The Cardiovascular System

3.2a The physiology of the cardiovascular system

This chapter focuses on the cardiovascular system, also known as the circulatory system. Like Section 3.1a, this section is divided into two parts. If you are using this book as a study course, then, because of its length, you are advised to work through the two parts in separate study sessions.

The blood in arteries is under high pressure because it has just been pumped from the heart. This is why a severed artery will spurt blood in pulses. It is from the arteries, and not the veins, that a Chinese medical practitioner can sense the state of Blood and Qi when taking the pulse.

One important disease of arteries, arterial thrombosis, has already been described in this text (see Section 2.2d).

VEINS

In contrast to arteries, the blood within the veins is under much lower pressure, and it is necessary for veins to contain valves to ensure that the flow of blood back to the heart remains one-way. Veins rely on the pumping action of the muscles to aid in the movement of blood back to the heart. This is partly why sedentary people may develop swollen ankles at the end of the day. Fluid accumulates in the tissues of the lower parts of the body when there is no motive force to propel it upwards against the force of gravity.

One important potentially life-threatening venous disease, deep vein thrombosis (DVT), has already been described (see Section 2.2d). The more common and benign condition of varicose veins is described later (see Section 3.2c).

CAPILLARIES

Capillaries are minute vessels with walls that are only one cell thick. A significant characteristic of capillaries is that they enable the rapid transfer of nutrients, gases and waste products to and from the blood across their very thin walls.

Capillaries were introduced in Section 2.2b, where inflammation was described. In inflammation, the gaps between the cells in the capillary wall widen, making the capillaries more permeable to tissue fluid.

The transfer of nutrients from the capillaries to the tissues

Healthy blood that has just passed through the digestive system will contain a high concentration of basic nutrients, including simple sugars, cholesterol, triglycerides, fatty acids, amino acids from proteins, vitamins and minerals. Once the blood has passed through the capillaries of the lungs, it will also contain a high concentration of oxygen. Thus the blood transports both the oxygen and glucose and other simple nutrients that will be used by the mitochondria of every cell in the body to produce the energy required for many vital cellular processes.

The body relies on the transport processes of diffusion, osmosis and active transport for nutrients and oxygen to enter the bloodstream from the tissues of the digestive system and the lung. These processes are described in detail in Section 1.1c.

Oxygen, simple sugars, fatty acids, vitamins and minerals all pass with ease through the single-celled wall of the capillary by diffusion. The movement is particularly rapid when it is from a region in which these nutrients are in high concentration to one in which there is a low concentration. For both oxygen in the lungs and nutrients in the tissues of the digestive system this movement enables these substances to readily enter the circulating fluid component of the blood, the plasma.

Conversely, when the nutrient and oxygen-rich plasma reaches other tissues in which the concentration of nutrients and oxygen may be relatively low, the nutrients and oxygen tend to move out of the blood and into the tissues. This transfer of nutrients from digested food in the intestines and of air from the lungs into the tissues happens within a few seconds. Because diffusion is a spontaneous process, this process is incredibly energy efficient, requiring no extra energy over that required by the heart to pump the blood.

In addition to this energy-efficient transfer of oxygen and nutrients into the tissues, another wonderful aspect of the process of diffusion is that it underpins the movement of waste products out of the tissues. As waste products such as urea from protein metabolism and carbon dioxide from respiration build up in the tissues, the tendency is for them to move back into the bloodstream, where they are kept at a relatively low concentration. The waste products can then be carried to the liver, kidneys, sweat glands and lungs, each of which is specially adapted to remove otherwise toxic by-products from the body.

Similarly, water moves into and out of the plasma with ease. In this case, the process is called osmosis. This term describes the movement of water from a region in which it is part of a dilute solution to one in which it is part of a more concentrated solution. This is the means by which water will move from a vase into a wilted (dehydrated) flower stem. In the same way, by osmosis, water will enter the plasma from the tissues if the plasma is more concentrated than the tissue fluid. Conversely, if the plasma is relatively well hydrated, water will move out of the plasma into the tissues. This is another very energy-efficient mechanism, and it ensures that water is transported from the digestive system to the tissues that need it.

One other factor needs to be considered in this description of the movement of nutrients into and out of the bloodstream, and this is the effect of the physical pressure of fluid in the vessels. Just as air in an inflated tire is under a measurable degree of pressure (which is what makes the tire feel inflated), so the blood in a blood vessel is under a degree of pressure. In the blood vessels the internal pressure varies according to the proximity to the heart. Arteries close to the heart contain blood that is being pumped by muscular contractions of the heart, and this blood is therefore under significant pressure. As the arteries branch into more and more numerous smaller vessels, the pressure within them gradually decreases, so that at the start of the capillary bed the pressure is much lower. Nevertheless, at the arterial end of the capillaries the pressure is still sufficient to force some fluid through the gaps between the cells of the capillary walls and into the tissues. This can be compared visually to squeezing an inflated bicycle tube punctured with numerous tiny holes; the greater the pressure applied, the more air that is able to escape through the holes. However, unlike the air escaping from a squeezed and punctured bicycle tube, the tendency for water to move out of the blood vessels by physical pressure is countered by the tendency of fluid to stay in the blood vessels under osmotic pressure. In health, a delicate balance between the two forces is established, ensuring that the tissues remain perfectly hydrated, becoming neither too dry nor too waterlogged.

