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Blood pressure is a vital physiological parameter that reflects the force exerted by circulating blood on the walls of the arteries. It plays a crucial role in ensuring that oxygen, nutrients, and hormones reach every tissue of the body while waste products are carried away for elimination. The heart, acting as a muscular pump, generates this pressure with each beat, and the vascular system distributes it throughout the body.

As one of the four essential vital signs, along with pulse, respiration, and temperature, blood pressure is a key indicator of overall health. It is not a fixed value but a dynamic measurement that changes with activity, emotions, posture, and even time of day. Despite these variations, the body maintains blood pressure within a narrow range to preserve normal circulation and organ function.

Monitoring blood pressure is of great clinical importance. Persistently elevated pressure, called hypertension, increases the risk of heart disease, stroke, and kidney damage, while abnormally low pressure, called hypotension, can cause dizziness, fainting, and impaired organ perfusion. Because of these serious health implications, blood pressure measurement has become a routine and indispensable part of medical examinations and preventive healthcare.

Definition of Blood Pressure

Blood pressure is defined as the lateral pressure exerted by circulating blood on the walls of arteries while it flows through them. This pressure is generated mainly by the pumping action of the heart and is maintained by the resistance offered by blood vessels, the volume of blood in circulation, and the elasticity of arterial walls. It is expressed in millimeters of mercury (mmHg), the standard unit used in clinical practice.

A blood pressure reading consists of two values. The first, called systolic pressure, is the maximum pressure recorded in the arteries during the contraction of the ventricles when blood is forcefully pumped into the aorta. The second, called diastolic pressure, is the minimum pressure recorded when the heart is relaxed and filling with blood. A reading of 120/80 mmHg means the systolic pressure is 120 mmHg and the diastolic pressure is 80 mmHg.

Blood pressure is not a constant parameter; it fluctuates with physical activity, emotional state, posture, and physiological conditions. Despite these changes, the body maintains it within a normal range to ensure a continuous flow of blood through the tissues.

Normal Blood Pressure Values

Normal blood pressure values are essential to understand because they reflect the efficiency of the cardiovascular system and help in assessing overall health. In a healthy adult at rest, the average normal blood pressure is considered to be around 120/80 mmHg. Here, 120 mmHg represents the systolic pressure, which is the maximum pressure in the arteries during contraction of the heart, and 80 mmHg represents the diastolic pressure, which is the minimum pressure when the heart relaxes between beats.

Regulation of Blood Pressure

The human body has highly developed control systems that regulate blood pressure and keep it within normal limits, ensuring a constant supply of blood to vital organs such as the brain, heart, and kidneys. Regulation is achieved through the combined actions of the nervous system, hormones, and the kidneys, which work together to balance cardiac output, vascular resistance, and blood volume.

Neural Regulation

The autonomic nervous system plays a rapid and direct role in controlling blood pressure. Specialized pressure-sensitive receptors, called baroreceptors, are located in the walls of the carotid sinus and the aortic arch. When blood pressure rises, these receptors are stretched and send signals to the cardiovascular centers in the medulla oblongata. This activates the parasympathetic system, which decreases heart rate and dilates blood vessels, lowering pressure. When blood pressure falls, baroreceptor activity decreases, stimulating the sympathetic system, which increases heart rate, strengthens the force of contraction, and constricts blood vessels, thereby raising pressure.

Chemoreceptors located in the carotid and aortic bodies also contribute by sensing changes in oxygen, carbon dioxide, and pH levels. Low oxygen or high carbon dioxide stimulates sympathetic activity, causing vasoconstriction and an increase in blood pressure to improve tissue perfusion.

Hormonal Regulation

Several hormones maintain blood pressure by acting on the heart, blood vessels, and kidneys. The most important system is the renin–angiotensin–aldosterone system (RAAS). When blood pressure or blood volume falls, the kidneys release renin, which leads to the formation of angiotensin II. This hormone causes powerful vasoconstriction and stimulates aldosterone secretion from the adrenal glands, increasing sodium and water retention, thereby raising blood volume and pressure.

Other hormones also contribute. Adrenaline and noradrenaline, secreted by the adrenal medulla, increase heart rate, cardiac output, and cause vasoconstriction during stress. Antidiuretic hormone (ADH or vasopressin), released from the posterior pituitary, promotes water reabsorption in the kidneys and constricts blood vessels, raising pressure. In contrast, the heart releases atrial natriuretic peptide (ANP) when blood pressure is high; it reduces sodium and water retention, dilates vessels, and lowers pressure.

Renal Regulation

The kidneys are long-term regulators of blood pressure. By adjusting the excretion or retention of sodium and water, they control blood volume, which directly influences arterial pressure. When blood pressure rises, the kidneys excrete more sodium and water, reducing blood volume and lowering pressure. When blood pressure falls, they conserve sodium and water, expanding blood volume and raising pressure.

