Overview
This lecture sets out why a circulation exists and how it is laid out. It starts from the transport problem (diffusion works only over microns, so bulk flow is needed over the metre-scale distances between lungs and tissues), then describes the two circuits in series with each other, the pulmonary circulation and the systemic circulation, and the mostly parallel arrangement of the systemic vascular beds with two functional exceptions. It then quantifies the system in two ways: where the cardiac output goes at rest and in exercise, and how blood volume, pressure, vessel number and diameter, total cross-sectional area and flow velocity differ between the classes of vessel. It closes with a labelled diagram of the heart.
Course context
- CVS physiology is delivered as 11 lectures, 3 practicals (cardiac cycle, ECG, BP) and 2 cases (haemorrhage, heart failure). Also listed: fainting / rhythm / MI, endocarditis.
- Sources given: HUBS texts (Peterson, Berne & Levy, Guyton, Vander), Ganong Review of Medical Physiology, Physiology at a Glance, and online material (Khan Academy, cvphysiology, Lectorio).
Why you have a cardiovascular system
- To provide the tissues of the body with an adequate supply of oxygen and nutrients.
- To remove unwanted by-products of metabolism from the tissues, carbon dioxide and H+ ions.
- To transport all of these substances, and others such as hormones and “heat”, by bulk flow, because diffusion would not be enough (unless you were an amoeba).
- Figure 1.3 (Levick) makes the distance argument explicit for O2 transport: diffusion operates over short distances at the lungs and at the tissues, bulk flow over the long distances in between.
- Lung capillary to gas: 0.3 µm (diffusion).
- Lungs to tissues: greater than 1 metre, 1,000,000 µm (convective transport, that is flow).
- Tissue capillary to cell: 10 µm (diffusion).
Components and the closed loop
- The heart, and blood vessels: arteries, arterioles, capillaries, venules, veins.
- 4 to 6 L of blood.
- The loop, as drawn: right heart, pulmonary artery, lungs (pulmonary capillaries, O2 taken up), pulmonary vein, left heart, aorta and branches, capillary beds of the body tissues, venae cavae, back to the right heart. The circuit diagram labels the capillary beds of the lungs and of all body tissues as the sites of gas exchange. Colour key: red is oxygen-rich and CO2-poor blood, blue is oxygen-poor and CO2-rich blood.
Pulmonary circulation
- Carries blood from the heart to the lungs and back: right ventricle to pulmonary arteries (deoxygenated), pulmonary capillaries where O2 is picked up and CO2 off-loaded, then oxygenated pulmonary venous blood to the left heart.
- ALL of the right heart output goes through the pulmonary circulation.
- The lungs (bronchi) also receive a blood supply from the systemic circulation, via the bronchial arteries.
- Why a separate pulmonary circuit is a good idea:
- the lungs do not have to compete with the organs of the systemic circulation for blood flow;
- the lungs are close to the heart;
- the pulmonary vasculature has a very low resistance to flow, so it is a low energy circuit.
Systemic circulation
- Carries blood from the heart to the body organs and tissues and back again: the left ventricle ejects oxygenated blood into the aorta and its branches (high pressure), O2 is off-loaded and CO2 picked up at the tissue capillaries, and deoxygenated systemic venous blood returns to the right heart.
Mostly in parallel
- The vascular beds of different tissues are arranged in parallel: brain; head and arms; airways (bronchi); heart (coronary artery); splanchnic circulation (liver, GI tract, spleen); renal system; trunk and legs.
- Only SOME of the cardiac output circulates through any one tissue at a given time.
- Two reasons this makes physiological sense:
- the total resistance of resistances arranged in parallel is much lower than if they were arranged in series;
- blood flow to individual organs can be controlled independently.
Two exceptions, in series
There are 2 exceptions to the “strictly in parallel” rule, for reasons of function.
- Splanchnic circulation: shown in series as gut, portal vein, liver (the highlighted region covers hepatic artery, portal vein and the associated capillaries).
- Renal system (kidney): shown in series. The slides highlight these components of the renal region of the vascular tree, without giving an order between them: renal arteries, afferent and efferent arterioles, glomeruli, peritubular capillaries.
Warning
The detailed vascular-tree schematic (Berne & Levy Fig 15.1) carries no accompanying bullet text on the slide where it is first shown, and is repeated later with no text at all, so it is not further explained in the lecture.
Distribution of blood flow: rest versus exercise
Table 14.1 (Petersen, Lecture Notes: Physiology 5th ed), distribution of blood flow within the systemic circulation at rest and in moderately vigorous exercise. Maximum cardiac output varies between individuals with age, fitness, nutritional status and training.
| Rest (L/min) | Rest (%) | Exercise (L/min) | Exercise (%) | |
|---|---|---|---|---|
| Cardiac output | 5.8 | 100 | 20 | 100 |
| Brain | 0.75 | 13 | 0.75 | 4 |
| Heart | 0.25 | 4 | 0.85 | 4 |
| Gut | 1.5 | 26 | 0.6 | 3 |
| Kidney | 1.0 | 17 | 0.5 | 3 |
| Skeletal muscle | 1.2 | 21 | 15 | 75 |
| Skin | 0.5 | 7 | 1.95 | 10 |
| Other | 0.6 | 10 | 0.35 | 2 |
The lecture annotates the pattern of change with three mechanisms:
- Vasoconstriction of feed blood vessels: gut and kidney (flow falls in exercise).
