Lecture 11: The vascular tree Flashcards
What are the control points for distribution of blood into the capillary beds?
Arterioles
What is the hydraulic filter? Function?
Combination of elastic conduits and high resistance terminals
Converts intermittent output of the heart to steady flow to capillaries (smoothing of blood flow)
Importance of compliance of aorta
During cardiac contraction, the aorta expands
During relaxation, the aortic valve closes, allowing maintenance of pressure in the arterial tree.
Potential energy is stored in the aortic wall
What happens if the aortic valve leaks?
Regurgitation
Pressure in the arterial tree is lost
Aorta compliance and ageing
As people get older, their aorta gets stiffer and stiffer (non-expansive)
Normal natural ageing event
What does a rigid aorta result in?
Less compliance
Most stroke volume will flow through the capillaries during systole with minimal flow during diastole.
This is inefficient and makes the heart work harder
What is vessel compliance?
aka Capacitance
Distensibility of a vessel
Increase in volume caused by a given increase in pressure
compliance = delta V / delta P
Effect of age on vessel compliance
Aortic compliance decreases with age. (delta V decreases)
The heart in an elderly person is less able to eject stroke volume into the arterial system, despite normal cardiac function.
Peak arterial pressure occurs progressively later in systole.
The rapid ejection phase is significantly prolonged vs that of a younger person’s arterial system.
Blood pressure throughout the circulatory system
See figure
Why is the minimum pressure in the aorta higher than that of the ventricles?
Aortic valve saves the pressure in the aorta (conserves energy)
Why is there higher systolic pressure in the systemic arteries than the aorta?
Phenomenon referred to as systolic amplification
Causes reflectance of pressure wave
Not completely understood
Why is there a drop in pressure at the arterioles?
Arterioles have a lot of muscle, allow little blood through when constricted
Potential energy is low
What happens to arterial pressure during diastole?
falls slowly as the aorta imparts it’s stored PE back into the system.
Blood continues to flow to the capillaries during diastole.
Why does mean arterial pressure or MAP (dotted lines in diagram) fall by a negligible fraction in the large and medium arteries?
1) They have larger diameter
2) Don’t offer much resistance to flow
3) little frictional drag
MAP as distance from heart increases
MAP decreases with distance from the heart
Blood moves to capillaries down a pressure gradient
Pulse pressure =
Systolic P - diastolic P
MAP =
MAP = Pd + 1/3(Ps-Pd)
Table showing BP in system
- note systolic amplification
See figure
Factors that determine MAP
Physical factors: Mean blood volume and Arterial compliance
Physiologic factors: cardiac output, peripheral resistance
What is blood volume?
A product of cardiac output and the rate of outflow from arteries to the resistance vessels
What is a pressure wave?
During cardiac ejection (a relatively violent event) a pressure or pulse wave
is transmitted down the aorta and its branches, with a relative velocity that is several times faster than actual blood flow.
What does taking the pulse at the wrist detect?
Detection of pressure wave and is due to the movement of the arterial wall as it absorbs the shock of cardiac systole
Variation of volume of pressure wave
Varies inversely with vascular compliance
How is the arterial blood pressure measured?
By a sphygmomanometer
Rubber bad is wrapped around upper arm over the brachial artery
One tube connects the inside of the bag with a manometer containing mercury
Another tube connects the inside of the bad to a hand operated pump and a release valve
How to use a sphygmanometer
Air is pumped into the rubber bag until the pressure in the cuff is greater than the pressure in the artery during systole
Artery is now closed during systole AND diastole
By releasing the valve, the pressure in the cuff is gradually reduced until maximum pressure in the artery just overcomes pressure in the cuff. Some blood begins to spurt through during systole, but artery is still closed during diastole
At this point, faint, rhythmic tapping sounds begin to be heard through the stethoscope
The height of mercury in millimetres is taken as the systolic blood pressure
Pressure in the cuff is reduced still further until it is just less than the lowest pressure in the artery towards the end of diastole. Blood flow is unimpeded during systole and diastole and the sounds stop. The height of the mercury indicates the diastolic blood pressure.
What are capillaries?
tubes of endothelial cells that branch from the arterioles
sites of O2 and nutrient exchange between the blood and cells.
Abundance of capillaries
Very abundant
Offer large surface area
How does exchange occur in the capillaries?
By diffusion
Enhanced by large surface area and increased time for diffusion (low velocity)
RBCs slowly squeeze through the thin-walled capillaries
See figure
Resistance of flow in capillaries vs arterioles
Resistance of flow is low in capillaries
Adjustment of capillary and arteriole calibre
Capillaries cannot adjust flow
Rely on arterioles for adjustments
What do capillary pores allow?
enable passage of small, water-soluble substances that can’t cross endothelial cell walls
Ions, glucose and amino acids permeate the pores
Where are the pores located in capillaries?
Pores are present at cellular junctions between endothelial cells.
See figure
How do lipid soluble substances cross the capillaries?
Dissolve through the lipid bilayer
Capillary pore size
differs with tissue type
Liver capillary pores are large
Brain capillary pores are non-existent.
Do proteins cross through the capillary pores?
Proteins are generally excluded
Except in the liver capillaries
What substance enlarges capillary pores?
Histamine
What structures are in between arterioles and capillaries?
Metarterioles
Where are the pre capillary sphincters? What do they do?
