Cardio Respiratory - Week 3 Circulatory Physiology Flashcards
Describe the circuitry of the circulatory system (2)
Consists of vessels arranged in series and parallels
Why is the circulatory system arranged in series and parallels (3)
Has important implications in terms of resistance, flow and pressure in blood vessels
How to calculate series and parallels (2)
Check notes week 3 cardio respiratory
Where is the greatest total cross sectional area? (1)
At the level of capillaries
Where is most of the blood located? (1)
Venous circulation
Describe the velocity of blood flow (2)
Slowest in the capillaries and most rapid in major arteries and the aorta
Describe the blood flow (1)
Equal at each level of the system
Describe the structure of blood vessel walls (4)
Consist of 3 distinct layers:
- tunica adventitia: connective tissues (collagen fibres)
- tunica media (smooth muscle and elastin)
- tunica intima (endothelium (squamous epithelium))
What are elastic arteries (1)
The aorta and its major branches
Describe the structure of elastic arteries (3)
Large diameter and low resistance pathways
Large amounts of elastin in the tunica media, allowing them to withstand and smooth out large fluctuations
Describe the structure of muscle arteries (5)
Deliver blood to specific organs
Smaller than elastic arteries
Diameters in the range 0.3mm - 10mm
Less elastin and more smooth muscle in the tunica media
Less distensible and more active in vasoconstriction - can help direct blood flow to where it is needed
Describe arterioles (4)
Deliver blood to capillary beds
Diameter ranges from 10 micrometer - 300 micrometer
Tunica media is almost entirely smooth muscle
Arteriole diameter regulates blood flow to capillary beds: responds to neural stimuli and local chemical influences
What are capillaries (1)
Smallest blood vessels - diameter 8-10 micrometer
What are the two basic type of capillaries (2)
Continuous capillaries
Fenestrated capillaries
Describe continuous capillaries (3)
Most common type
Endothelila cells have tight junctions between them
Intercellular clefts exist: these allow the limited passage of fluid and small solutes
Describe fenestrated capillaries (3)
Endothelial cells have oval pores or fenestrations
Much more permeable to fluids and small solutes
Found where active absorption and filtrate formation occurs
What are capillary beds (5)
Interweaving networks
Microcirculation - blood flow through capillary beds
True capillaries are the actual exchange vessels
In most beds, a vascular shunt called a metaarteriole bypasses the true capillaries
Capillary flow is regulated by a ring of smooth muscle fibres called a pre capillary sphincter
What are venous vessels (1)
Carry blood from capillaries back to the heart
Describe venules (4)
Smallest of the venous system and formed when capillaries unite
Diameter: 8-100 micrometer
Wall of smaller venules consist solely of endothelium
Larger venules possess a sparse tunica media and tunica adventitia
What are veins? (4)
3 distinct tunicae, but their walls are always thinner and their lumens are always larger than those of the corresponding arteries
Veins accommodate a large blood volume (~65% of the total and are referred to as capacitance vessels
Low pressures require special adaptations to ensure that blood continues to flow towards the heart:
valves (see later)
Describe systemic blood pressure (6)
A fluid driven through a circuit of closed vessels operates under pressure
The closer to the pump, the greater the pressure
Blood flows along a pressure gradient
Pressure results when flow is opposed by resistance
Systemic blood pressure is greatest in the aorta and declines throughout the circulation to reach 0mmHg in the right atrium
The steepest drop in pressure occurs in the arterioles, which offer the greatest resistance to flow
What affects arterial blood pressure (2)
The compliance (distensibility) of the elastic arteries near the heart
The volume of blood being forced into these arteries at a particular point in time
What is the pulsatile flow of blood in the major arteries (2)
Volume of blood being forced into arteries varies during the cardiac cycle
So the arterial blood pressure also varies (increases/decreases)
Pulsatile pressure is progressively damped by the elasticity of the arterial walls and the frictional resistance of the small arteries and arterioles, so that capillary blood flow is essentially non-pulsatile
What is the dicrotic notch? (1)
Closure of aortic valve
What is systolic pressure (BPs) (2)
Highest arterial pressure
Corresponds to the systolic phase of the cardiac cycle
What is diastolic pressure (BPd) (2)
Lowest arterial pressure
Corresponds to the diastolic phase of the cardiac cycle
What is pulse pressure (1)
(BPs - BPd) Typically 40mmHg
What is Mean Arterial Pressure (MAP)? (2)
Calculated as:
BPd + (Pulse Pressure/3)
Typically (80 + 40/3) = 93mmHg
What is aortic stenosis? (1)
Stiffening of the aortic valves
Typically characterised by the pressure at the top of the pressure wave
(check graph in notes week 3 cardiorespiratory)
What is aortic incompetence? (1)
Leaky valve
What is the highest point in pressure cycle? (1)
Systolic pressure - ventricles contracting
What is the lowest point in pressure cycle? (1)
Diastolic pressure - relaxing
Describe capillary blood pressure (2)
