels while blood pressure remains relatively constant.
Describe the changes in pressure across the circulation
Pressure falls across the circulation due to viscous (frictional) pressure losses.
Small arteries and arterioles present most resistance to flow.
Pressures follow the same pattern in the pulmonary circuit, but at lower pressures.
Why is it essential that the blood vessels are tube resistant
Essential as they can direct the flow of blood, by altering the resistance of the vessels- vascular shunt
What does the resistance to flow depend on
- Fluid viscosity (n, eta)
- The length of the tube (L).
- Inner radius of the tube (r)
What is the equation for resistance to blood flow
𝑅=8𝐿𝜂/𝜋𝑟4
By this equation, resistance is inversely proportional to r4 so doubling the radius, decreases the resistance by 16x.
What is the effect of halving the radius on flow
Halving the radius decreases the flow 16 times
Flow (Q)= MBP/R
▪ So flow is: o Inversely proportional to resistance. o Proportional to the difference in pressure between 2 points
What is the most important factor in the resistance equation
The resistance to blood flow depends upon three variables: o Fluid Viscosity (n) – NOT FIXED (but usually stays constant) o Length of tube (t) – FIXED (length of blood vessel is constant) o Inner radius of the tube (r) – NOT FIXED (main determinant)
The MOST important factor in the equation is the correlation of radius!
What is the difference between cardiac output at rest and during exercise and how is this achieved
Rest: cardiac output approx 5 L/min
Exercise: cardiac output approx 20 L/min
▪ As the arterioles and arteries can vasoconstrict/dilate, they have the ability to reroute blood supply to more appropriate organs. ▪ This can be seen here during exercise were the skeletal muscles receive a ~30fold increase in muscular blood flow
Decreased volume of blood stored in veins by increasing venous return and thus cardiac output is also responsible.
Describe what is meant by laminar blood flow
Velocity of the fluid is constant at any one point and flows in layers
Blood flows fastest closest to the centre of the lumen.
– Blood GENERALLY exhibits this. ▪ In a normal circulation, flow is laminar. ▪ The blood flows in layers or streamlines which rarely interfere with each other. (flows more slowly at sides as the red blood cells encounter resistance as they hit the vessel wall).
Can laminar blood flow be heard
You CANNOT hear laminar flow with a stethoscope, only turbulent flow (sounds of korotkoff). o We utilise this in BP readings.
Describe turbulent blood flow
Blood flows erratically, forming eddys, and is prone to pooling
Associated with pathophysiological changes to the endothelial lining of the blood vessels..
o Changes occur due to changes in shear stress. ▪ Velocity of fluid is NOT constant at any one point
In parts of the vessel (atheroma), this may occur.
Why is the velocity faster in the centre in laminar flow
Note how the velocity is faster in the centre of the lumen due to adhesive forces on the fluid by the vessel lining.
How do we calculate the shear rate
Shear RATE = the velocity gradient (difference between the highest and lowest velocity in the lumen). 𝑆ℎ𝑒𝑎𝑟 𝑅𝐴𝑇𝐸(𝑠)= 𝜕𝑣 /𝜕𝑟
Gradient of tangent to parabolic velocity profile.
How do we calculate shear stress
Shear STRESS = the shear rate multiplied by the viscosity. 𝑆ℎ𝑒𝑎𝑟 𝑆𝑇𝑅𝐸𝑆𝑆(𝜏)= 𝑠n
What is the significance of shear stress
It governs how well endothelial cells work, and consequently laminar flow.
Shear stress disturbs endothelial function which is important for maintaining laminar flow. o HIGH shear stress = GOOD. o LOW shear stress = BAD
What is the effect of high shear stress on the endothelium
Promotes endothelial cell survival and quiescence
Cells aligned in direction of flow
Secretions promote vasodilation and anticoagulation- normal secretions
What is the effect of low shear stress on the endothelium
Promotes endothelial proliferation
Promotes endothelial proliferation, apoptosis and shape change
Secretions promote vasoconstriction, coagulation and platelet aggregation
What can cause blood to change from laminar to turbulent
Shear rate and stress decrease with age — turbulent
Turbulent flow in carotid artery gets worse with age.
Why do we measure blood pressure on the upper arm
Blood pressure usually measured on upper arm as its easily accessed and at heart-level
Describe how we measure blood pressure
Cuff placed on patient’s upper arm
Cuff pressure is increased until it overcomes arterial pressure (120mmHg) hence occluding the artery.
Pressure is then decreased below arterial pressure- turbulent flow- can be heard- each ‘tick’ represents each heart beat-SBP
Cuff pressure decreases until artery (brachial) is fully open- DBP- LAMINAR flow
What is pulse pressure equal to
SBP-DBP
What is Mean Arterial Pressure (MAP) equal to
DBP +1/3PP
Once the aortic valve closes, why does the pressure in the aorta decrease less rapidly than the ventricle
This can be explained by the elasticity of the aorta and large arteries which act to “buffer” the change in pulse pressure.