Just under half of the volume of the blood is made up of blood cells that are too large to move between the gaps in the capillaries. A good proportion of the composition of the remaining plasma consists of dissolved protein molecules, which are also too large to pass through the gaps in the capillaries. Therefore, the movement of fluid across capillary membranes is selective for the remaining small molecules, such as water itself, salt and sugars. This is the property of semi-permeability, as first described in Section 1.1c.

An understanding of these forces that affect the movement of fluid between the blood and the tissues is a helpful foundation for the study of the important condition of edema, described in Section 3.2f.

To summarize, the balance of these forces is such that the net movement of fluid is out of the capillaries at the arterial end and into the capillaries at the venous end. This is ideal to suit the body’s needs. It means that nutrients and oxygen tend to move out into the tissue fluid from the arterial end, and wastes and carbon dioxide tend to move into the venous end of the capillaries. This is how the blood deposits its nutrients into the cells that need them, and transports away the wastes from the tissues at the capillaries.

The pathway of the arteries

Figure 3.2a-I illustrates the pathway of the major arteries that lead away from the heart. The diagram shows how the blood is directed from the heart to all parts of the body. The arch of the largest blood vessel in the body, the aorta, is shown leaving the top of the heart and then descending to run deep in the body, through the diaphragm to the pelvis. Here, it splits into two main branches, which supply blood to each leg.

Figure 3.2a-I The pathway of the arteries from the heart

The structure of the heart

It might be appreciated from this description that the flow of blood in the body can be compared to a figure-of-eight, with the heart situated at the crossing point of the lines of the eight. In this delicate but powerful pumping system, both the right and the left heart contract together in a single heartbeat. This results in a double pumping action, which means that the pulse of flow of blood within the lungs and body occurs at exactly the same time, and the clapping of the four valves closing in turn makes the heart sounds that are audible by means of a stethoscope.

In summary, it is important to understand that the route of the flow of blood through the two halves of the heart is as follows:

•oxygenated blood from the lungs enters the left heart

•the left heart pumps oxygenated blood to all the tissues of the body

•tissues absorb oxygen so that the blood becomes deoxygenated

•deoxygenated blood from the body returns to the right heart

•the right heart pumps deoxygenated blood to the lungs

•blood absorbs oxygen from the lungs so that it becomes oxygenated

•oxygenated blood from the lungs enters the left heart

…and so on.

The blood supply to the heart

It might appear surprising that the heart itself requires a blood supply. However, the heart muscle is working continuously for a full lifetime, and requires a consistent supply of oxygen and nutrients. These high demands cannot be met by the blood flowing through the chambers of the heart alone.

Figure 3.2a-VI illustrates how vessels, known as coronary arteries (derived from cor, the Latin root meaning heart), branch from their origin at the aorta to wrap around the heart muscle. An understanding of this blood supply will be good preparation for the study of coronary heart disease (see Section 3.2e), which is the most important cause of death after cancer in developed countries.

Figure 3.2a-VI The blood supply to the heart

The conduction system of the heart

The conduction system of the heart is what enables the heart to contract in such a way that a wave of contraction moves from the top of the heart to the tip (apex). This means that the blood within the heart is pumped smoothly through the two chambers of the two sides.

The conduction system consists of strands of specialized cardiac muscle cells, which run from an electrically active node situated in the muscular wall between the atria in the upper part of the heart. These cells branch out from this region to connect to all parts of the heart muscle. This node (more specifically known as the sinoatrial node) is the heart’s own pacemaker. The cells of this pacemaker have an internal rhythm, giving them the property of being able to contract at a rate of just over once every second. Because of the membranous joints between cardiac cells, the contraction of the cells in the pacemaker can stimulate the contraction of neighboring cells in the conduction system, and so on, until the whole heart muscle has contracted.

In the heart, the sympathetic nerves cause the pacemaker to contract more rapidly, and the heart muscle to contract more forcefully (in Chinese medicine this would be interpreted as a Yang response). The parasympathetic nerves cause the pacemaker to contract more slowly, and the heart muscle to contract less forcefully (a Yin response).

Emotional states such as fear and anxiety and also physical exercise all lead to an increased heart rate as a result of activation of the SNS. Other factors that contribute to a relatively increased heart rate include body temperature, being female and young age (infancy and childhood).