- Vasodilation of feed blood vessels: heart, skeletal muscle and skin (flow rises in exercise).
- Flow autoregulation: brain (absolute flow is unchanged at 0.75 L/min from rest to exercise).
Flow against oxygen consumption
- In some organs and tissues blood flow correlates well with oxygen consumption: skeletal muscle, liver and gut, brain.
- In others there is less correlation: skin, kidneys, heart muscle. The reasons given are that skin flow serves thermoregulation, kidney flow serves water and waste excretion, and heart muscle is not well supplied with blood, that is, it could really do with a bit more, which makes it vulnerable.
- Resting comparison of the two pie charts:
| Tissue | O2 consumption (%) | Cardiac output (%) |
|---|---|---|
| Liver and gut | 30 | 25 |
| Kidneys | 6 | 20 |
| Brain | 18 | 13 |
| Heart | 10 | 4 |
| Skeletal muscle | 20 | 20 |
| Skin | 2 | 8 |
| Other | 14 | 10 |
Distribution of blood volume
- The highest volume of blood is found in the veins and venules.
- Figure 1.11 (Folkow and Neil, via Levick), distribution of blood volume in a resting man of 5.5 L: systemic veins and venules 60 to 70%, pulmonary circulation 10 to 12%, systemic arteries 10 to 12%, heart 8 to 11%, systemic capillaries 4 to 5%.
- The systemic veins and venules are labelled the capacitance vessels.
- Fig 16.6 (Petersen) gives % blood volume by vessel class at rest, supine. Systemic: aorta 2, arteries 10, arterioles 3, capillaries 6, venules 10, veins 40, venae cavae and right heart 4. Pulmonary: arteries and arterioles 5, capillaries 3, venules and veins 8, left heart 4.
Pressure
- Ventricle: 0 to maximum (the graph shows the left ventricle spiking from 0 to about 120 mmHg).
- Arteries: pulsatile, from diastolic to systolic (large arteries about 80 to 120 mmHg; a dashed trace shows the lower-amplitude pulmonary artery pressure).
- Arteriole: the major pressure drop, with loss of pulsatility. These are the resistance vessels.
- Capillaries, venules and veins: non-pulsatile, at low pressure.
Number and diameter of vessels
- On the arterial side there is only one aorta; the number of arteries increases rapidly with distance from the heart while their diameter falls.
- At the level of the arterioles there are about 0.16 x 10^9 vessels.
- There are about 5 x 10^9 capillaries.
- Diameter of the aorta about 2.5 cm; diameter of a capillary only 8 µm.
- On average there are more blood vessels on the venous than on the arterial side: venules about 0.5 x 10
- On the venous side, as one approaches the heart the number of vessels falls and their diameters rise; there are 2 main veins (venae cavae), diameter about 3.2 cm.
Total cross-sectional area
- The total cross-sectional area of the arterial tree rises from the aorta to the arterioles.
- At the level of the capillaries the total cross-sectional area is stated as 1000 cm². (In the Fig 16.6 graph the peak at the capillaries is plotted at roughly 3000 to 4000 cm² in both circuits.)
- The total cross-sectional area of the venules and veins is higher than that of the arterioles and arteries.
Blood flow velocity
- Blood flow is the same in all parts of the cardiovascular system; blood flow velocity varies.
- Aorta: velocity fluctuates from 0 to a maximum of about 150 cm/sec.
- Arteries: mean velocity about 20 cm/sec.
- Capillaries: slows down, 0.05 cm/sec.
- Veins: speeds up again, 15 cm/sec.
Structure of the heart (Figure 1.4)
Labelled diagram of the mammalian heart; pink denotes oxygenated blood. The two printed label columns of the figure read as follows.
- Left-hand label column: innominate or brachiocephalic artery, ascending aorta, superior vena cava, pulmonary artery, right atrium, coronary sinus, tricuspid valve, inferior vena cava, right ventricle.
- Right-hand label column: aortic arch, to left lung, pulmonary veins, left coronary artery in sinus of Valsalva, left atrium, mitral valve, chordae tendineae, papillary muscle, left ventricle, interventricular septum, apex.
- AoV marks the aortic valve and PuV the pulmonary valve, at their respective valve positions.
Where the course goes next
The lecture closes by posing the questions the following lectures address: what happens each time the heart beats, why the heart beats, how the vascular system provides and controls perfusion, and how the system adapts.
Self-test
- State the three functions of the cardiovascular system given in this lecture.
- Using the distances in Figure 1.3, explain why O2 transport between lungs and tissues cannot rely on diffusion.
- List the classes of blood vessel named in the lecture and give the total blood volume.
- Describe the route of blood through the pulmonary circulation from the right ventricle, saying what happens to the gases at each stage.
