Every inch of our body needs a constant and plentiful supply of oxygen and nutrients. To reach them, the blood in our veins needs to be under a certain amount of pressure.
If pressure is too low, blood may not sufficiently reach the extremities. If, on the other hand, it is too high, it may result in damage to the heart and blood vessels, which can cause problems throughout the body.
What is blood pressure?
Just as the air inside a balloon exerts pressure on the balloon walls, the blood within blood vessels exerts pressure on the vessel walls. Blood pressure can vary, depending on the amount of blood, the force with which the heart pumps it, and the resistance of the vessels through which it travels.
Think of a hosepipe - the pressure within the pipe depends on the amount and force of the water coming out of the tap and the width of the pipe itself. If the pipe becomes narrowed, or the tap is turned up, the pressure in the pipe will increase.
In the body, the tap is representative of the heart muscle. The heart exerts a pumping action through rapid cycles of contraction (systole) and relaxation (diastole). This is a little like turning the tap on and off.
The blood in the arteries will therefore be under a different pressure at different times during the cardiac cycle, depending on whether the heart is contracting or relaxing.
Systolic and diastolic pressures
Systolic pressure, measured in the arteries when the heart is contracting, is higher than diastolic pressure. This is when the heart is relaxed.
Blood pressure is measured in millimetres of mercury: 'mmHg'. Normal blood pressure would be a systolic pressure of 120 mmHg, and a diastolic pressure of 80 mmHg. This is usually expressed as '120 over 80' or written as 120/80.
It is, however, rare for anybody to give such ideal measurements. For adults, a normal blood pressure reading can be anywhere from 120/70 to 140/90.
Blood pressure control
Various factors affect the pressure of the blood, from the time of day to the volume of the blood itself. Your body is usually able to keep blood pressure within safe limits, but sometimes changes in lifestyle or general health make this difficult. This can cause your pressure to become consistently higher or lower than normal, which can lead to major health complications.
Two systems in the body are responsible for the control of blood pressure under day-to-day conditions: the cardiovascular system and the renal system. Blood pressure is constantly monitored by receptors around the body. Any abnormalities will lead to these systems being alerted.
The cardiovascular system
The cardiovascular system comprises a pump (the heart), pipes (blood vessels), and fluid (blood).
Fascinating facts Adults have in the region of 60,000 miles of blood vessels in their bodies, through which the heart pumps more than 10 pints of blood per minute.
It is estimated that a person's heart will beat approximately 2,700,000,000 times in one lifetime. |
There are 3 main types of blood vessel:
- arteries, which carry nutrient-rich blood to the tissues and are large vessels with thick muscular walls
- capillaries, found within the tissues and only one cell thick to allow easy exchange of nutrients and waste products
- veins, large vessels with thinner less-muscular walls than arteries and whose function is to carry 'used' blood back to the heart.
Unlike in the other vessels, the blood in the arteries is largely under the influence of the beating heart. This can be felt as a pulse at various points around the body.
How the cardiovascular system affects blood pressure
The amount of blood that the heart is pumping, the force with which it pumps, and the diameter of the vessels all have an effect on pressure.
The heart can respond to or cause changes in blood pressure by increasing the frequency and force of its contractions.
The muscular walls of arteries and veins can expand and contract to control the flow of blood around the body. If pressure drops, some of these vessels will constrict to prevent blood from flowing in less essential areas. This causes more blood and therefore more pressure in areas which need to be maintained, such as the brain.
These effects can be seen during exercise, when blood pressure drops due to sweating. The water and salt that sweat is made up of is taken from the blood, so the volume of blood drops and with it the pressure.
Imagine letting some air out of our balloon: the pressure inside will get lower as air is lost.
In order to maintain the pressure needed at the muscles and brain during exercise, the heart begins pumping harder and faster, and blood vessels to organs that are non-essential during exercise, such as the stomach and liver, constrict to prevent blood flow.
After exercise, once these recovery systems return to normal, the pressure will remain low until drinking and eating restore blood volume.
Renal system
The kidneys play an important part in the control of blood pressure. Under normal circumstances, the their main function is to filter waste products, excess water, and salts out of the blood, which are excreted in th eform of urine.
Excess water and salt will increase blood volume and cause elevated pressure. Removal of these thereby helps maintain blood pressure.
Similarly, too little water and salt in the blood will lead to the blood pressure becoming lowered. If this happens, the kidney acts to retain as much as possible in the blood to increase blood volume.
High blood pressure (hypertension)
A person consistently giving readings of 140/90 or higher would be considered to have high blood pressure, also known as hypertension. Systolic pressures between 130 and 139 and diastolic pressures between 85 and 89 normally need to be watched carefully.
Classifying blood pressure
| Systolic | Diastolic | Condition |
| Less than 130 and | Less than 85 | Normal blood pressure |
| 130-139 or | 85-89 | High-normal blood pressure |
| 140-159 or | 90-99 | Stage 1 (mild) hypertension |
| 160-179 or | 100-109 | Stage 2 (moderate) hypertension |
| 180-209 or | 110-119 | Stage 3 (severe) hypertension |
| 210 or higher or | 120 or higher | Stage 4 (very severe) hypertension |
If your systolic and diastolic blood pressures are at different levels, the higher level is your blood pressure class.
Primary versus secondary hypertension
High blood pressure can either be 'primary', known as 'essential', or 'secondary'.
Primary hypertension is when there is no obvious cause for the elevation.
Secondary hypertension is when the blood pressure is high as a result of another (primary) illness.
What causes blood pressure?
Although some factors which contribute to the risk of developing high blood pressure are beyond our control, such as age, gender and genetic predisposition, more important contributory factors, such as diet, exercise and smoking can be modified and used effectively in the battle against high blood pressure.
As mentioned earlier, hypertension can occur as a result of another condition. Such primary illnesses include diabetes, kidney disorders, hormonal imbalance, or pregnancy. Relieving the primary condition is often enough to eliminate the problem.
In approximately 95% of cases, however, primary illness is not the cause of the rise in blood pressure. Such instances are known as essential hypertension, the major cause being a narrowing of arteries which increases pressure in blood vessels in the same way as narrowing the end of the running hosepipe.
This is usually a result of a build-up of cholesterol in the arteries, causing fatty deposits to stick to arterial walls. One manifestation of high blood pressure is that vessels become even more narrowed due to thickening of the walls, thus a vicious cycle begins and every part of the body is at risk.
Implications for health
Even mild hypertension over a long period will damage both the heart muscle and blood vessels all around the body. This will have varying consequences depending on the tissue.
Kidneys and eyes are often the worst affected. The blood vessels become 'leaky' under the strain and blood contaminates the surrounding areas. Damaged blood vessels will also result in the tissue becoming under-supplied, which can hinder its growth and function.
02/06/2009