The elasticity of a vessel is related to its compliance.
The dichrotic notch is due to the rebound phenomenon and is due to the elastic aorta
Why is blood pressure not equal to the difference between DBP and SBP
The cardiac cycle consists of different lengths of times of each phase.
Describe arterial compliance
During ejection, blood enters the aorta and other downstream elastic arteries faster than it leaves them (40% of SV is stored by the elastic arteries)
When the aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation
If arterial compliance decreases (arteries become stiffer; e.g. with age), the damping effect of the Windkessel effect is reduced and the pulse pressure increases.
Describe the Windkessel effect
▪ The dampening effects of the aorta is often called a ‘Windkessel’ effect. ▪ If arterial COMPLIANCE (the elasticity of the arteries) decreases (such as with age, so the arteries become stiffer), the damping effect of the Windkessel is reduced and the PULSE PRESSURE will INCREASE.
Elasticity of the aorta maintains blood flow, expands when blood is fliing it, to maintain diastolic BP
What is flow determines by
o Pressure differences between two points. o Transmural pressure (pressure inside the vessel)
What is transmural pressure created by
Created simply by blood moving through the vessel- puts tension on vessel- Law of LaPlace
What is the relationship between transmural pressure and tension
▪ The relationship between transmural pressure and wall tension can be de determined by the LAW OF LAPLACE. 𝑇=𝑃 𝑥 𝑟
What is the relationship between transmural pressure and volume
The relationship between transmural pressure and vessel volume is then called the compliance (depends on vessel elasticity).
Describe circumferential stress
The circumferential stress (sigma) depends on vessel wall thickness. 𝜎= 𝑃 𝑥 𝑟/ ℎ
What does a persistently high circumferential stress lead to
Persistent high circumferential stress causes vessel distension, which leads to aneurysms.
Describe aneurysms
Over time, vessel walls can weaken causing a balloon-like distension
Pathological example of the Law of Laplace. Vascular aneurysms increase radius of the vessel. This means that for the same internal pressure, the inward force exerted by the muscular wall must also increase.
However, if the muscle fibres have weakened, the force needed cannot be produced and so the aneurysm will continue to expand …
often until it ruptures.
This pathology and the underlying physical forces involved also holds for the formation of diverticuli in the gut.
Describe the compliance properties of arteries and veins
The relationship between the transmural pressure and the vessel volume is called the compliance and depends on vessel elasticity
Venous compliance is 10 to 20 times greater than arterial compliance at low pressures.
The elasticity of veins and arteries vary (compliance) and thus: ▪ Veins are highly compliant at LOW pressures. ▪ Arteries are compliant over a wider range of pressures. This means small changes in venous pressure distend veins and increase blood volume stored in them.
Describe the effect of smooth muscle contraction on venous compliance
When you stimulate the nerves to the smooth muscle of the veins, causing vasoconstriction, you decrease venous volume and increase venous pressure → due to venous compliance.
Describe the effect of gravity on the hydrostatic pressure in the lower limbs when standing
Standing up increases hydrostatic pressure in the legs as a result of gravity
120 cm H20 ~ 80 mmHg
(h·ρ·g) height x density x gravitational
▪ When standing, gravity increases pressure in the lower limbs which due to compliance of veins, increases volume of blood in these vessels (T=Pr). ▪ This reduces CO which if not compensated for = fainting
Why don’t we faint upon standing
However, we don’t faint upon standing up due to a number of mechanisms: ▪ Activation of SNS (sympathetic nervous system) – constricts veins and arteries to increase TPR and maintain BP. ▪ Standing up causes a slight increase in HR to increase contractility to allow more blood to return to the brain.
Describe how the effect of the gravitational force varies in different vessels
At any particular location the pressure gradient from left heart to right heart is maintained to ensure unidirectional flow
The extent to which the force of gravity increases the hydrostatic pressure in the vessels varies with height but can be approximated to ~ 100 mmHg- this is the pressure difference
Major effect of gravity is on distensible veins in leg and the blood within them
Describe the skeletal muscle pump
Muscle Pump – Return of blood in the upright posture is assisted by contraction of skeletal muscle (A) which compresses veins. ▪ Valves in veins only ensure uni-directional flow
Describe the respiratory pump
During inhalation, there is a relative negative pressure in the thorax which effectively sucks blood into the central veins by reducing extra-vascular pressure in the thorax and increasing it in the abdominal cavity (greater negative intrathoracic pressure)(differences in pressure = flow).
What causes varicose veins
Incompetent valves cause
dilated superficial veins in
the leg (varicose veins
What does prolonged elevation of venous pressure cause
Prolonged elevation of venous pressure (even with intact compensatory mechanisms) causes oedema in feet
When does the viscosity of the blood change
In pathophysiology’s and at high heights.
