Blood

I

Introduction

Blood, fluid substance that circulates in the arteries and veins of the body. Blood is bright red or scarlet when it has been oxygenated in the lungs and passes into the arteries; it becomes bluish red when it has given up its oxygen to nourish the tissues of the body and is returning to the lungs through the veins and the tiny vessels called capillaries. In the lungs, the blood gives up the carbon dioxide wastes it has taken from the tissues, receives a new supply of oxygen, and begins a new cycle. This movement of blood is brought about by the coordinated activity of the heart, lungs, and blood vessels.

II

Composition of Blood

Blood is composed of a yellowish fluid, called plasma, in which are suspended the millions of cells that constitute about 45 per cent by volume of whole blood. It has a characteristic odour and a relative density between 1.056 and 1.066. In an average healthy adult, the volume of blood is one-eleventh of the body weight, or between 4.5 and 6 litres (5 and 6 qt).

A great portion of the plasma is composed of water, a medium that facilitates the circulation of the many indispensable factors of which blood is composed. A cubic millimetre of human blood contains about 5 million red corpuscles called erythrocytes; 5,000 to 10,000 white corpuscles called leucocytes; and 200,000 to 300,000 platelets called thrombocytes. The blood also carries many salts and organic substances in solution.

A

Erythrocytes

The red corpuscles, or red blood cells, are round discs, concave on two sides, and approximately 7.5 thousandths of a millimetre in diameter. In humans, and most other mammals, the mature red blood cell contains no nucleus; in some vertebrates, it is oval, and nucleated. Haemoglobin, a protein in the red blood cells, is the most prevalent of the special blood pigments that transport oxygen from the lungs to the body cells, where it picks up carbon dioxide for transport back to the lungs to be expired.

B

Leucocytes

The white blood cells are of two principal types: the granular, which have multilobed nuclei, and the nongranular, which have rounded nuclei. The granular leucocytes include neutrophils, which ingest and destroy bacteria; eosinophils, which increase and become active in the presence of certain infections and allergies; and basophils, which secrete the anticoagulant heparin and the substance histamine, which stimulates inflammation. The nongranular leucocytes are the lymphocytes and the less numerous monocytes, both associated with the immune system. Lymphocytes have an important role in producing antibodies and in cellular immunity. Monocytes ingest nonbacterial foreign substances, usually during chronic infection.