heart rate and mobilize glucose into the blood among other things. Collectively,
adrenaline and noradrenaline (and dopamine) are known as catecholamines. The outer
portion of the adrenal gland, called the adrenal cortex, produces mineralocorticoids and
glucocorticoids. The former hormones act on the kidneys to conserve salt and water
by returning them to the blood during urine formation. The latter are secreted when we
encounter stressors in order to help the body respond appropriately. One of the most
important glucocorticoids is cortisol (corticosterone in rodents) and as such it is frequently
referred to as the ‘stress hormone’ and measured in studies of psychological stress.
Cardiovascular system
The central function of the cardiovascular system is to ensure that oxygen (with other
nutrients) is transported to all the organs of the body and that carbon dioxide (as well
as other waste products) is removed from each of the body’s cells. The blood is the
vehicle that transports the oxygen with the heart and blood vessels allowing the blood
to be carried around the body. The heart, the centre of the cardiovascular system, is
made of muscle and ‘beats’ or ‘pumps’ approximately 100,000 times per day. The main
muscular outer part of the heart, which contains the cardiac veins and arteries, is called
the myocardium. The heart has four chambers: the two upper chambers are known
as atriums and the two lower ones are called ventricles. In the cardiovascular system,
veins carry blood to the heart and myocardium and the arteries carry blood from the
heart and myocardium.
We can follow the journey of blood flow by considering where it enters the heart.
Blood enters the right atrium deficient of oxygen and full of waste products (carbon
dioxide) and is bluish in colour. Once the atrium is full, the blood is pushed into the
right ventricle, which then contracts, thus pumping the blood from the heart towards
the lungs, where it becomes oxygenated (and red in colour). Once oxygenated, the
blood travels to the left atrium in the heart and is passed into the left ventricle before
it is pumped into the general circulation via the aorta (a large artery). Before returning
to the heart, some of the blood is cleansed of waste products by passing through the
kidneys (where the waste products are filtered out and excreted in urine) and the liver
(where nutrients, e.g. simple sugars, are stored and harmful bacteria are removed).
The cardiovascular system is a closed system and therefore it always contains some
pressure. Blood pressure is the force exerted by the blood on the artery walls and has
two components:
1 Diastolic blood pressure is the resting level in the arteries in between contractions.
2 Systolic blood pressure is the maximum pressure in the arteries when the heart pumps.
An individual’s blood pressure is described using two numbers representing both
the systolic and diastolic components and is expressed in units known as millimetres
of mercury (mmHg; e.g. 126 over 70 or 126/70 mmHg). A number of factors increase
blood pressure including temperature, weight, posture and food intake. Psychological
factors, such as chronic stress, have also been found to be associated with the
development of high blood pressure (or hypertension), which is known to damage
the heart and the arteries. We will consider the links between stress, blood pressure
and cardiovascular disease in more detail later.
22 BIOLOGICAL BASES OF HEALTH AND ILLNESS