The blood vessels

The ‘vascular system’ is the term used to describe the vessels alone. It consists of a network of tubes lined with endothelium (a form of epithelial tissue). These tubes are at their widest as they leave the heart, and then divide, like the branches of a tree, to form thousands of tiny capillaries within the tissues. The capillaries then meet up again to form wider and wider tubes, which return the blood back to the heart.

Blood vessels that lead blood away from the heart are called ‘arteries’ and those that lead blood back again to the heart from the tissues are called ‘veins’. ‘Arterioles’ and ‘venules’ are the names given to the smallest types of arteries and veins.

The transfer of nutrients from 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 (see Chapter 3.1a for a reminder of the origin of the basic nutrients and how they are processed by the digestive system). Once the blood has passed through the capillaries of the lungs, it will also contain a high concentration of oxygen. Chapter 1.1b explained how oxygen is one of the essential ingredients which, together with glucose and other simple nutrients, is 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 Chapter 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 occurs so that these substances 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 transfer 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 (see Q3.2a-2).

Similarly, water moves into and out of the plasma with ease. In this case, the process is called ‘osmosis’. This is the term used to describe 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.

The ‘desire’ for water to move from a region of low concentration to one of high concentration is described in scientific terms as ‘osmotic pressure’. This pressure relates directly to the difference in concentration between two regions, and so to the tendency that water will have to move between these regions. When the osmotic pressure is high (e.g. in a dehydrated plant stem), a lot of water can be encouraged to move (in this case against the downward pull of gravity and into the stem). Thus, in the body, if a dehydrated person drinks some water, there will be a rapid movement of water from the digestive system, through the plasma and into the dehydrated tissues, all under the ‘pull’ of the increased osmotic pressure of the concentrated tissue fluids.

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 tyre is under a measurable degree of pressure (which is what makes the tyre feel inflated), so is the blood in a blood vessel 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.

It is worth remembering for the present that 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 semipermeability, as first described in Chapter 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 oedema, which is described in Chapter 3.2f (see Q3.2a-3).

To summarise, 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 (see Q3.2a-4 and Q3.2a-5).

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.

Figure 3.2a.II shows how the major branches of the aorta in the neck, the common carotid arteries, branch to supply blood to the head. One branch, the internal carotid artery, divides to enter the skull and supply the brain with blood (see Q3.2a-6).

Figure 3.2a-II • The pathway of the left common carotid artery.

Now turn back to Figure 3.2a-I, which shows how arterial branches from the aorta divide down each limb to take blood to the hands and the feet. Compare this with Figure 3.2a-III, which shows the pathways of the veins that return the blood to the heart, ending in two major vessels called the ‘superior vena cava’ and ‘inferior vena cava’. Note how the basic pathways of the veins mirror those of the arteries. This is the pattern for almost all parts of the body, in that an artery delivering blood to a body part runs very close to the vein that is draining the blood away from that part.

Figure 3.2a-III • The pathway of the veins leading back to the heart.

The main difference between the positions of arteries and veins is that some veins tend to run closer to the surface of the body, whereas arteries generally lie much deeper. The veins that run close to the surface are called ‘superficial veins’. All the blue-coloured vessels that can be seen through the skin in areas such as the back of the hand, the crooks of the elbows and the neck are superficial veins, not arteries. If these vessels are palpated, no pulse will be felt, and it will be found that they are very easy to compress.

In addition to superficial veins, there is also a system of ‘deep veins’. The blood draining from the surface of the body runs first into the superficial veins, and then enters the deep veins through vessels that penetrate the deeper tissues called ‘perforators‘. It is important to be clear about the distinction between superficial and deep veins in order to understand the pathology of the condition of varicose veins, described in Chapter 3.2c.

The structure of the heart

The heart is a fist-sized organ that sits just to the left of midline in the chest (see Figure 3.2a-I). It is situated between the two lungs, and together these organs fill the space that overlies the diaphragm (the thoracic cavity; Figure 3.2a-IV). These organs are so closely related that the inside surfaces of the lungs bears the impression of the shape of the heart upon them.

Figure 3.2a-IV • The organs associated with the heart in the thoracic cavity.

The heart is largely made up of cardiac muscle (the myocardium), and is divided into four chambers. It might be helpful now to turn back to the description of the tissue types given in Chapter 1.2d for a reminder of the characteristics of cardiac muscle, and how the cardiac muscle cells communicate with each other through specialised ‘joints’ (see Q3.2a-7).

