Cardiovascular Physiology: Regulation of Blood Pressure and Blood Flow

Cardiovascular Regulation

Overview of Blood Pressure and Blood Flow Regulation

The cardiovascular system maintains homeostasis by regulating blood pressure and blood flow to meet the metabolic demands of tissues. This regulation involves multiple factors, including heart function, blood vessel properties, blood composition, and neurohormonal control.

• Mean Arterial Pressure (MAP): The average pressure in the arteries during one cardiac cycle. It is a key determinant of tissue perfusion.

• Cardiac Output (CO): The volume of blood the heart pumps per minute. It is calculated as:

$CO = \text{Heart Rate} \times \text{Stroke Volume}$

• Total Peripheral Resistance (TPR): The resistance to blood flow offered by all of the systemic vasculature, primarily determined by arteriolar radius.

• Relationship: Mean arterial pressure is determined by cardiac output and total peripheral resistance:

$MAP = CO \times TPR$

Determinants of Cardiac Output

• Heart Rate (HR): The number of heartbeats per minute. Influenced by autonomic nervous system activity (sympathetic increases HR, parasympathetic decreases HR).

• Stroke Volume (SV): The amount of blood ejected by the left ventricle in one contraction. Influenced by venous return, contractility, and afterload.

• Venous Return: The flow of blood back to the heart, affecting preload and thus stroke volume.

• Sympathetic Stimulation: Increases heart rate and contractility, enhancing cardiac output.

• Epinephrine: A hormone that mimics sympathetic effects, increasing heart rate and contractility.

Determinants of Total Peripheral Resistance (TPR)

• Arteriolar Radius: The primary determinant of TPR. Vasoconstriction increases resistance; vasodilation decreases resistance.

• Blood Viscosity: Determined by the number of red blood cells (hematocrit). Increased viscosity raises resistance.

• Vasoconstrictors: Substances such as vasopressin (antidiuretic hormone), angiotensin II, and sympathetic nervous system activity increase arteriolar tone and resistance.

• Vasodilators: Factors such as local metabolites (e.g., in skeletal muscle during exercise) decrease resistance.

Neurohormonal Regulation

• Sympathetic Nervous System: Increases heart rate, contractility, and arteriolar constriction.

• Parasympathetic Nervous System: Decreases heart rate.

• Vasopressin (ADH): Promotes water retention and vasoconstriction, increasing blood pressure.

• Renin-Angiotensin-Aldosterone System (RAAS): Increases blood volume and vasoconstriction, raising blood pressure.

• Passive Bulk-Flow Fluid Shifts: Movement of fluid between vascular and interstitial compartments helps maintain blood volume and pressure.

Blood Composition and Its Effects

• Red Blood Cells: The number of red blood cells affects blood viscosity and thus resistance.

• Plasma Volume: Changes in plasma volume (e.g., due to fluid shifts or hormonal effects) influence venous return and cardiac output.

Summary Table: Major Factors Affecting Blood Pressure

Factor	Effect on Blood Pressure	Mechanism
Heart Rate	Increase	Increases cardiac output
Stroke Volume	Increase	Increases cardiac output
Arteriolar Radius	Decrease (vasodilation)	Decreases TPR
Blood Viscosity	Increase	Increases TPR
Sympathetic Activity	Increase	Increases HR, contractility, and vasoconstriction
Vasopressin/Angiotensin II	Increase	Vasoconstriction and fluid retention
Red Blood Cell Number	Increase	Increases viscosity and TPR

Example: Exercise and Blood Pressure Regulation

• During exercise, sympathetic activity increases, raising heart rate and contractility.

• Local vasodilation in skeletal muscle decreases resistance in those areas, while overall TPR may be maintained or slightly decreased.

• Cardiac output increases to meet the metabolic demands of active muscles.

Additional info: The notes reference Chapters 14 and 15 for more details on the renin-angiotensin-aldosterone system and fluid shifts. These systems are crucial for long-term regulation of blood pressure and volume.