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