The healthy heart is lined with endocardium, a smooth epithelial tissue lining that is continuous with the lining of the blood vessels. The outside of the heart is protected by a tough fibrous sac called the ‘pericardium’. In between the pericardium and the heart organ is a very small space containing fluid. This fluid lubricates the movement between the heart and the pericardium, and contains immune cells. The remainder of the organ consists of the naturally contractile myocardium (see Q3.2a-8).

To understand the action and some of the structural disorders of the heart, it is important to understand that the heart is divided into two halves: the right half deals with the flow of the blood from the body and to the lungs, and the left half deals with the flow of the blood from the lungs and to the body. Each half has two chambers known as the ‘atria’ (plural of ‘atrium’) and the ‘ventricles’, and together these form the four chambers of the heart. There are two valves situated in each half of the heart. Their function is to ensure that the blood flow is always one way. Figure 3.2a-V illustrates the position and shape of the four chambers and the valves of the heart, and the direction of blood flow through the two halves of the heart.

Figure 3.2a-V • The interior of the heart and its blood flow.

The flow of blood through the heart

The double structure of the heart means that blood can flow through the heart twice in each full circulation of the body. It might help when reading the following description of the flow of the blood to use Figure 3.2a-V to visually trace the pathway the blood takes.

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 the ‘coronary arteries’ (‘cor’ is 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 (Chapter 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 specialised cardiac muscle cells, which run from a specialised ‘node’ situated in the 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 joints between cardiac cells, the contraction of the cells in the pacemaker can stimulate the contraction of neighbouring cells in the conduction system, and so on until the whole heart muscle has contracted.

Various factors can affect the rate at which the pacemaker cells contract, which explains why people can be aware of changes in their heart rate from time to time. The most important factor is the nerve supply to the heart. The nerve supply to the heart is part of the autonomic nervous system (described in more detail in Stage 4.1). The autonomic nervous system plays an essential role in the maintenance of homeostasis in the body, and very probably in the mechanism of many of the effects of acupuncture. For the time being, all that need be understood about this system is that it deals with the control of the involuntary aspects of the function of the body, such as breathing, digestion and sweating. It is conventionally seen as two opposing systems; the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). Most parts of the body are under the influence of both these systems.

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) (see Q3.2a-10).

Emotional states, such as fear and anxiety, and exercise all lead to an increased heart rate as a result of activation of the sympathetic nervous system. 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, 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 move with the physical blood (see Q3.2a-11).

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’. In conventional medicine, the ability to use a stethoscope to interpret these changes in detail is considered a skill that requires many years of practice.

The blood pressure

The blood pressure is simply a measure of the force that the bulge of blood in the arteries exerts on the walls of the arteries. Blood pressure 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.

The blood pressure oscillates up and down during the heartbeat. When the heart reaches the peak of contraction, the blood pressure also reaches a peak, and this is called the ‘systolic blood pressure’. When the heart relaxes after its contraction, the blood pressure drops down to its lowest point, the ‘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 essential to ensure a steady flow of nutrients to all parts of the body. In times of physical stress the blood pressure rises, but this is appropriate to the body’s needs. It is only if the 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 Chapter 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 the blood pressure, as they both lead to an increased and more forceful heart rate.

The second factor that increases the blood pressure is the amount of tension in the muscles of the artery walls. If the artery walls are tightly constricted, then 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 the 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 pressures than older people, and women tend to have lower blood pressures than men.

Control of blood pressure

It now might be appreciated that the 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 most important system that controls the blood pressure is the autonomic nervous system. The muscles within the small artery walls are supplied by sympathetic nerves, and thus it is the sympathetic nervous system (SNS) that deals with increasing the heart rate. In the small blood vessels, the sympathetic nerves, when activated, 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 (see Q3.2a-12).

Two of the many chemicals that are integral to the activation of the SNS are the hormones adrenaline and noradrenaline. A release of adrenaline from the adrenal glands will lead to an increased heart rate, constriction of the artery walls and an increase in blood pressure. This is explained in more detail in Section 5.1 (see Q3.2a-13).

When the SNS is not activated, the action of the parasympathetic nervous system (PNS) becomes dominant. In this situation, the heart rate slows down, the blood vessels relax and the blood pressure drops. A faint often occurs as a result of a strong emotional response to a shock (different to fear) or even to excessive joy. This has the effect of activating the PNS. The result of this is that the heart rate suddenly slows and the blood pressure drops. This leads to a sudden drop in the supply of blood and nutrients to the brain, and the person will feel dizzy and may very briefly lose consciousness. However, the act of fainting, and the drop in blood pressure that accompanies it, then stimulates the SNS, which immediately reverses the situation. A hallmark of a faint is that the person very quickly begins to regain consciousness (within seconds) as the blood pressure rises and the blood supply returns to the brain.

