Blood Pressure

BLOOD PRESSURE

Blood pressure is the force exerted by circulating blood on the walls of blood vessels, especially arteries. It plays a vital role in ensuring continuous blood flow to all tissues and organs.

If blood pressure becomes too high, blood vessels may get damaged, leading to:

• Formation of blood clots
• Rupture of blood vessels
• Internal bleeding

If blood pressure becomes too low, blood flow through tissue beds may be insufficient. This is particularly dangerous for vital organs such as the heart, brain, and kidneys, which require a constant supply of oxygenated blood.

The systemic arterial blood pressure, commonly called arterial blood pressure, is mainly produced due to the discharge of blood from the left ventricle into the already filled aorta.

Blood pressure varies depending on:

• Time of the day
• Body posture
• Age and gender
• Physical activity and rest

Blood pressure usually falls during rest and sleep. It generally increases with age and is often slightly higher in women than in men.

Types of Blood Pressure

Blood pressure is of two main types:

1.     Systolic Blood Pressure

2.     Diastolic Blood Pressure

Systolic Blood Pressure

Systolic blood pressure is the maximum pressure exerted on arterial walls. It occurs during systole, when the heart contracts and ejects blood into the arteries.

The normal range of systolic pressure is approximately 100–120 mm Hg.

Diastolic Blood Pressure

Diastolic blood pressure is the minimum pressure present in the arteries. It occurs during diastole, when the heart relaxes between two contractions.

The normal range of diastolic pressure is approximately 60–80 mm Hg.

Normal Blood Pressure Values:

PULSE PRESSURE

Pulse pressure is the difference between systolic and diastolic blood pressure.

Formula:

Pulse pressure = Systolic pressure − Diastolic pressure

Using average values:
Pulse pressure = 120 mm Hg − 80 mm Hg = 40 mm Hg

Pulse pressure gives an indication of the force generated by the heart during contraction.

FACTORS DETERMINING BLOOD PRESSURE

Blood pressure mainly depends on cardiac output and peripheral resistance.

Blood Pressure = Cardiac Output × Peripheral Resistance

Any change in these factors can alter blood pressure, although the body usually activates compensatory mechanisms to maintain normal levels.

Cardiac Output (CO)

Cardiac output is determined by:

• Heart rate
• Stroke volume

Factors that increase or decrease heart rate or stroke volume will directly affect cardiac output and blood pressure.

An increase in cardiac output raises both systolic and diastolic pressure.
An increase in stroke volume raises systolic pressure more than diastolic pressure.

Peripheral or Arteriolar Resistance

Arterioles are the smallest arteries and have a tunica media rich in smooth muscle, which responds to nerve and chemical stimulation.

Constriction and dilation of arterioles are the main determinants of peripheral resistance.

• Vasoconstriction → increases blood pressure
• Vasodilation → decreases blood pressure

With aging, elastic tissue in the tunica media is replaced by inelastic fibrous tissue, leading to a rise in blood pressure.

Autoregulation

Systemic blood pressure continuously rises and falls depending on activity level and body position.

However, organs can independently regulate their local blood flow and pressure, irrespective of systemic blood pressure.

This ability is known as autoregulation, and it protects tissues from sudden fluctuations in blood pressure.

CONTROL OF BLOOD PRESSURE (BP)

Blood pressure is regulated by two main mechanisms:

1.     Short-term control

2.     Long-term control

Short-term Control

Short-term regulation works on a moment-to-moment basis and involves:

• Baroreceptor reflex
• Chemoreceptors
• Circulating hormones

Long-term Control

Long-term regulation involves control of blood volume, mainly by:

• Kidneys
• Renin–angiotensin–aldosterone system

Baroreceptors

Baroreceptors are stretch-sensitive receptors that provide important input to the vasomotor center.

They are located in:

• Arch of the aorta
• Carotid sinuses

As the aorta leaves the left ventricle, it forms an arch and then descends through the thoracic and abdominal cavities.

Baroreceptors respond to stretch or distension of blood vessel walls and are therefore also called stretch receptors.

A change in blood pressure activates the baroreceptor reflex, which produces negative feedback responses to restore blood pressure to normal.

Chemoreceptors

Chemoreceptors are nerve endings located in:

• Carotid bodies
• Aortic bodies

They are primarily involved in the control of respiration.

Chemoreceptors are sensitive to changes in:

• Carbon dioxide levels
• Oxygen levels
• Blood pH

Central chemoreceptors are present on the brain surface in the medulla oblongata and measure the chemical composition of the surrounding cerebrospinal fluid.

Chemoreceptor input strongly influences the cardiovascular center only when:

• Severe respiratory disturbance occurs
• Arterial blood pressure falls below 80 mm Hg

Higher Centres in the Brain

Input to the cardiovascular centre (CVC) from higher brain centres is influenced by emotional states such as:

• Fear
• Anxiety
• Pain
• Anger

These emotions can stimulate changes in blood pressure.

The hypothalamus controls body temperature and influences the CVC by adjusting the diameter of blood vessels in the skin.

This mechanism plays an important role in heat loss and heat retention.

PULSE

The pulse is a wave of distension and elongation felt in the wall of an artery each time the left ventricle ejects blood into the circulation.

It represents the rhythmic expansion of arteries and provides valuable information about heart function and blood flow.

Clinical Significance of Pulse: