This is the first article in a new series on the anatomy and physiology of ageing. The series explores normal age-related changes that occur in the anatomy and physiology of the major organ systems
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Citation: Knight J, Nigam Y (2008) Exploring the anatomy and physiology of ageing: part 1 - the cardiovascular system. Nursing Times; 104: 31, 26-28.
Authors: John Knight is lecturer in biomedical science; Yamni Nigam is lecturer in biomedical science; both at the School of Health Studies, Swansea University.
Due mainly to advances in healthcare, the average lifespan of the UK population is rising - many 60-year-olds can expect a further 25 years of healthy life. However, our knowledge of the ageing process remains limited.
The ageing process is largely determined by genetic factors. It is also heavily influenced by environmental factors such as diet, exercise, exposure to micro-organisms, pollutants and ionising radiation. That is why two people of the same age may differ markedly in terms both of physical appearance and physiology. Gender also plays a part. In most developed countries women typically outlive men by 7-10 years.
The cardiovascular system
The cardiovascular system is the body’s major transport system. Its most important role is to deliver oxygenated blood, nutrients and chemical signals, such as hormones, to the organs and tissues. It also transports carbon dioxide to the lungs and waste products, such as urea and uric acid, to the kidneys for elimination. As well as this, the system plays a major role in thermoregulation, distributing and dissipating heat throughout the body.
A healthy, efficient cardiovascular system is essential for systemic health. Therefore, as the system ages and becomes less efficient, there will be a negative impact on all other major organ systems.
This article explores the major anatomical and physiological changes that occur within the cardiovascular system in normal ageing.
At birth the arteries are elastic, flexible and compliant, and this is essential for optimal cardiac function and health.
During ventricular systole (contraction), blood is ejected into the pulmonary and systemic circuits and the arteries stretch, reducing the resistance to blood flow. As the body ages, blood vessels (particularly arteries) lose their elasticity and become less compliant.
Changes to the tunica media
The tunica media is composed of layers of smooth muscle, and is connected to and under the influence of the vasomotor centre within the medulla oblongata in the brain stem. The vasomotor centre controls vascular tone and plays an important role in regulating blood pressure, controlling vasodilation and vasoconstriction.
A gradual thickening within the tunica media of large and medium-sized arteries occurs with age (Fig 1). This is associated with an increase in the number and density of collagen fibres, making the artery increasingly rigid and less compliant (Alberto et al, 2003). The vessels also display fracturing of the elastic (elastin) components and often varying degrees of calcification.
Changes to the endothelium
The endothelium is the innermost layer of blood vessels and is in direct contact with the blood. It is composed of a single layer of squamous epithelial cells which, in children and young adults, are regular and smooth, offering minimal resistance to blood flow.
With age, the endothelial layer begins to display irregularly shaped cells and is often thickened due to the presence of smooth muscle fibres that have migrated from the tunica media. This thickening not only contributes to a reduction in arterial elasticity and compliance but also reduces the lumen size, further increasing resistance to blood flow (Fig 1).
To overcome reduced arterial compliance and increased peripheral resistance, the ventricles of the heart must pump with greater force.
The myocardium (muscular layer of the heart) responds in much the same way as other muscles exposed to increased load - by enlargement and hypertrophy.
Typically, the left ventricle increases in thickness by around 30% between the ages of 20 and 80 (Fig 2) and there is a gradual increase in cardiac weight (Gudipati and Labovitz, 1990).
There is a steady decrease in the number of cardiac myocytes (cardiac muscle cells), while those remaining are often enlarged and the myocardium displays increased levels of collagen (Alberto et al, 2003).
Functional cardiac changes
One of the most obvious changes in cardiac function is a linear decrease in the maximal heart rate achievable during exercise. In young healthy children, a maximal heart rate of around 220 beats per minute (bpm) is normal following vigorous exercise. With age, this rate falls roughly in line with the formula 220 minus age in years, so by 60 years it is around 160bpm. It is thought that this reduction is primarily due to changes in the heart’s conductive system.
The cardiac conductive system
Studies on the heart’s pacemaker (sinoatrial node) have revealed losses by cell death (apoptosis) of 50-75% of pacemaker cells by the age of 50.
While the number of cells in the atrioventricular node remains relatively constant, there is fibrosis and cellular death in the atrioventricular bundle (bundle of His). These changes in conductive tissues may reduce the efficiency of cardiac conduction and contribute to the decline in maximal heart rate and increase in arrhythmias observed with age (Alberto et al, 2003).
Changes in blood pressure
Systolic blood pressure gradually increases with age - the average in males is around 126mmHg at 25 years and 140mmHg at 60. This is thought to reflect the decrease in elasticity and lumen diameter within the arterial tree and the associated hypertrophy of the left ventricle (Fig 2). In addition, small arteries and arterioles become less responsive to vasodilator cues with age, further increasing peripheral resistance.
In the absence of any pathology, diastolic pressure changes very little with age.
Baroreceptor function and postural hypotension
Following a change in posture, such as moving from sitting to standing, blood drains into the lower extremities and the blood pressure falls.
This hypotension is immediately detected by the baroreceptors (blood pressure sensors) within the aortic arch and carotid sinus. This information is relayed to the cardiac and vasomotor centres within the medulla oblongata.
The cardiac centre responds by increasing the heart rate and the vasomotor centre initiates vasoconstriction, restoring normal blood pressure, ensuring adequate blood flow to the brain and preventing postural hypotension and syncope (fainting).
In older people, baroreceptor reflexes are less efficient - it is thought that the thickening of the arterial walls interferes with their ability to measure the degree of stretch (blood pressure) within the vessel accurately. This can lead to postural hypotension and fainting. Postural hypotension is reported to affect 30-50% of people aged over 75 (Docherty, 1990).
Ageing is often associated with a general reduction in activity and fitness. However, exercise at any age can be beneficial, and encouraging older people to remain active and take regular exercise maximises cardiovascular function well into old age (Montague et al, 2005).
Alberto, U. et al (2003) Invited review: ageing and the cardiovascular system. Journal of Applied Physiology; 95: 2591-2597.
Gudipati, C.V., Labovitz, A.J. (1991) Effects of ageing on left ventricular structure and function. American Heart Journal; 121: 3 (Pt 1), 871-875.
Docherty, J.R. (1990) Cardiovascular responses in ageing: a review. Pharmacological Review; 42: 103-125.
Montague, S.E. et al (2005) Physiology for Nursing Practice. Oxford: Elsevier.