Blood as Circulating Medium

Blood as Circulating Medium is the characterization of blood as the specialized fluid tissue that is continuously propelled through the cardiovascular system, functioning as the primary transport vehicle for gases, nutrients, hormones, waste products, and cellular defense components between the tissues of the body and the organs responsible for exchange, processing, and elimination.

Composition of Blood as a Transport Fluid

Plasma as the Liquid Component

Plasma, the liquid component of blood, is composed predominantly of water and serves as the solvent medium in which proteins, electrolytes, nutrients, hormones, and waste products are dissolved or suspended for transport. Plasma proteins, including albumin, globulins, and fibrinogen, contribute to oncotic pressure, immune defense, and clotting capacity respectively.

Formed Elements

Suspended within plasma are the formed elements of blood: erythrocytes, which carry oxygen and carbon dioxide; leukocytes, which provide immune surveillance and defense; and platelets, which participate in hemostasis. The relative proportion of these formed elements to plasma volume, expressed as hematocrit, influences the physical properties of blood as it flows.

Gas Transport Function

Oxygen Carriage

Hemoglobin within erythrocytes reversibly binds oxygen in the pulmonary capillaries, where oxygen tension is high, and releases it in peripheral tissue capillaries, where oxygen tension is low, enabling blood to serve as the principal vehicle for oxygen delivery from the lungs to metabolically active tissue.

Carbon Dioxide Removal

Blood transports carbon dioxide from tissues back to the lungs through three mechanisms: dissolved directly in plasma, bound to hemoglobin as carbaminohemoglobin, and, predominantly, converted to bicarbonate ion in plasma, a form that also contributes to the buffering of blood pH.

Nutrient and Metabolic Transport

Delivery of Substrates

Blood carries absorbed nutrients, including glucose, amino acids, and lipids, from the digestive system to tissues throughout the body, and distributes stored substrates mobilized from the liver and adipose tissue during periods of fasting or increased metabolic demand.

Removal of Metabolic Waste

Metabolic waste products, including urea, creatinine, and uric acid, are transported through the blood from tissues of production to the kidneys and liver for filtration, processing, and elimination, preventing their accumulation to harmful concentrations.

Regulatory and Signaling Transport

Hormone Distribution

Endocrine hormones released from glands are carried through the bloodstream to distant target tissues, allowing blood to function as the communication medium of the endocrine system and enabling coordinated regulation of processes across the entire body.

Thermoregulatory Function

Blood transports heat generated by metabolically active tissues to the skin and other surfaces where it can be dissipated, or conserves heat by redistributing flow away from the periphery, contributing to the maintenance of stable core body temperature.

Defense and Hemostatic Transport

Immune Cell Circulation

Leukocytes circulate within the blood, continuously surveying the vasculature and able to be rapidly recruited from the bloodstream into tissue at sites of infection or injury, making blood the principal conduit through which immune cells reach affected regions of the body.

Hemostatic Component Delivery

Platelets and clotting factors circulate in an inactive state within the blood, ready to be rapidly activated and delivered to a site of vascular injury, where they participate in the formation of a hemostatic plug to limit blood loss.

Blood Flow as the Enabling Mechanism

The transport functions of blood depend entirely on its continuous circulation, driven by the pumping action of the heart and distributed through the vascular network, without which the diffusional and convective processes that deliver and remove substances from tissue would be far too slow to sustain the metabolic demands of a large, multicellular organism.