Surround capillaries
Contraction of sphincters reduces blood flowing into capillary bed
Myogenic tone is high (capillaries do not carry blood during resting conditions)
See figure
What is the pre capillary sphincter muscle sensitive to?
Metabolic changes
What is the interstitial fluid? How is it a site of exchange?
The interstitial fluid (IF) is an intermediary between the blood and tissue cells.
Exchange between RBC’s and target cells is via the IF
How much of the ECF is interstitial fluid?
80 %
Other 20% is blood plasma
Type of exchange between interstitial fluid and plasma membranes
Active: carrier-mediated
Passive: diffusion down concentration gradients
What is bulk flow? What substances cross the capillary wall this way?
Constituents in a fluid move through the wall in bulk (no discrete movement of single components)
Fluid is pushed through the wall from higher pressure to lower pressure
Na, K, glucose, amino acids
What is ultrafiltration?
fluid (ultrafiltrate) moves from the capillary lumen (site of higher hydrostatic pressure = p) to the interstitial fluid (IF) outside the capillary (pi-p
See figure
What happens to plasma proteins during ultrafiltration?
Plasma proteins remain in the capillary, unable to pass through the pores
What is reabsorption?
Reverse bulk flow (IF to capillary lumen): inward- driving pressure exceeds an outward opposing pressure across the capillary wall.
What are the driving forces for bulk flow?
Hydrostatic (fluid) pressure
Colloid osmotic pressure (oncotic pressure)
What is the significance of bulk flow?
It regulates the distribution of ECF between plasma and IF
What is the outward pressure at the arteriolar end of the capillary?
High
What is the inward pressure at the venular end of the capillary?
Greater than the outward pressure
See figure
Importance of lymphatic system
There is more fluid ultrafiltrated than fluid reabsorbed, so there is extra fluid, which is removed by the lymphatics
What is lymph?
Interstitial fluid that enters a lymphatic vessel
Contains escaped plasma proteins and bacteria that are not reclaimed by the blood plasma
How is lymph propelled into larger lymphatic vessels?
Skeletal muscle and smooth muscle propels lymph into larger lymphatic vessels
Lymph is eventually combined with venous blood near the heart
Functions of the lymphatic system
Defense against disease by the lymph nodes
Transport of absorbed fat from the digestive tract
What is edema?
When there is excessive interstitial fluid in tissues?
Causes of edema?
Reduced concentration of plasma proteins (starvation)
Increased permeability of the capillary walls… allows more plasma proteins to pass from blood fluid to interstitial vessel
Increased venous pressure (blood dams up in the venous side, increases venular capillary hydrostatic pressure = less reabsorption)
Blockage of lymphatic vessels (filariasis is a parasite that invades the lymph and causes elephantiasis)
Role of veins
Blood flows from the capillaries to the veins.
Veins collect blood for its return to the heart.
Resistance in veins
Veins have large radii and offer low resistance to the flow of blood
Velocity of blood flow in veins vs capillaries
The velocity of blood flow increases in the veins (vs capillaries), as cross-sectional area is relatively low.
Thickness of vein walls
Thinner than arteries
Can be distended
Capacitance vessels, serve as blood reservoir (hold 60% of blood volume)
What is venous return?
volume of blood entering each atrium per minute.
What is the mean pressure of the venous circulation?
17 mm Hg
What are the three main components of the venous system?
Sphlanic, cutaneous and muscle veins
How are capacity and pressure of veins adjusted?
Adjusted by contraction of smooth muscle within their walls
Splanchnic veins are particularly active in this function
Control central blood volume
See figure
What is the central blood volume?
Effective regulation of blood volume in the right heart, pulmonary vessels and left heart
Controlled by systemic veins
Compliance of veins vs arteries
Arterial walls are thicker and less distensible vs those of systemic veins.
Veins are, on average, more distensible than arteries by a factor of 8.
What does compliance of veins allow them to do?
Veins are important as blood reservoirs
They readily alter their net volume capacity with only slight alterations in intraluminal pressure.
Five major factors for enhancement of venous return?
- Increased sympathetic stimulation of the veins
- Increased skeletal muscle contraction
- Valves inside veins prevent return of blood to point of constriction by muscle pump - blood does not flow backward
- Cardiac suction
- Respiration (breathing)
See figure
How does increased sympathetic stimulation of the veins enhance venous return? What mediates this?
contraction of the smooth muscle in the wall of the veins = venoconstriction.
Stored blood is mobilized to the heart.
a-adrenoceptors mediate smooth muscle cell contraction.
How does increased skeletal muscle contraction enhance venous return?
Skeletal muscle contraction compresses veins and increases venous pressure/venous return.
NB in exercise and to counter the effect of gravity.
Gravity contributes a great deal to absolute pressure
How does cardiac suction enhance venous return?
Heart creates suction by expansion of the ventricular chamber due to the presence of “internal resistor” within the cardiac extracellular matrix.
How does respiration (breathing) enhance venous return?
Creates subatmospheric pressure during inspiration in the chest cavity (5 mm Hg less), drawing blood toward the heart.
Influence of gravity on pressure
Pascal’s 3rd law for hydrostatic fluid at rest
p = sigma x g x h
sigma = density (g/ml)
g = force produced by acceleration of gravity
h = height of column (cm)
Pressure in the arteries below heart level is higher by a factor of (sigma)gh (gravitational potential)
potential energy is essentially = pressure.
See figure
Where is arterial pressure usually measured?
Measured at a point at the same level as the heart
(arm if sitting)
Because gravity has such a strong effect
