The body usually contains about 5 litres of blood. Its constituents include a
fluid known as plasma and a variety of cells. As well as the various exogenous
cells carried in the blood (nutrients, oxygen, etc.), it produces its own cells.
These are manufactured by stem cells in the bone marrow. Three different
types of cell are produced:
- Erythrocytes (or red blood cells) transport oxygen around the body. In
them, oxygen combines with haemoglobin in the lungs and is transported
to cells in need of oxygen, where it is released, allowing cell respiration.
- Phagocytes and lymphocytes (or white blood cells; see above) include the
immune system’s B cells and T cells described earlier in the chapter.
- Platelets are cells that respond to damage to the circulatory system. They
aggregate (form a clot) around the site of any damage and prevent loss of
blood from the system. They are also involved in repair to damage within
the arteries themselves and contribute to the development of atheroma.
We consider this process later in the chapter.
The heart has two separate pumps operating in parallel. The right side of the
heart is involved in the transportation of blood to the lungs; the left side
pumps blood to the rest of the body (Figure 8.6). Each side of the heart has
two chambers (Figure 8.7), known as atria and ventricles. The right atrium
Figure 8.6 The flow of blood through the heart.
relating to things
outside the body.
a mature blood cell that
contains haemoglobin to
carry oxygen to the
tiny bits of protoplasm
found in the blood that
are essential for blood
clotting. These cells bind
together to form a clot
and prevent bleeding at
the site of injury.
228 CHAPTER 8 • THE BODY IN HEALTH AND ILLNESS
takes deoxygenated blood from veins known as the superior and inferior
vena cava and pumps it into the right ventricle. Blood is then pumped into
the pulmonary artery, taking it to the lungs, where it picks up oxygen in its
haemoglobin cells. Oxygen-laden blood then returns to the heart, entering
through the left atrium. It is then pumped into the left ventricle, and then
into the main artery, known as the aorta, which carries blood to the rest of
The rhythm of the heart is controlled by an electrical system. This is initiated
by an electrical impulse generated in a region of the right atrium called
the sinoatrial node. This impulse causes the muscles of both atria to contract.
As the wave of electricity progresses through the heart muscle and nerves, it
reaches an area at the junction of the atria and ventricles known as the atrioventricular
node. This second node then fires a further electrical discharge
along a system of nerves including the bundle of His and Purkinje fibres (see
Figure 8.7), triggering the muscles of both ventricles to contract, completing
the cycle. Although the sinoatrial node has an intrinsic rhythm, its activity is
largely influenced by the autonomic nervous system.
The electrocardiogram (ECG) is used to measure the activity of the heart.
Electrodes are placed over the heart and can detect each of the nodes firing
and recharging. Figure 8.8 shows an ECG of a normal heart, indicating the
electrical activity at each stage of the heart’s cycle.
n The P wave indicates the electrical activity of the atria firing – the time
needed for an electrical impulse from the sinoatrial node to spread
throughout the atrial musculature.
n The QRS complex represents the electrical activity of the ventricles
n The T wave represents the repolarization of the ventricles.
Figure 8.7 Electrical conduction and control of the heart rhythm.
the main trunk of the
carrying blood from the
left side of the heart to
the arteries of all limbs
and organs except the
THE CARDIOVASCULAR SYSTEM 229
When the heart stops beating or its electrical rhythm is completely irregular
and no blood is being pushed around the body, doctors may use a
defibrillator to stimulate a normal (sinusoidal) rhythm.
Blood pressure has two components:
- the degree of pressure imposed on the blood as a result of its constriction
within the arteries and veins – known as the diastolic blood pressure
- an additional pressure as the wave of blood pushed out from the heart
forms flows through the system (our pulse) – known as the systolic blood
This pressure is measured in millimetres of mercury (mmHg), representing
the height of a tube of mercury in millilitres that the pressure can push up
(using a now old-fashioned sphygmomanometer). This varies around the
body, being at its highest close to the heart and at its lowest as the blood
re-enters the heart. To standardise the measurement of blood pressure, it is
usually measured at the top of the arm. Healthy levels of blood pressure
are an SBP below 130–140 mmHg and a DBP below 90 mmHg (written as
130/90 mmHg: see also the discussion of hypertension later in the chapter).
A number of physiological processes are involved in controlling blood
pressure. Those of particular interest to psychologists involve the autonomic
nervous system. The brainstem (and the hypothalamus) receives continuous
information from pressure-sensitive nerve endings called baroreceptors
situated in the carotid arteries and aorta. This information is relayed to a
centre in the brainstem known as the vasomotor centre. Reductions in blood
pressure or physical demands such as exercise that require increased blood
pressure causes activation of the sympathetic nervous system. Sympathetic
activation results in increases in the strength and frequency of heart contractions
(via the activity of the sinoatrial and atrioventricular nodes) and a
Figure 8.8 An electrocardiograph of the electrical activity of the heart (see text for
a machine that uses
an electric current to
stop any irregular and
dangerous activity of the
heart’s muscles. It can
be used when the heart
has stopped (cardiac
arrest) or when it is
beating in a highly
sensory nerve endings
that are stimulated by
changes in pressure.
Located in the walls of
blood vessels such as
the carotid sinus.
the main artery that
takes blood from the
heart via the neck to
230 CHAPTER 8 • THE BODY IN HEALTH AND ILLNESS
contraction of the smooth muscle in the arteries. Together, these actions
result in an increase in blood pressure, and allow sustained flow of blood to
organs such as the muscles at times of high activity. Parasympathetic activity
results in an opposing reaction.
How do caffeine, smoking and drinking alcohol influence blood pressure?
People who drink significant amounts of alcohol have a one and a half to two
times greater risk of developing hypertension than those who do not drink.
The association between alcohol and high blood pressure is particularly
noticeable when the alcohol intake exceeds five drinks per day. In addition,
the connection is a dose-related phenomenon. In other words, the more
alcohol that is consumed, the stronger is the link with hypertension (e.g.
Saremi, Hanson, Tulloch-Reid et al. 2004).
Smoking also increases the risk of heart disease and stroke in people who
already have hypertension, although it is not strongly associated with the
development of hypertension. Nevertheless, smoking a cigarette can produce
an immediate, temporary rise in the blood pressure of 5 to 10 mmHg.
However, steady smokers may actually may have a lower blood pressure
than non-smokers. This is because the nicotine in the cigarettes causes a
decrease in appetite, which leads to weight loss. This, in turn, lowers the
blood pressure. Stopping smoking can lead to increased food consumption
and increased weight (e.g. Janzon, Heblad, Berglund et al. 2004).
Finally, there is extensive evidence that normal levels of caffeine can
increase blood pressure. Overall, the impact of dietary caffeine on population
BP levels is likely to be modest – probably about 4 mmHg of diastolic
blood pressure and 2 mmHg of systolic blood pressure. Despite this relatively
modest increase in blood pressure, James (2004) estimated that they
could contribute to 14 percent of premature deaths resulting from coronary
heart disease and 20 percent of those from a stroke.
Are the risks associated with these types of behaviour something that
those involved in public health should act upon? Should we place health
warnings on coffee, tea, alcohol? Should advertising warn that coffee – and
other caffeine-containing drinks – can do you harm? What are the limits to
health promotion and how the state advises our consumption of potentially
harmful substances – tea, coffee, caffeine supplements, alcohol, cigarettes,
marijuana? After all, any limit is going to be arbitrary, and something we
should all consider.