Blood Properties

Blood is an opaque red fluid, freely flowing but denser and more viscous than water. The characteristic colour is imparted by hemoglobin, a unique iron-containing protein. Hemoglobin brightens in colour when saturated with oxygen (oxyhemoglobin) and darkens when oxygen is removed (deoxyhemoglobin). For this reason, the partially deoxygenated blood from a vein is darker than oxygenated blood from an artery. The red cells constitute about 45 percent of the volume of the blood, and the remaining cells (white cells and platelets) less than 1 percent. The fluid portion, plasma, is a clear, slightly sticky, yellowish liquid. After a fatty meal plasma transiently appears turbid. Within the body the blood is permanently fluid, and turbulent flow assures that cells and plasma are fairly homogeneouslymixed.

When blood is shed, physicochemical changes are initiated that cause the blood to coagulate (see bleeding and blood clotting ). The blood clot consists of microscopic strands of a complex protein, called fibrin, forming a gel in which the erythrocytes and other cells are entrapped. When the clot shrinks, or retracts, it squeezes out an incoagulable yellowish fluid, which is called the serum. An anticoagulant can be added to the shed blood to prevent clot formation, thereby maintaining the blood in a fluid state. When blood treated in this way is undisturbed, the cells gradually settle because they are denser than the plasma; the red cellsgo to the bottom, the white cells and platelets form a thin white layer (buffy coat) overlying the red cells, and the plasma appears in the upper portion of the container. Rapid segregationof cells and plasma may be accomplished by the use of a centrifuge, a machine in which rapidrotation accelerates sedimentation by increasing gravitational forces.

The total amount of blood varies with age, sex, weight, body build, and other factors, but a rough average figure for adults is about 60 millilitres per kilogram of body weight. An averageyoung male has a plasma volume of about 35 millilitres, and a red cell volume of about 30 millilitres, per kilogram of body weight. There is little variation in the blood volume of a healthy person over long periods, although each component of the blood is in a continuous state of flux. In particular, water rapidly moves in and out of the bloodstream, achieving a balance with the extravascular fluids (those outside the blood vessels) within minutes. The normal volume of blood provides such an adequate reserve that appreciable blood loss is well tolerated. Withdrawal of 500 millilitres (about a pint) of blood from normal blood donorsis a harmless procedure. Blood volume is rapidly replaced after blood loss; within hours plasma volume is restored by movement of extravascular fluid into the circulation. Replacement of red cells is completed within several weeks. The vast area of capillary membrane, through which water passes freely, would permit instantaneous loss of the plasma from the circulation were it not for the plasma proteins, in particular, the plasma albumin. Capillary membranes are impermeable to albumin, the smallest in weight and highest in concentration of the plasma proteins. The osmotic effect of the plasma albumin retains fluid within the circulation, opposing the hydrostatic forces that tend to drive the fluid outward into the tissues.