Physiology 3120 Lecture Notes - Lecture 35: Blood Pressure, Pulmonary Artery, Blood Vessel
2 May 2018
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Human Physiology Lecture 35
Vascular Function
General Organization
- Two Circulatory Systems:
o 1. Pulmonary Circulation; carry blood to + from lungs
o 2. Systemic Circulation; carry blood to + from rest of body
- Tissue beds are arranged in both parallel system & in series with one another
Cross sectional area and velocity of blood flow
- Mainly systemic circulation
- Aorta large arteries small arteries arterioles capillaries venules veins vena cava
o BLOOD FLOW IS LEFT TO RIGHT
- Lumen = inner diameter of the vessels
o Decreasing lumen diameter starting off with the aorta (big vessel), then begins to increase
again after the capillaries
▪ Keep dividing into smaller and smaller vessels
▪ NOTE: VENA CAVA IS LARGER THAN THE AORTA
- If you have 5L leaving the heart from the left ventricle, then 5L has to come back to the heart
o No blood leaves, and no extra blood comes in
- Consecutive branches of the arterial system become smaller and smaller
o Constantly branching the vessels into more and more vessels
- The proliferation of more and more blood vessels
- Combination of DECREASING vessel size, but INCREASING vessel number
- Leads to a massive increase in total cross section area
o Total cross section area: taking into account ALL the blood vessels
▪ Aorta: large vessel but there is only one of it, therefore the total cross sectional area
is going to be small
▪ Capillaries: very tiny vessels, but since there are SO many, the total cross sectional
area is huge
- As you progressively get to smaller vessels, the total cross sectional area increases
o Reach a maximum in the capillaries
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- The same total blood flow (5L/min) passes each level
o Blood flow leaving the left ventricle is almost the same as the blood flow coming back to the
right side of the heart
o Blood flow through 1 aorta is going to be same as the blood flow in all the arteries
▪ ALL HAS TO BE 5L
- Blood velocity (cm/sec) is steadily decreasing (since same total blood flow passes each level of
vasculature)
o Aorta: velocity in the single vessel is going to be VERY high
o As the vessels divide into smaller vessels, the velocity of the blood is going to decrease
o Blood velocity reaches a MINIMUM in the capillaries
o Note: there is the reverse trend on the venous side of circulation, with the velocity increasing
from capillary to vena cava
o Velocity is highest in: aorta + large veins
- Where blood vel. is slowest (in the capillaries)
o The greatest surface area
▪ Increasing the SA of diffusion, increases the rate of diffusion
o The shortest diffusion distances
▪ The capillaries are VERY thin – do not have a great distance for molecules to diffuse
across the capillaries
o Exchange takes place!
▪ Blood is moving very slowly through the capillaries lots of time for exchange of
carbon dioxide, oxygen, nutrients, waste
▪ Area has a very large area for the diffusion to take place
- Trends then reverse
o Total cross sectional area decreases
o Vessels are beginning to reunite
o Get fewer vessels
o Blood velocity begins to increase – vessels are reuniting as one vessel
Pressure and Resistance Throughout Systemic Circulation
- Fall in pressure in each part of the systemic circulation is directly proportional to the vascular
resistance (diameter of the vessels)
- Aorta and Large Arteries:
o Very little resistance to blood flow so the pressure remains high
o Pulsatile – 120, dropping to 80
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▪ Heart is contracting = systolic pressure (120)
• Systolic pressure is directly related to the force of contraction
▪ Heart is relaxing = diastolic pressure (80)
• Diastolic pressure is related to the resistance the blood encounters as it
flows throough the blood vessels
o Not a huge drop in blood pressure because there is VERY little resistance the blood
encounters as it moves through the vessels
o Since the heart is pumping blood to the aorta, the pressure is VERY high; 100 mmHg average
- Small arteries
o The fall in pressure begins where the resistance to flow begins to slightly increase
o Blood encounters resistance; blood coming from smaller diameter vessels
- Arterioles
o The greatest resistance to flow accounting for about half the resistance in the entire systemic
circulation
o A LOT of resistance
- Capillaries
o The pressure drop is about 20 mmHg
▪ Slight drop in blood pressure – some resistance encountered
- Venous circulation
o Small but significant amount of resistance
▪ Small pressure drop
o By the time blood reaches the right atrium the pressure approaches zero mmHg
▪ Between 0-5 mmHg
- As the blood passes through the systemic circulation, pressure falls progressively to almost 0 mmHg
by the time it reaches the right atrium
- Pressure drop is directly proportional to vascular resistance
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Pulmonary circulation; carry blood to + from lungs: 2. Systemic circulation; carry blood to + from rest of body. Tissue beds are arranged in both parallel system & in series with one another. Cross sectional area and velocity of blood flow. Aorta large arteries small arteries arterioles capillaries venules veins vena cava: blood flow is left to right. If you have 5l leaving the heart from the left ventricle, then 5l has to come back to the heart: no blood leaves, and no extra blood comes in. Consecutive branches of the arterial system become smaller and smaller: constantly branching the vessels into more and more vessels. The proliferation of more and more blood vessels. Combination of decreasing vessel size, but increasing vessel number. As you progressively get to smaller vessels, the total cross sectional area increases: reach a maximum in the capillaries. Where blood vel. is slowest (in the capillaries: the greatest surface area, the shortest diffusion distances.
