Blood Pressure – Its Maintenance and Regulation
Introduction
Blood pressure is the force that blood exerts on the walls of blood vessels. Systemic arterial blood pressure ensures the essential flow of blood into and out of organs. Maintaining normal blood pressure is critical: high pressure can damage vessels, while low pressure can compromise blood flow to vital organs such as the heart, brain, or kidneys.
Blood pressure varies with time of day, posture, gender, and age. It falls at rest and during sleep, increases with age, and is usually higher in women than men.
Systolic and Diastolic Pressures
Systolic pressure: The pressure produced when the left ventricle contracts and pushes blood into the aorta (≈120 mmHg).
Diastolic pressure: The pressure in arteries during complete cardiac diastole (≈80 mmHg). The difference between systolic and diastolic pressure is called the pulse pressure.
Blood pressure is measured using a sphygmomanometer, expressed as systolic over diastolic pressure.
Elasticity of Arterial Walls
Large arteries contain substantial elastic tissue. When the left ventricle ejects blood, the aorta expands and then recoils, pushing blood forward into systemic circulation. This distension and recoil maintain diastolic pressure during cardiac diastole.
Factors Determining Blood Pressure:
Cardiac Output
Cardiac output depends on stroke volume and heart rate. Increases in cardiac output raise both systolic and diastolic pressures; increases in stroke volume raise systolic pressure more than diastolic.
Peripheral (Arteriolar) Resistance
Arterioles have smooth muscle that constricts or dilates in response to neural and chemical signals. Vasoconstriction increases blood pressure, while vasodilation decreases it. Aging replaces arterial elastic tissue with fibrous tissue, raising blood pressure.
Autoregulation
Organs can adjust blood flow independently of systemic blood pressure. This protects tissues from systemic pressure fluctuations, especially the kidneys and brain.
Regulation of Blood Pressure:
Blood pressure is regulated by:
- Short-term control: moment-to-moment adjustments via baroreceptors, chemoreceptors, and circulating hormones.
- Long-term control: regulation of blood volume via kidneys and the renin–angiotensin–aldosterone system (RAAS).
Short-Term Blood Pressure Regulation:
The cardiovascular centre (CVC) in the medulla and pons integrates signals from baroreceptors, chemoreceptors, and higher brain centres, controlling heart rate and vessel diameter.
Baroreceptors
Located in the aortic and carotid sinuses, baroreceptors sense stretch. Increased pressure activates parasympathetic output to slow the heart and inhibit sympathetic output, causing vasodilation. Low pressure triggers sympathetic stimulation to raise heart rate and constrict vessels.
Chemoreceptors
Found in carotid and aortic bodies, chemoreceptors detect CO2, O2, and pH changes. Severe disruption triggers sympathetic responses, increasing blood pressure and improving tissue perfusion.
Higher Brain Centres
Emotional states like fear, pain, or anger influence the CVC. The hypothalamus adjusts blood vessel diameter in the skin to regulate body temperature.
Long-Term Blood Pressure Regulation:
Long-term control involves the RAAS, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP). RAAS and ADH increase blood volume, raising pressure. ANP reduces sodium and water retention, lowering pressure.
Pressure in the Pulmonary Circulation
Pulmonary blood pressure is lower than systemic pressure due to extensive capillaries in the lungs. Pressures above 25 mmHg can cause pulmonary oedema. Autoregulation ensures blood flows through well-oxygenated air sacs.
Detailed Notes
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