adult would be around 120/80 mmHg.
The two figures obtained each time a blood pressure measurement is taken represent:
The systolic BP: This is the upper figure which corresponds to the time during the cardiac cycle when the ventricles are undergoing systole (contraction) and blood is being ejected.
The diastolic BP: This is the lower figure and corresponds to the time during the cardiac cycle when the ventricles of the heart are undergoing diastole (relaxation) and no blood is being ejected.
Currently NICE (National Institute for Health and Care Excellence) recognises BP readings of 140/90 mmHg or higher as being indicative of hypertension (high blood pressure). It is estimated that hypertension affects at least a quarter of all adults in the UK and over half of all adults in the UK over the age of 60. Hypertension is a major preventable cause of mortality in the UK, increasing the risk of MI, stroke (CVA), heart failure, chronic kidney disease and cognitive decline (NICE, 2018).
A normal BP is essential to maintain tissue perfusion (blood supply) throughout the body from the top of the scalp to the tips of the toes. BP can be affected by many parameters, but a normal BP depends on having a healthy heart to ensure adequate cardiac output (CO) and healthy blood vessels to ensure adequate blood flow. The blood vessels provide a collective resistance to blood flow with the total resistance offered by all the blood vessels in the body known as the peripheral resistance (PR).
In simple terms BP can be thought of as a product of multiplying the CO and the PR:
BP = CO × PR
As we will explore below, BP can be altered by changing the heart rate to change CO or by altering the diameter of blood vessels to change the PR.
Control of BP
The human body has a variety of elaborate homeostatic mechanisms to ensure that BP is maintained within its normal range. BP control can be broadly split into neural mechanisms, which allow BP to be altered rapidly within seconds, and hormonal mechanisms, which play a key role in the medium- to long-term control of BP.
Neural control of BP
Two specialised regions are located within the medulla oblongata (inferior portion of the brain stem) which can rapidly either raise or lower the BP to match the body’s current needs. The cardioregulatory centre or cardiac centre regulates the heart rate and hence the cardiac output (CO). The vasomotor centre regulates vascular tone by controlling the diameter of blood vessels (vasodilation or vasoconstriction). By regulating vascular tone, the vasomotor centre is able to increase or decrease the peripheral resistance (PR).
Both the cardiac and vasomotor centres require a continuous ‘real-time’ measurement of the current BP. This is achieved using specialised stretch receptors called baroreceptors which are located in the walls of the aortic arch and carotid sinuses (bulbous regions of the carotid arteries in the neck). Measuring the degree of stretch gives a good measure of current BP, with more stretch equating to a higher BP and less stretch indicative of a lower BP. In humans the aortic arch baroreceptors relay information to the cardioregulatory and vasomotor centres via the vagus nerve and the carotid sinus baroreceptors via the glossopharyngeal nerve (Figure 3.10).
Figure 3.10 Baroreceptor response
The baroreceptor responses
Neural control of blood pressure is particularly important for changes in posture or when engaging in strenuous physical activities. The baroreceptor responses highlighted below are classic examples of homeostatic negative feedback (Chapter 2) where deviations from normal systolic and diastolic BP are resisted and BP is normalised.
Many things can cause a drop in BP but a common cause is suddenly standing up (Figure 3.11).
Figure 3.11 Neural responses to decreased BP
In the situation in Figure 3.11, gravity will pull arterial blood downwards (arteries have no valves), leading to a significant drop in BP. If BP is not restored rapidly, there is a risk of reducing cerebral blood flow, potentially leading to dizziness and fainting (syncope). This phenomenon is termed postural hypotension or orthostatic intolerance and becomes more common with age. An understanding of postural hypotension is essential for nurses, so to develop your understanding further read through Janet’s case study before attempting Activity 3.4.
Case study: Janet – postural hypotension
Janet is a 71-year-old woman who has recently returned home following a long 10-day stay in hospital recovering from a severe bout of pneumonia. Janet has hypertension which has for many years been treated successfully using a combined beta blocker and diuretic. When Janet stood up to make herself a cup of tea she promptly fainted, hitting her head on a coffee table as she fell. Fortunately, her partner was with her and able to drive her quickly to A&E for stitches and a full assessment.
As a nurse, you should be able to link your knowledge of human anatomy and physiology to patient histories.
Activity 3.4 Evidence-based practice and research
From what you have learnt in this chapter, describe what is likely to have caused Janet’s faint. Could this be related to her recent hospital stay or blood pressure medication?
You should now have a good understanding of the nature of postural hypotension and the risk of falls associated with this condition. Although postural hypotension becomes more common in older people, high blood pressure affects a much greater proportion of the population.
Increased BP may occur as a result of disease (e.g. atherosclerosis), stress or simply through increased sodium (salt) consumption (Figure 3.12). Hormonal mechanisms tend to be more effective in reducing elevated BP; however, neural mechanisms also play a more immediate role.
Figure 3.12 Neural responses to increased BP
In addition to the rapid neural adjustments to BP that are required as a result of postural changes, it is essential that BP is controlled and maintained over the longer term. It is here that hormonal mechanisms play the dominant role.
Hormonal control of blood pressure
Hormones are chemical signals which are transported to their sites of action in the blood (Chapter 5). A multitude of hormones are involved in regulating blood pressure and there is much synergy (working together) between these and also much interplay between the hormonal mechanisms and neural mechanisms described above.
Antidiuretic hormone (ADH)
Also known as vasopressin, ADH is a neuropeptide hormone (small protein produced by nerve cells) that is synthesised in the hypothalamus. Once produced, ADH is transported along the axons of hypothalamic