Local (Autoregulation) Mechanisms

Tissues and organs also regulate their own blood supply through autoregulation. For example, when a tissue becomes active, local metabolic changes such as low oxygen and increased carbon dioxide cause vasodilation, increasing blood flow to meet demand. Endothelial cells lining the blood vessels also release substances like nitric oxide, which causes relaxation of smooth muscle and lowers vascular resistance.

Factors Affecting Blood Pressure

Blood pressure is not a fixed value; it fluctuates throughout the day depending on internal physiological states and external environmental conditions. These variations are usually temporary and help the body adapt to different situations. However, when certain factors persist or become exaggerated, they may contribute to long-term changes and even cause hypertension or hypotension. The main factors influencing blood pressure are described below.

Age

Blood pressure changes with age. In infants and children, values are lower because of smaller body size and lower vascular resistance. As a person grows, blood pressure gradually increases. In older adults, systolic pressure often rises due to reduced elasticity of arterial walls, while diastolic pressure may remain stable or decrease slightly.

Sex

In early adulthood, females tend to have slightly lower blood pressure than males of the same age, largely due to hormonal influences. After menopause, women often experience a gradual rise in blood pressure, sometimes surpassing that of men.

Body Weight

Overweight and obesity are directly linked to higher blood pressure. Increased fat tissue demands more blood supply, which raises cardiac output and vascular resistance, leading to elevated pressure.

Exercise and Physical Activity

During physical activity, systolic blood pressure rises significantly to meet the increased oxygen demand of muscles, while diastolic pressure may remain unchanged or decrease slightly. Regular exercise, however, contributes to long-term lowering of resting blood pressure by improving heart efficiency and vascular elasticity.

Emotions and Stress

Emotional states such as anxiety, anger, excitement, and fear stimulate the sympathetic nervous system. This leads to increased heart rate, stronger contractions, and vasoconstriction, all of which temporarily raise blood pressure. Chronic stress may contribute to sustained hypertension.

Sleep and Rest

During deep sleep, blood pressure naturally falls as the metabolic demands of the body decrease. Lack of adequate sleep or sleep disorders, such as sleep apnea, can lead to persistently elevated blood pressure.

Posture

Blood pressure varies with body position. It is usually higher when standing compared to lying down because the body must work against gravity to maintain circulation. In some individuals, sudden standing can cause a temporary drop in blood pressure, a condition known as orthostatic hypotension.

Diet

High salt intake increases sodium retention, raising blood volume and pressure. Diets high in saturated fats may also contribute to vascular stiffness and hypertension. On the other hand, diets rich in fruits, vegetables, and potassium help lower blood pressure. Excessive alcohol and caffeine can also elevate pressure.

Environmental Factors

Cold temperatures cause vasoconstriction, leading to increased pressure, while warm temperatures promote vasodilation, lowering pressure.

Pathological Conditions

Certain medical conditions such as kidney disease, endocrine disorders (thyroid or adrenal abnormalities), and cardiovascular diseases can significantly influence blood pressure.

Measurement of Blood Pressure

Measurement of blood pressure is an essential clinical procedure used to assess the state of the cardiovascular system. It provides valuable information about how effectively the heart is pumping blood and how much resistance the arteries offer to blood flow. Accurate measurement is important for diagnosis, treatment, and monitoring of conditions such as hypertension and hypotension.

Units of Measurement

Blood pressure is measured in millimeters of mercury (mmHg). Two readings are always recorded:

• Systolic pressure, which represents the force exerted when the heart contracts.
• Diastolic pressure, which represents the force exerted when the heart relaxes.

The reading is expressed in the form of systolic over diastolic, for example 120/80 mmHg.

Instruments Used

The most common instrument used is the sphygmomanometer, which may be of three types:

1. Mercury sphygmomanometer, considered the gold standard for accuracy.
2. Aneroid sphygmomanometer, which uses a dial and requires regular calibration.
3. Digital sphygmomanometer, widely used at home and in clinics for quick readings.

A stethoscope is also used in the auscultatory method to listen to arterial sounds.

Abnormalities of Blood Pressure

Abnormal blood pressure refers to values that consistently fall outside the normal range, either too high or too low. Both conditions can have serious consequences on health and may indicate underlying medical issues. Understanding these abnormalities is crucial for timely diagnosis, management, and prevention of complications.

Hypertension (High Blood Pressure)

Hypertension is defined as a persistent elevation of blood pressure above 140/90 mmHg in adults.

Hypotension (Low Blood Pressure)

Hypotension is defined as a blood pressure reading consistently below 90/60 mmHg. While mild hypotension may not cause symptoms and can be normal in healthy individuals, severe hypotension can lead to inadequate blood flow to vital organs, causing dizziness, fainting, shock, or organ failure.