1.3 Blood as Circulating Medium Foundation

Blood as Circulating Medium Foundation is the body of fundamental principles describing blood as the fluid tissue that occupies and moves through the cardiovascular system, serving as the physical medium by which gases, nutrients, hormones, and waste products are transported between the heart, vasculature, and tissues. It establishes the composition, physical properties, and functional roles of blood that underlie its behavior as it circulates under the pressures and flows generated by cardiovascular activity.

Composition of Blood

Cellular Components

Blood contains three principal cellular elements, red blood cells that carry oxygen, white blood cells that participate in immune defense, and platelets that contribute to clotting, each suspended within a fluid matrix.

Plasma as the Fluid Component

Plasma constitutes the liquid portion of blood, composed largely of water along with dissolved proteins, electrolytes, nutrients, and signaling molecules that must be transported to and from tissues.

Proportional Relationship Between Components

The relative proportion of cellular material to plasma, commonly expressed through hematocrit, influences the physical properties of blood and varies with physiological state and health.

Physical Properties Relevant to Circulation

Viscosity and Its Determinants

Blood viscosity, influenced primarily by red blood cell concentration, affects the resistance encountered as blood moves through vessels, forming one of the variables that determines flow according to fundamental fluid dynamic relationships.

Volume and Its Distribution

The total volume of blood within the circulation is distributed unevenly between arterial, capillary, and venous compartments, with the venous system holding the largest proportion at rest due to its high compliance.

Behavior as a Non-Uniform Fluid

Unlike a simple uniform fluid, blood exhibits properties dependent on vessel diameter and flow velocity, particularly in small vessels where cellular components alter local viscosity and flow patterns.

Functional Roles as a Circulating Medium

Transport of Respiratory Gases

Blood functions as the medium for transporting oxygen from the lungs to tissues and carbon dioxide from tissues back to the lungs, a role carried out primarily through hemoglobin contained within red blood cells.

Distribution of Nutrients and Waste

Blood carries absorbed nutrients from the digestive system to tissues throughout the body, while simultaneously transporting metabolic waste products toward the organs responsible for their elimination.

Transport of Regulatory Molecules

Hormones and other signaling molecules travel through the blood to reach distant target tissues, allowing blood to function not only as a transport medium but as a communication pathway integrating physiological regulation across the body.