Physiology 3120 Lecture Notes - Lecture 33: Venae Cavae, Blood Pressure, Vascular Resistance
15 May 2018
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Lecture 33 - Vascular function (blood vessels)
Cross sectional area and velocity of blood flow
- Blood in the systemic circulation flows from the aorta → vena cava → right side of the heart
- We are constantly branching our blood vessels into more and more vessels
- Consecutive branches of the arterial system become smaller and smaller AND
o The proliferation of more and more blood vessels
o = leads to a massive increase in total cross section area (lumen area of the vessles)
- Aorta is the largest vessel but there is only one of it = total cross sec. area is small
- Capillaries are tiny vessels and one has a very small cross sectional area
o When you consider how many there are, the total cross sectional area is huge
- As you get into smaller vessels, the total cross sectional area increases
- Highest cross sectional area is in the capillaries
- There is a decrease in lumen diameter starting with the aorta (only a single one)
o Vessels divide into smaller ones (arteries)
o After the capillaries the lumen diameter increases
o Vena cava is larger than the aorta
- Blood flow is the CO (at rest: 5L)
o If 5L are leaving the heart at the left ventricle, 5L return at the right side
- Blood flow leaving left ventricle is the same as the blood flow coming back to the right side
of the heart
- Blood flow through one aorta is the same as blood flow through all of the arteries (5L)
o Blood flow through all of the capillaries is 5L
- The same total blood flow (5L/min) passes each level
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- Blood velocity (cm/sec) is different and is steadily decreasing
- Velocity in single vessels (aorta) is very high but as vessels divide into more and more
smaller vessels, the velocity of blood going through each individual vessel decreases
- Blood velocity reaches a minimum in the capillaries
- Where blood velocity is slowest (capillaries)
o Greatest surface area for diffusion
▪ Increasing surface area for diffusion increases the rate of diffusion
o Shortest diffusion distances (walls are very thin)
o = Exchange takes place
o Slow velocity allows for a lot of time for exchange
o Highest total cross sectional area
- After the capillaries, trends then reverse
o Total cross sectional area decreases because vessels reunite (fewer vessels)
o Blood velocity increases because vessels reunite into one vessel
Pressure and resistance throughout systemic circulation at rest
- Systemic circulation: left side of the heart (aorta) → right side (vena cava)
- Fall in pressure in each part of the systemic circulation is directly proportional to the
vascular resistance (resistance the blood encounters as it moves through the vessels)
- Pressure is pulsatile in the aorta and large arteries because:
o Blood is being pumped out of the heart as the heart is contracting during ventricular
systole = systolic pressure
o Diastolic pressure: heart relaxes during ventricular diastole
- Blood pressure continues to drop throughout the circulation until the vena cava in the right
atrium where it drops to almost 0 mmHg
- Aorta and Large Arteries
o Pressure is high and pulsatile
▪ Goes up to 120 (systolic pressure) and drops back to 80 (diastolic pressure)
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o There is not a huge drop in blood pressure is seen because there is very little
resistance that the blood encounters as it moves through the vessels
o Very little resistance to blood flow so the pressure remains high
- Small arteries
o The fall in pressure begins where the resistance to flow begins to slightly increase
o Blood is beginning to encounter resistance, partially because of the smaller
diameter vessels
- Arterioles
o The greatest resistance to flow accounting for about half the resistance in the entire
systemic circulation
o HUGE drop in pressure
o Where blood encounters greatest resistance
- Capillaries
o The pressure drop is about 20 mmHg (slight drop in pressure)
- Venous circulation
o Small drop in blood pressure but significant amount of resistance by the time blood
reaches the right atrium the pressure approaches zero mmHg
o By the time the blood reaches the right atrium, the blood is between 0-5 mmHg
- Pressure drop is directly proportional to vascular resistance
- Total Peripheral Resistance (TPR) = total resistance throughout entire systemic
circulation
o 50% of TPR is in arterioles (LARGE pressure drop)
o 20% in aorta and arteries
o 20% in capillaries
o 10% in veins
Pressure and resistance throughout pulmonary circulation
- The pulmonary circulation is a 'low pressure-low resistance' circuit
- Pressure drops in the pulmonary circulation are very different from the systemic circulation
- Approx. Pressures in Pulmonary Circulation (mmHg):
o Pulmonary artery = 15
o Pulmonary capillaries = 8
o Left atrium = 5
- Pulmonary circulation is a much lower pressure system
o There is still a pressure drop but you do not start nearly as high
- Difference in pressure is a result of the difference in wall thickness of the ventricles
- Left vent wall thickness VS right vent wall thickness
o Right ventricle is not as thick as the left ventricle and cannot generate nearly as
much pressure when it contracts
o Heart does not contract as forcefully and does not generate as much pressure
o Left ventricle can generate more pressure
o Accounts for the huge change in pressure
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