Introduction to Cardiovascular Physiology: Structure, Function, and Blood Components

Lecture 1: Introduction to Cardiovascular Physiology

Why Do Humans Require a Circulatory System?

The circulatory system is essential for multicellular organisms to efficiently transport materials between cells and the external environment. Unlike single-celled organisms, which rely on diffusion, larger organisms require a specialized system to overcome limitations of distance and metabolic demand.

• Diffusion Limitations: In single-celled organisms, diffusion is sufficient for exchange of nutrients and waste. In multicellular organisms, most cells are not in direct contact with the environment, making diffusion inadequate.

• Transport Functions: The cardiovascular (CV) system transports gases (O2, CO2), nutrients (energy substrates), waste products, hormones (for communication), immune cells (defense), and heat (temperature regulation).

• Homeostasis: The CV system maintains homeostasis by matching delivery of oxygen and nutrients to tissue metabolic needs.

Example: During exercise, increased oxygen demand by muscles is met by increased blood flow, demonstrating the system's ability to adjust to metabolic needs.

Transport in the Cardiovascular System

The cardiovascular system facilitates exchange of substances between cells, blood, and the external environment. Transport occurs through specialized organs and tissues.

Transport Type	From	To
Entering the Body	Lungs	All cells
Entering the Body	Intestinal tract	All cells
Moved from Cell to Cell	Some cells	Liver for processing
Moved from Cell to Cell	Hormones (endocrine cells)	Target cells
Leaving the Body	All cells	Kidneys, skin, lungs

Additional info: This table summarizes the major routes of transport for gases, nutrients, hormones, and waste products in the body.

Overall Design of the Cardiovascular System

Main Structural Components

The cardiovascular system consists of a series of tubes (blood vessels) filled with fluid (blood) and connected to a pump (the heart). It is organized as a closed circuit, allowing blood to recirculate throughout the body.

• Heart: The central pump, divided into right and left sides, each serving different circulations.

• Blood Vessels: Tubes that transport blood; include arteries, veins, and capillaries.

• Blood: The circulating fluid, composed of plasma and cells.

Pulmonary and Systemic Circulations

The heart operates as two pumps:

• Right Heart: Pumps oxygen-depleted blood to the lungs (pulmonary circulation).

• Left Heart: Pumps oxygen-rich blood to the rest of the body (systemic circulation).

Arteries carry blood away from the heart, while veins return blood to the heart.

Parallel Arrangement of Vascular Beds

Major vascular beds are arranged in parallel, which prevents changes in blood flow to one organ from significantly affecting flow to others. This arrangement allows for independent regulation of blood supply to different tissues.

• Example: During vigorous exercise, blood flow increases to muscles without compromising flow to other organs.

Principles of Blood Flow and Resistance

Blood Flow Equation

Blood flow to an organ is determined by the pressure gradient and vascular resistance. The relationship is described by the following equation:

• Blood Flow ($Q$): Directly proportional to the pressure difference ($riangle P$) between arteries and veins, and inversely proportional to resistance ($R$).

$Q = \frac{\triangle P}{R}$

• Cardiac Output: The total blood flow from the heart into the arteries.

• Vascular Resistance: Primarily determined by the radius of arterioles; resistance increases as radius decreases.

$R \propto \frac{1}{r^4}$

Additional info: Small changes in arteriolar radius have large effects on resistance and thus on blood flow.

Main Components of Blood and Their Functions

Plasma

Plasma is the liquid component of blood, making up about 55% of its volume. It consists mainly of water, proteins, salts, and organic molecules.

• Water: ~92% of plasma; serves as a solvent and medium for transport.

• Proteins: ~7%; includes albumin, globulins, and fibrinogen, which are important for osmotic balance, immunity, and clotting.

• Other Solutes: ~1%; includes electrolytes, nutrients, hormones, and waste products.

Cellular Elements

• Red Blood Cells (Erythrocytes): Specialized for oxygen transport via hemoglobin.

• White Blood Cells (Leukocytes): Involved in immune defense and response to pathogens.

• Platelets: Cell fragments derived from megakaryocytes; essential for blood clotting.

Hemoglobin in red blood cells binds oxygen for delivery to tissues and assists in carbon dioxide transport back to the lungs.

Summary Table: Main Components of Blood

Component	Percentage	Main Function
Plasma	~55%	Transport of nutrients, hormones, waste; maintains osmotic balance
Red Blood Cells	~45%	Oxygen and carbon dioxide transport
White Blood Cells	<1%	Immune defense
Platelets	<1%	Blood clotting

Additional info: Blood composition and function are critical for maintaining tissue health and responding to injury or infection.