At the arterial end of the capillary bed, the blood pressure is about 40 mmHg
At the venous end of the capillary bed, the pressure has dropped to about 20 mmHg
Why is capillary pressure important (3)
It must be low otherwise the fragile capillary walls will rupture
It must be closely controlled since it regulates the extent of filtration of solute-containing fluids into the interstitial space
Describe venous blood pressure (4)
Venous pressure is steady and does not change significantly during the cardiac cycle
The pressure gradient over the whole venous system is only about 20mmHg (as compared to 60mmHg over the length of the arterial system)
Venous pressure is normally too low to ensure that blood is returned to the heart at the same rate as the heart is attempting to pump blood into the systemic arterial system
This imbalance is not sustainable for more than a few heart beats, so functional adaptations exist to promote adequate venous return
What are the factors influencing venous return? (3)
The respiratory pump
The ‘skeletal muscle pump’
Describe the respiratory pump (4)
Inspiration increases abdominal pressure and compresses the abdominal veins
Since venous valves prevent the backflow of blood, this forces blood towards the heart
Since inspiration also decreases thoracic pressure, thoracic veins expand, further aiding the movement of blood towards the right atrium
During expiration, the decrease in abdominal pressure facilitates venous flow from the lower body, and the increase in thoracic pressure forces blood into the right atrium
Describe the skeletal muscle pump (3)
When skeletal muscles (particularly those of the leg) contract, they compress the deep veins and propel blood towards the heart
The valves distal to the point of compression are closed by the back-flowing blood
What is sphygmomanometry? (1)
Sphygmomanometry is used to measure systemic arterial blood pressure
What is Korotkoff sounds? (2)
If an distensible cuff is inflated to a pressure between BPs and BPd, the smooth, laminar flow of blood through the occluded artery is interrupted, resulting in Korotkoff Sounds
What are the two indirect methods of measuring blood flow? (2)
Doppler ultrasound or laser-doppler flow meters
Venous occlusion plethysmography
What is Doppler ultrasound or laser-doppler flow meters? (2)
Velocity: based on the frequency shift of an ultrasound beam projected onto a vessel
Vessel Width: B-mode Doppler
What is venous occlusion plethysmography (2)
At 40 mm Hg venous drainage is occluded but arterial inflow is not affected. The forearm swells change in volume dV / change in time dt = Flow
What is Darcy’s law? (3)
States that flow (Q) (in the steady state) is linearly proportional to the pressure difference (P1 – P2) between two points
The proportionality constant (K), is hydraulic conductance
The reciprocal of K is resistance (R ), which arises from internal friction within the moving fluid
What is mean velocity? (3)
Flow divided by total cross sectional area
Since total cross sectional area increases as the blood enters the microcirculation, mean velocity falls progressively
Total flow is not altered: it remains equal to the cardiac output at each level of the vascular system
What drives blood flow? (1)
An energy gradient
In the right atrium, what is the difference between aortic pressure and venule and why is this important? (2)
Aortic pressure averages about 90mmHg, whereas the pressure in the vena cava is close to zero at the entrance to the right atrium
This creates a pressure gradient (effectively equal to the MAP) which represents the overall driving force pushing blood through the systemic circuit
What is the problem with Darcy’s law? (1)
Darcy’s law predicts that blood should flow from the foot to the aorta through the arteries
What is Bernoulli’s Theory? (1)
Flow between points A and B is proportional to the difference in the fluid’s mechanical energy between A and B
What is pressure energy? (1)
Pressure Energy: Pressure x Volume (PV)
What is potential energy? (1)
Potential Energy: Fluid Mass (Density x Volume V) x Height (h) x Gravitational Force (g). Equals pVhg
What is kinetic energy? (1)
Increases in proportion to the velocity of flow squared (2). Equals pV.v^2/2
What is mechanical energy? (1)
Mechanical Energy (per unit volume) = P + pgh +pv^2/2
How does the mechanical energy differ from aorta to the feet? (1)
The greater potential and kinetic energy of blood in the aorta outweighs the greater pressure in the arteries of the foot
What are the different flow in circulation and where do they occur? (3)
Laminar flow - occurs in normal arteries and veins
Turbulent flow - occurs in the ventricles and, sometimes, in the ascending aorta of healthy subjects & sites of stenosis
Single-file flow - occurs in the capillaries
Describe laminar flow and give the equation (3)
The lamina in contact with the vessel wall is held stationary by molecular cohesive forces (i.e. it has 0 velocity)
Check notes week 3 cardio respiratory
Describe turbulence and give Reynold’ds number (3)
Turbulence is encouraged by high fluid velocity (), large tube diameter (D) and high fluid density () and discouraged by high fluid viscosity ()
Check notes week 3 cardio respiratory
Describe single-file flow (3)
Check notes week 3 cardio respiratory
What is resistance to flow? (3)
