Blood pressure is the pressure of blood in the circulatory system. There is an arterial blood pressure, a capillary blood pressure, and a venous blood pressure. Unless specified, blood pressure means arterial blood pressure.
1. Systolic and Diastolic Blood Pressure
At every systole of the heart, a large amount of blood is pumped into the arteries and the pressure increases steeply. Some of the extra blood pumped is accommodated by the stretching or compliance of the arteries which are distensible. Even so, the pressure rises considerably during systole and an average reading is 120 mm Hg. This is called the systolic blood pressure.
When the heart relaxes (diastole), no fresh blood is pumped into the arterial system. Some amount of blood escapes through the arterioles into the capillaries and therefore the pressure falls. Because arterioles prevent the arteries from being completely emptied out and also because a considerable volume of blood is accommodated by the compliant arteries, blood pressure does not fall to zero but remains at 80 mm Hg till the next systole. This is called the diastolic blood pressure. The blood pressure is expressed as 120/80 mm Hg, which means a systolic BP of 120 mm Hg and a diastolic BP of 80 mm Hg.
2. Normal Blood Pressure
The average blood pressure in the population is 120/80 mm Hg. This is considered to be normal. A blood pressure of 140/90 mm Hg is considered as the higher side of normal and blood pressure of 90/60 is considered to be the lower side of normal.
3. Measurement of Blood Pressure
pressure can be measured by putting a needle inside the artery. However, this is injurious. Therefore it is indirectly measured by an instrument known as the sphygmomanometer. It is commonly called the blood pressure instrument. Newer methods of blood pressure measured by electronic instruments have not been successful because they are prone to error.
4. Factors Controlling Blood Pressure
The 2 most important factors which control blood pressure are the cardiac output and the peripheral resistance.
The higher the cardiac output, the higher is the blood pressure. Cardiac output depends on the efficiency of the left ventricle and the blood volume. The peripheral resistance is mainly dependent on the state of contraction of the arterioles also known as the arteriolar pinchcock. When the arteriole is constricted, the peripheral resistance is high. When the arteriole is dilated, the peripheral resistance is low.
The blood pressure depends on the diameter of the Merioles raised to power 4. Very small changes in arteriolar diameter can cause huge changes in peripheral resistance and blood pressure. Therefore the most important factor controlling the blood pressure is the peripheral resistance. Each of the factors will be taken in greater detail later.
5. Regulation of Blood Pressure
As mentioned earlier the most important factors regulating blood pressure (BP) are cardiac output (CO) and peripheral resistance (PR), Of these factors PR is by far the more important.
Peripheral Resistance (PR)
PR is primarily determined by the state of contraction of the arteriole. What are the factors which influence the arteriole?
(a) The most important factor is a chemical substance which causes contraction of the arteriole, angiotensin II. Angiotensin II is formed by the action of an enzyme which converts angiotensin I to angiotensin II. The enzyme is called angiotensin converting enzyme or ACE. The precursor of angiotensin is renin, chemical substances produced in the kidney.
(b) Other chemical substances can also cause contraction of the arterioles, e.g. sympathomimetic amines such as nor adrenaline, but they are of lesser importance.
(c) Metabolites can cause relaxation of arterioles but these are essentially local or regional and do not influence BP significantly.
Cardiac output is influenced by many factors. In the case of a normal heart, the most important factor is blood volume. Blood volume is normally 5-6 liters. It depends essentially on the function of the kidney in controlling Na+ (sodium) excretion. When Na+ is excreted, water is lost in the urine. Substances which cause retention of Na+ cause increase of blood volume because Na+ (osmotically) retains water. The most important hormone which retains Na+ is aldosterone produced in the adrenal glands. When blood volume increases, the volume of venous return of blood to the heart increases. As a result, the amount of blood pumped out by the heart (cardiac output) increases and blood pressure rises.
For the body to exercise the regulatory mechanisms to maintain a constant blood pressure, there are baroreceptors (pressure receptors) in the carotid arteries in the neck. Information is transmitted by the baroreceptors to the vasomotor center in the brain which sets into motion, the mechanisms which regulate blood pressure.