This is also what occurs when a person receiving acupuncture experiences needle shock, which very occasionally occurs during a first treatment. Although alarming for the practitioner, this type of faint is usually nothing to worry about, as long as the patient starts to regain consciousness immediately.

Measurement of blood pressure

Although blood pressure can be roughly assessed by palpation of the pulse, the way to obtain a reliable numerical measure of the blood pressure is by means of an instrument called the ‘sphygmomanometer’. This consists of an inflatable fabric cuff, which is wrapped around a limb, usually the upper arm, and a device to measure the pressure within the cuff.

The pressure can be assessed by a simple manually operated pressure gauge, similar to those used to measure car-tyre pressure, in which case the equipment is called an ‘aneroid sphygmomanometer’. Increasingly, in medical settings an ‘oscillometric’ assessment is made, which involves a digitally processed result drawn from the mean arterial pressure. This requires an automated digital sphygmomanometer. The British Hypertension Society currently recommends a wide range of oscillometric models for reliable assessment of blood pressure.

However, for accurate measurement, many doctors still consider the bulky mercury-containing sphygmomanometer to provide the gold standard for measurement. For this reason, the numerical value of blood pressure is conventionally expressed in millimetres of mercury (mmHg). A normal systolic pressure falls within the range 100–140 mmHg, and a normal diastolic pressure falls within the range 65–90 mmHg. The blood pressure is expressed as a combination of these two values. For example, a blood pressure of 120/80 mmHg (‘120 over 80’) would be considered to be a normal blood pressure, and one of 150/105 mmHg would be considered to be a high (albeit only moderately raised) blood pressure.

In blood pressure assessment the cuff is inflated to a pressure high enough to totally compress the arteries within the limb. This will be equivalent to a pressure higher than the systolic pressure, and can be in the range of 200–250 mmHg. At this pressure the compression of the artery leads to a total cessation of blood flow through the artery, and the pulse can no longer be felt or picked up by an oscillometer. This stage is often uncomfortable for the patient, but this level of pressure should be maintained only for a moment.

In aneroid assessment, a stethoscope is placed over the artery that is distal to the cuff. This is usually at the brachial artery, the pulse of which can be palpated at the elbow (in the region of the acupoint Quze PC-3). The pressure in the cuff is then gradually reduced. As the pressure in the cuff falls to that of the systolic pressure, squirts of blood can be forced through the now partially compressed artery. This is heard as a pulsatile rushing noise through the stethoscope. The systolic pressure is recorded as the pressure of the cuff when the rushing noises are first heard.

As the pressure in the cuff is reduced further to close to the diastolic pressure, more and more blood can pass through the brachial artery. As the blood flows more easily, the rushing noises become increasingly soft. When the pressure in the cuff reaches the diastolic pressure, all the blood can pass freely through the vessel and the sounds disappear altogether. The diastolic pressure is recorded as the pressure of the cuff when the rushing noises stop altogether.

The British Hypertension Society has written guidelines for health professionals on how to take accurate blood pressure measurements and how to treat high blood pressure. These guidelines ask for careful repeated measurements, although this ideal may be rarely achieved in real practice because of time constraints. However, this is not a trivial issue; an inaccurate measurement will lead to a false assumption about the true level of blood pressure, and can lead to inappropriate treatment.

The 2004 British Hypertension Society Guideline (IV) 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 2 mmHg per second) so that the rushing sounds can be accurately matched with the cuff pressure. The blood pressure measurement should be repeated after 1–2 minutes and the first reading discarded. If more readings are taken then the mean of these readings should be calculated to give the final measurement.

This description of blood pressure assessment has been given in detail because many complementary therapists routinely assess the blood pressure of their patients.

Physical examination of the vessels and circulation

The physical examination of the vessels and circulation involves the stages listed in Table 3.2b-I (see Q3.2b-1).

Table 3.2b-I The stages of the physical examination of the blood vessels and circulation

Blood tests

Blood tests can be performed to exclude certain conditions that can contribute to disease of the vessels. The most common tests to be performed are serum tests to check for raised blood lipids (such as cholesterol) and glucose (indicative of diabetes). The erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) (see Chapter 2.4d) may be measured to exclude the rare inflammatory conditions that may affect the blood vessels. A full blood count (FBC) is performed to exclude anaemia, because in anaemia the oxygen-carrying capacity of the blood is markedly reduced and this can worsen the symptoms of vascular disease. For this reason, it is important to diagnose and treat anaemia in someone with disease of the blood vessels.