Resistance (R) to steady flow along a straight, cylindrical tube is proportional to tube length (L) and fluid viscosity () and inversely proportional to tube radius raised to the 4th power (r4)
Check notes Week 3 cardio respiratory
What is Poiseuille’s Law? (3)
Check notes week 3 cardio respiratory
Describe arteriole resistance (3)
The greatest fall in pressure is in the arterioles, therefore, the greatest resistance to flow is in the arterioles, despite the large total arteriole XSA (cross-sectional area)
Check notes week 3 cardio respiratory
How is resistance to blood flow in a circuit arranged? (3)
Collectively, the arteries, arterioles, capillaries, venules and veins are arranged in series
With the exception of the aorta and pulmonary trunk, each vessel is arranged in parallel with other vessels of the same type
Hence, resistances to blood flow are arranged in both series and parallel circuits
Describe resistance in series and parallel circuits (2)
Total resistance in a circuit increases if series units are added
Total resistance in a circuit decreases if parallel units are added
What is compliance and give the equation for compliance? (2)
The compliance of a vessel is defined as the change in volume per unit change in distending pressure (stretchability of the vessels)
The distending pressure acting on a vessel is the pressure inside minus the pressure outside (i.e. the transmural pressure)
Check notes Week 3 cardio respiratory
Describe compliance in arteries and veins and draw graph (4)
Veins have a higher compliance (or capacitance) than arteries because they are thin-walled and easily stretched
This means that they can accommodate a large increase in blood volume in response to a small increase in blood pressure (meaning that they are good at storing volume)
Veins therefore act as volume reservoirs, unlike arteries which function as pressure reservoirs
Check notes week 3 cardio respiratory
Describe the force in the blood vessel wall equation (3)
Check notes week 3 cardio respiratory
What is the law of LaPlace and give the equation (2)
Check notes week 3 cardio respiratory
How does wall tension compare against pressure, radius and wall thickness (3)
Wall tension increases with internal pressure and vessel radius and decreases with increasing wall thickness
Describe the thickness of the walls in arteries and why this is the case (2)
In large arteries, Pressure and resistance are large, so the wall needs to be thick in order to compensate
Describe the thickness of the walls in capillaries and why this is the case (2)
In capillaries, Pt is quite low and r is very small; this allows the walls to be very thin
Describe the thickness of the walls in veins and why this is the case (2)
In veins, the Pressure is low, although resistance is still large: since the walls are thin, significant tension is still generated
Describe where vessel rupture is greatest (1)
The likelihood of vessel rupture is greatest in the elastic arteries (e.g. the aorta): Rupture of the aorta is a relatively common (and usually fatal) medical event
Define microcirculation (2)
Defined as the circulation of blood through the smallest blood vessels: the arterioles, capillaries and venules
Where are capillaries most dense? (1)
Density is highest in metabolically active tissues such as skeletal muscle
What does blood flow in capillary depend on? (1)
Blood flow in capillaries is not uniform and depends primarily on the contractile state of the arteriolar smooth muscle
What is auto regulation? (2)
The intrinsic adjustment of blood flow to a tissue or specific vascular bed such that the flow meets the local requirements at any given point in time
How are changes in local blood flow brought about? (2)
By varying the diameter of the arterioles supplying the local capillary beds
By altering the degree of contraction of the precapillary sphincters which regulate flow through the true capillaries
What are intrinsic control mechanisms classed as? (2)
Metabolic
Myogenic
Describe metabolic control (and its relationship with blood flow) and draw the graph (2)
As the rate of metabolism increases blood flow increases
Since this relationship is observed when the perfusion pressure is kept constant, and maintained in the absence of autonomic nerve input, it is characterised as an intrinsic property of the microcirculation
Look at notes week 3 cardio respiratory
Describe O2 as a factor in metabolic auto regulation (2)
When the metabolic rate of a tissue increases such that local O2 consumption exceeds delivery, a local hypoxia results
This causes a relaxation of the nearby arteriolar smooth muscle
Look at notes week 3 cardio respiratory
What are the other factors that are involved in metabolic auto regulation? (2)
Products of metabolism
Substances synthesised within the vascular endothelium
How are products of metabolism involved in metabolic autoregulation (2)
Products of metabolism (such as CO2, H+, K+ and adenosine) diffuse from the surrounding tissue and cause relaxation of vascular smooth muscle
How are the substances synthesised within the vascular endothelium involved in metabolic autoregulation (2)
Substances synthesised within the vascular endothelium (e.g. prostacyclin and nitric oxide) also diffuse into the adjacent smooth muscle, where they usually mediate vasodilation.