The generation of the pulse

The pulse wave, which is of interest to practitioners both of Chinese medicine and conventional medicine, is a direct consequence of the rhythmic pumping of the heart. As blood is forced through the chambers of the heart by the wave of contraction from the base to the apex of the heart, the arteries that leave the heart bulge with the increased amount of blood that has been forced into them, and this distension is propagated down the length of the major arteries. Arteries are elastic vessels that can stretch, but are not flaccid. This bulging is felt on palpation at any artery as a pushing outward of the vessel wall, and this bulge is what is known as the pulse.

When conventional medicine practitioners assess the pulse they use quite firm pressure. In this way they can feel the physical force of the blood in the artery. Practitioners of Chinese medicine apply very little pressure to assess the pulse. This is because they are less concerned with the physical force of the blood and more with the energetics of the vital substances of Blood and Qi, which are believed to move with the physical blood.

The heart sounds

Practitioners of conventional medicine use a stethoscope to listen to the heart sounds. In a healthy person, the heart sounds are like the familiar repeated “lub-dub” used so often to heighten emotion in televised hospital dramas. As mentioned previously, the heart sounds are produced by the heart valves closing as the blood is forced through them. In each chamber, one valve closes before the other, thus producing the “lub-dub” sound.

Different conditions of the heart lead to very subtle changes in these sounds. A valve that does not close properly will give a muffled sound as blood leaks through it, and this is known as a murmur.

Part II: Blood pressure and the Chinese medical view

Blood pressure

Blood pressure is simply a measure of the force that the bulge of blood in the arteries exerts on the walls of the arteries. It can be assessed approximately by palpation of the artery. An artery that feels very tense during the pulse can reflect a high blood pressure, whereas a weak, flaccid pulse can reflect a low blood pressure.

Blood pressure oscillates up and down during the heartbeat. When the heart reaches the peak of contraction, blood pressure also reaches a peak, and this is called systolic blood pressure. When the heart relaxes after its contraction, blood pressure drops down to its lowest point, diastolic blood pressure. When a conventional doctor palpates the force of the pulse, information about the difference between systolic (peak) and diastolic (trough) pressure can also be obtained.

Adequate blood pressure is necessary for ensuring a steady flow of nutrients to all parts of the body. In times of physical stress blood pressure rises, but this is appropriate to the body’s needs. It is only if blood pressure remains at too high a level for the body’s requirements for too long that long-term damage to the circulatory system can occur. The condition of high blood pressure (hypertension) is explored in more detail in Section 3.2d.

Two important factors combine to affect the blood pressure. The first is the amount of blood being pumped by the heart at any one time. A rapid and forceful heartbeat will lead to a higher blood pressure than a slow and relaxed heartbeat. A frightening experience or strenuous exercise will both temporarily put up blood pressure, as they both lead to an increased and more forceful heart rate.

The second factor that increases blood pressure is the amount of tension in the muscles of the artery walls. If the artery walls are tightly constricted, the pressure of the blood being forced through them will be raised. There are many factors that cause the artery walls to constrict. One familiar cause is extreme cold, which causes the small arteries in the skin to constrict. This leads to cold flesh, numbness and a bluish discoloration. This is an important reflex in the body, because it means that the heat of the blood is not lost so readily from the skin, and is kept deep in the interior where it is vital for the effective function of the organs.

It is important for homeostasis that blood pressure remains generally within a certain range. It has been found that most healthy individuals have comparable ranges of blood pressure. Young people tend to have lower blood pressure than older people, and women tend to have lower blood pressure than men.

It might now be appreciated that blood pressure has to be responsive to a wide range of conditions, which include emotional stress, exercise, pain and blood volume.

As with the heart rate, the autonomic nervous system (ANS) is central in the control of blood pressure. The muscles within the small artery walls are supplied by sympathetic nerves, and thus it is the SNS that deals with increasing the heart rate. In the small blood vessels, activated sympathetic nerves lead to constriction. Conditions such as emotional stress, exercise, pain and loss of blood volume activate the SNS and lead to constriction of the vessels.

The guidance states that the patient should be seated and should be relaxed. The arm should be resting on a surface so that it is at the level of the heart. The cuff pressure should be deflated very slowly (no more than 2mmHg per second) so that the rushing sounds can be accurately matched with the cuff pressure. The blood pressure should be measured in both arms, and if one of the two readings is consistently higher, this should be considered to be the accurate reading. If the blood pressure measurement is high, it should be repeated after 1–2 minutes and the first reading discarded if the second reading is lower. If more readings are taken, the lowest of these is used to give the final measurement. If the clinic reading is high, the advice is that the doctor should arrange for home readings to be obtained to ensure that a “white-coat effect” (see Section 3.2b) has not raised the blood pressure to higher than its usual level. The gold standard is that the patient is given an automated ambulatory blood pressure monitor to assess blood pressure throughout their day, but the guidelines accept that readings from a simple home monitor may also be valid in the diagnosis of raised blood pressure.