25.01 Introduction

The circulatory system has a number of important physiological functions. These include transport of metabolites, hormonal and temperature regulation and protection against disease. All of the molecules needed for normal cellular metabolism are transported in the blood. Red blood cells carry oxygen to tissues and remove carbon dioxide waste gases. In addition, the blood carries nutrients from digestion and absorption of foodstuffs to target cells and removes excess water and unwanted waste products of cellular metabolism to the kidney for excretion. Blood also transports hormones and other cellular control molecules from site of synthesis to their site of action and is involved in body temperature regulation. Furthermore the circulatory system aids in protection against foreign invasion by toxin or bacteria through protective white blood cells (leukocytes), whilst clotting mechanisms protect against blood loss when the blood vessels of the circulatory system are damaged.

Blood comprises two portions, a cellular component termed the “formed elements” and a “fluid component” termed plasma. The plasma is a pale yellow fluid that consists of water, dissolved solutes, soluble metabolites, hormones, antibodies and other proteins including the albumins and globulins which are used to transport molecules around the body and fibrinogen, which is an important clotting factor. Within the “formed elements” are the cellular components of blood including the erythrocytes (used to transport oxygen via haemoglobin), platelets (involved in blood clotting) and the leukocytes (which comprise neutrophils, eosinophils, basophils, monocytes and lymphocytes and whose roles include phagocytosis and the release of antibodies as an immune response).