Define Blood Pressure. Enumerate the various factors affecting the blood pressure. How is the blood pressure regulated in the long term ?

Definition of Blood Pressure

Blood pressure is typically expressed in millimeters of mercury (mmHg) as systolic/diastolic pressure. Systolic pressure represents the maximum pressure during ventricular contraction, while diastolic pressure represents the minimum pressure during ventricular relaxation. Normal blood pressure is generally considered to be less than 120/80 mmHg.

Factors Affecting Blood Pressure

Numerous factors influence blood pressure, which can be broadly categorized as follows:

1. Cardiac Output (CO)

• Heart Rate (HR): Increased HR elevates CO and thus BP.
• Stroke Volume (SV): Increased SV (amount of blood ejected per beat) also increases CO and BP. SV is affected by preload, afterload, and contractility.

2. Peripheral Vascular Resistance (PVR)

• Vessel Diameter: Vasoconstriction increases PVR and BP; vasodilation decreases PVR and BP.
• Blood Viscosity: Higher viscosity increases PVR.
• Vessel Length & Number of Vessels: Longer vessels and fewer vessels increase PVR.

3. Blood Volume

• Increased blood volume elevates BP, while decreased blood volume lowers BP.
• Factors affecting blood volume include fluid intake, renal function, and hormonal regulation.

4. Blood Viscosity

• Higher hematocrit (percentage of red blood cells) increases viscosity and BP.
• Plasma protein concentration also influences viscosity.

5. Arterial Compliance (Elasticity)

• Reduced arterial compliance (stiffness) increases systolic BP and decreases diastolic BP.
• Aging and atherosclerosis contribute to reduced arterial compliance.

6. Hormonal Factors

• Angiotensin II: Potent vasoconstrictor, increases BP.
• Aldosterone: Increases sodium and water reabsorption, increasing blood volume and BP.
• Adrenaline & Noradrenaline: Increase HR, SV, and vasoconstriction, raising BP.
• Atrial Natriuretic Peptide (ANP): Promotes sodium and water excretion, lowering blood volume and BP.

Long-Term Blood Pressure Regulation

Long-term blood pressure regulation primarily involves the kidneys and hormonal systems. The mechanisms are:

1. Renal-Body Fluid System

The kidneys play a central role in regulating blood volume and, consequently, blood pressure. They achieve this through:

• Pressure Natriuresis & Diuresis: Increased BP leads to increased sodium and water excretion by the kidneys, reducing blood volume and BP.
• Renin-Angiotensin-Aldosterone System (RAAS): Decreased BP stimulates renin release from the kidneys. Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II by ACE. Angiotensin II causes vasoconstriction, aldosterone release, and increased sodium and water reabsorption, ultimately increasing BP.

2. Hormonal Regulation

• RAAS (as described above): A key long-term regulator.
• ANP: Released by the atria in response to atrial stretch (increased blood volume). ANP opposes the effects of RAAS, promoting sodium and water excretion and vasodilation.

3. Nervous System Modulation

While the nervous system primarily mediates short-term BP regulation, it also influences long-term control by affecting renal sympathetic nerve activity, which impacts renin release and sodium reabsorption.

Short-Term Regulation	Long-Term Regulation
Baroreceptor reflex	Renal-Body Fluid System
Chemoreceptor reflex	RAAS
Autonomic Nervous System	ANP