What is the endothelium-derived relaxing factor (EDRF)? (1)
Nitric oxide (NO)
What is myogenic? (1)
Relates to the muscle
Describe myogenic control (4)
As pressure increases, flow increases
At some point, flow plateaus somewhat
Then as pressure continues to rise, flow increases
Since Flow = (Pressure Gradient/Resistance), the observed relationship indicates that the increased pressure gradient must be met by an increase in resistance if flow is to remain constant
Describe the mechanism of the myogenic response (3)
Single-unit smooth muscle responds to passive stretch with an opposing contraction
This response keeps tissue perfusion fairly constant in the face of most variations in systemic arterial blood pressure
Look at week 3 cardio notes
What is long-term auto regulation (2)
If the nutritional and/or oxygen demands of a tissue chronically exceed delivery, long-term autoregulation may develop over a period of weeks to months
The response is due both to an increase in the number of microcirculatory vessels supplying blood to the tissue (angiogenesis), and to the enlargement of existing vessels
What is transcapillary Solute Exchange? (1)
Exchange of substances between tissue and capillaries
What are lipophilic solutes? (2)
Including O2 and CO2
Enter or leave the capillary via the transcellular route
What are Hydrophilic solutes? (2)
Can cross through the intercellular clefts, which have a diameter of ~60Å
What can and what cannot cross the clefts of capillaries? (2)
Whilst these clefts can easily be traversed by water, ions and small organic solutes, albumin (70Å diameter) and other plasma proteins cannot cross
Describe the Exchange of Fluid between Capillaries and Tissues (4)
The distribution of fluid between the plasma and the interstitial fluid is in a state of dynamic equilibrium
Hydrostatic pressure
Osmotic pressure
Look at week 3 cardio notes for further details
What are the starling forces? (1)
Hydrostatic and osmotic forces acting on capillaries
What does the lymphatic system do? (1)
Responsible for returning the fluid (along with any leaked plasma protein) due to starling forces back to the circulation
What are the 3 basic functions of the lymphatic system? (3)
It transports interstitial fluid back into the vascular system
It transports fat from the small intestine to the blood
Its cells (lymphocytes) contribute to the body’s immunological defences
Describe the lymphatic capillaries (2)
These microscopic, closed-ended tubes form vast networks in the intercellular spaces
Their walls consist of endothelial cells with porous junctions, allowing fluid to enter down a pressure gradient, along with proteins, micro-organisms and absorbed fat
Describe the larger lymphatic vessels (2)
The walls of these vessels are similar to the venous walls; they also contain valves to prevent backflow
Fluid movement occurs as a result of peristaltic contractions of the vessel walls and is assisted by the skeletal muscle pump
Describe lymphatic drainage (3)
Most of the systemic circulation is drained by lymphatic vessels that converge to form the thoracic duct, which drains into the left subclavian vein
The pulmonary circulation drains via the right lymphatic duct into the right subclavian vein
Describe the link between Interstitial Fluid Pressure and Lymph Flow (3)
Linear relationship between the hydrostatic pressure exerted by the interstitial fluid and the rate of lymphatic drainage
This relationship breaks down if the accumulation of interstitial fluid leads to abnormally high pressures. Under these circumstances, lymphatic drainage is inadequate and oedema may result
Check notes for week 3
Describe oedema (2)
Oedema occurs when the rate of fluid filtration out of a capillary bed exceeds the ability of the local lymphatic drainage to return the fluid to the vascular system
Why does oedema occur (2)
It results either from an abnormally high rate of fluid filtration, or from a decreased rate of lymphatic drainage
What are the 4 major causes of oedema (4)
Hydrostatic oedema
Low plasma oncotic pressure
Permeability oedema
Impaired lymphatic drainage
Describe hydrostatic oedema (5)
Commonly caused by venous congestion or occlusion
Left-sided heart failure leads to increased left atrial and pulmonary venous pressures. The resulting increase in pulmonary capillary hydrostatic pressure causes excessive fluid filtration in the pulmonary vascular bed, leading to pulmonary oedema
Right-sided heart failure causes similar problems in the systemic vascular beds (particularly in the lower extremities and the digestive system)
Localised venous occlusion causes a localised oedema (e.g. thrombophlebitis)
Other causes of oedema (3)
Low plasma oncotic pressure
Permeability oedema
Imapired lymphatic drainage
Check notes week 3