- Give the three reasons the lecture offers for a separate pulmonary circulation being a good idea.
- What fraction of right heart output passes through the pulmonary circulation, and what other blood supply do the lungs receive?
- List the systemic vascular beds the lecture names as arranged in parallel.
- Explain the two advantages of arranging systemic vascular beds in parallel.
- Name the two exceptions to the strictly parallel rule, and state the components the slides list for each.
- Compare the flow to gut, kidney, skeletal muscle and brain at rest and in moderately vigorous exercise, in L/min and as a percentage of cardiac output.
- For gut and kidney, for heart, skeletal muscle and skin, and for brain, state the mechanism the lecture annotates on the exercise table.
- Which tissues show poor correlation between resting blood flow and oxygen consumption, and what reason does the lecture give for each?
- Distinguish the share of blood volume held by the systemic veins and venules from that held by the systemic capillaries, and give the term applied to the former.
- Describe how pressure changes from the left ventricle to the veins, and state where pulsatility is lost and which vessels are called the resistance vessels.
- Give the approximate numbers and diameters of vessels at the extremes of the arterial and venous trees.
- Describe how total cross-sectional area changes from the aorta to the arterioles, and give the value stated for the capillaries.
- What does the lecture say about blood flow through the different parts of the CVS, and what velocities does it give for the aorta, arteries, capillaries and veins?
- Predict what happens to the percentage of cardiac output reaching the gut if a person moves from rest to moderately vigorous exercise, and name the vascular change responsible.
Answers
Reveal answers
- To provide the tissues with an adequate supply of O2 and nutrients; to remove the unwanted by-products of metabolism, CO2 and H+ ions; to transport these and other substances such as hormones and heat by bulk flow.
- Diffusion covers only very short distances: 0.3 µm from lung capillary to gas and 10 µm from tissue capillary to cell. The distance between lungs and tissues is greater than 1 metre (1,000,000 µm), which is covered by convective transport, that is bulk flow.
- Arteries, arterioles, capillaries, venules and veins, plus the heart; 4 to 6 L of blood.
- Right ventricle to the pulmonary arteries carrying deoxygenated blood, then the pulmonary capillaries where O2 is picked up and CO2 off-loaded, then oxygenated pulmonary venous blood back to the left heart.
- The lungs do not have to compete with the systemic organs for blood flow; the lungs are close to the heart; the pulmonary vasculature has a very low resistance to flow, making it a low energy circuit.
- All of the right heart output. The lungs (bronchi) also receive a systemic supply through the bronchial arteries.
- Brain; head and arms; airways (bronchi); heart via the coronary artery; splanchnic circulation (liver, GI tract, spleen); renal system; trunk and legs.
- The total resistance of resistances in parallel is much lower than the same resistances in series, and blood flow to individual organs can be controlled independently.
- The splanchnic circulation, shown in series as gut, portal vein, liver (hepatic artery, portal vein and associated capillaries highlighted); and the renal system, whose highlighted components are the renal arteries, afferent and efferent arterioles, glomeruli and peritubular capillaries, listed without a stated order.
- Gut 1.5 L/min (26%) at rest, 0.6 L/min (3%) in exercise. Kidney 1.0 L/min (17%) to 0.5 L/min (3%). Skeletal muscle 1.2 L/min (21%) to 15 L/min (75%). Brain 0.75 L/min (13%) to 0.75 L/min (4%). Cardiac output rises from 5.8 to 20 L/min.
- Gut and kidney: vasoconstriction of the feed blood vessels. Heart, skeletal muscle and skin: vasodilation of the feed blood vessels. Brain: flow autoregulation.
- Skin, kidneys and heart muscle. Skin flow serves thermoregulation; kidney flow serves water and waste excretion; heart muscle is not well supplied with blood, so it could do with more and is vulnerable.
- Systemic veins and venules hold 60 to 70% of the blood volume of a resting 5.5 L circulation, systemic capillaries only 4 to 5%. The veins and venules are the capacitance vessels.
- The ventricle swings from 0 to maximum (about 120 mmHg in the left ventricle); the arteries are pulsatile between diastolic and systolic (about 80 to 120 mmHg); the arterioles produce the major pressure drop and pulsatility is lost there, which is why they are the resistance vessels; capillaries, venules and veins are non-pulsatile.
- One aorta of about 2.5 cm diameter; about 0.16 x 10^9 arterioles; about 5 x 10^9 capillaries of 8 µm diameter; about 0.5 x 10^9 venules; 2 venae cavae of about 3.2 cm diameter.
- It rises from the aorta to the arterioles, and is stated as 1000 cm² at the level of the capillaries (the Fig 16.6 graph plots the capillary peak at roughly 3000 to 4000 cm²). Venules and veins have a higher total cross-sectional area than arterioles and arteries.
- Blood flow is the same in all parts of the CVS while velocity varies: aorta 0 to about 150 cm/sec, arteries mean about 20 cm/sec, capillaries 0.05 cm/sec, veins 15 cm/sec.
- It falls from 26% to 3% of cardiac output (1.5 to 0.6 L/min), through vasoconstriction of the feed blood vessels.