What is hypertension?
Hypertension is the medical term for elevated blood pressure. This is a higher than normal pressure within the blood vessels as blood travels through it. High blood pressure in the short term does not cause any significant damage in the body and may even go unnoticed. However, prolonged elevation of the blood pressure can lead to a host of diseases affecting primarily the cardiovascular system and having secondary effects on almost every organ and system in the body.
Normal and High Blood Pressure
The accurate definition of hypertension is the elevation of arterial blood pressure – pressure against the arterial walls. Pressure within the arteries ensures that there is sufficient force to propel oxygen-rich blood to all the tissues in the body. It also ensures that this force is transmitted through to the veins so that the oxygen-deficient blood can return back to the heart for re-oxygenation at the lungs.
The pressure at which the blood has to be maintained without causing damage to the blood vessels or body is commonly referred to as the normal blood pressure. A systolic pressure of 120 mm Hg and diastolic pressure of 80 mm Hg is considered as a normal blood pressure in adults (120/80 mm Hg). It can be slightly higher or lower and still remain within a normal range.
If it rises significantly above this then it is defined as hypertension according to the criteria below. If the blood pressure is significantly lower than the normal value then it is defined as hypotension (low blood pressure).
With regards to the actual pressure and values, the following criteria need to be present for a diagnosis of hypertension to be established.
- a systolic pressure, which is the pressure in the blood vessels during contraction of the heart, exceeding 139 mm Hg.
- a diastolic pressure, which is the pressure in the blood vessels while the heart is relaxing and the ventricles filling with blood, exceeding 89 mm Hg.
Ideally, three readings showing an elevated blood pressure of 140/90 mm Hg should be recorded in order for hypertension to be diagnosed.
Types of Hypertension
Hypertension can be broadly divided into benign and malignant.
- Benign hypertension includes primary (essential) hypertension and secondary hypertension.
- Primary hypertension is also known as essential hypertension or idiopathic hypertension. The exact cause is unknown although the disease mechanism has been established to a large degree and a variety of hypotheses exists as to why it occurs.
- Secondary hypertension is a consequence of certain diseases.
- Benign hypertension, primary or secondary, can lead to a host of complications over several years or even decades.
- Malignant hypertension is also known as accelerated hypertension and accounts for a minority of hypertension cases. It is a sudden and severe form of hypertension which if left untreated can lead to death within one or two years.
Although the cause of primary hypertension is not fully understood, its close link to obesity and often improvement after weight loss may suggest one or more of the following mechanisms :
- Cardiac output increased as blood needs to be distributed to a larger body mass.
- Vascular resistance caused by constriction of the arteries (vasoconstriction) as a result of sympathetic activity and possibly further contributed to by hormonal influence associated with increased fat stores.
- Salt and water retention is due to greater reabsorption from the renal tubules (kidney) and normal mechanisms for water-electrolyte balance may be disrupted in obesity.
Primary hypertension may therefore be due to a combination of one of more of the factors above. A related concept that is important to understand is the renin-angiotensin system which may lead to vasoconstriction as well as salt and water reabsorption. This system exists to help the body stabilize the blood pressure in the event of a drop in pressure. However, in patients with primary hypertension, this system appears to be overactive. The effects of the renin-angiotensin system is to cause vascular resistance and increase salt and water resistance.
Explain High Blood Pressure
The simplest way to explain the concept of hypertension is to consider the analogy of the garden hose or hose pipe. The water needs to exit the hose at a certain pressure which will allow it to reach its destination. The force of the water spraying out at the end is proportional to the pressure within the pipe.
- The more a faucet is opened, the higher the pressure of the water and faster the speed of the spraying water. This is related to the cardiac output component of blood pressure explained above.
- If the pipe is wider, the pressure is reduced and the water exits at a slower speed. If the pipe is narrower, the pressure within the pipe is greater. This aspect is related to the increased vascular resistance (vasoconstriction).
- If a larger than normal volume of water is pushed through a pipe, then the pressure increases just as is the case with water retention.