The circulating blood is of fundamental importance in maintaining homeostasis. This life-giving fluid brings nutrients and oxygen to the cells and carries away waste. The heart pumps blood continuously through a closed system of vessels. Blood is classified as a connective tissue because it consists of cells suspended in an intercellular background material, or matrix. Blood cells share many characteristics of origination and development with other connective tissues. However, blood differs from other connective tissues in that its cells are not fixed in position; instead, they move freely in the plasma, the liquid portion of the blood. Whole blood is a viscous (thick) fluid that varies in color from bright scarlet to dark red, depending on how much oxygen it is carrying. (It is customary in drawings to color blood high in oxygen as red and blood low in oxygen as blue.) The blood volume accounts for approximately 8% of total body weight. The actual quantity of circulating blood differs with a person’s size; the average adult male, weighing 70 kg. (154 pounds), has about 5 liters (5.2 quarts) of blood.

Functions of the Blood

The circulating blood serves the body in three ways: transportation, regulation, and protection.
* Oxygen from inhaled air diffuses into the blood through thin membranes in the lungs and is carried by the circulation to all body tissues. Carbon dioxide, a waste product of cell metabolism, is carried from the tissues to the lungs, where it is breathed out.
* The blood transports nutrients and other needed substances, such as electrolytes (salts) and vitamins, to the cells. These materials enter the blood from the digestive system or are released into the blood from body reserves.
* The blood transports the waste products from the cells to sites where they are removed. For example, the kidney removes excess water, acid, electrolytes, and urea (a nitrogen-containing waste). The liver removes blood pigments, hormones, and drugs, and the lungs eliminate carbon dioxide.
* The blood carries hormones from their sites of origin to the organs they affect.

* Buffers in the blood help keep the pH of body fluids steady at about 7.4. (The actual range of blood pH is 7.35 to 7.45.) Recall that pH is a measure of the acidity
or alkalinity of a solution. At an average pH of 7.4, blood is slightly alkaline (basic).
* The blood regulates the amount of fluid in the tissues by means of substances (mainly proteins) that maintain the proper osmotic pressure. Recall that osmotic pressure is related to the concentration of dissolved and suspended materials in a solution. Proper osmotic pressure is needed for fluid balance.
* The blood transports heat that is generated in the muscles to other parts of the body, thus aiding in the regulation of body temperature.

* The blood is important in defense against disease. It carries the cells and antibodies of the immune system that protect against pathogens.
* The blood contains factors that protect against blood loss from the site of an injury. The process of blood coagulation, needed to prevent blood loss, is described later.

Blood Constituents

The blood is divided into two main components (Fig. 9-1). The liquid portion is the plasma. The formed elements, which include cells and cell fragments, fall into three categories, as follows:
* Erythrocytes, from erythro, meaning “red,” are the red blood cells, which transport oxygen.
* Leukocytes, from leuko, meaning “white,” are the several types of white blood cells, which protect against infection.
* Platelets, also called thrombocytes, are cell fragments that participate in blood clotting.
Figure 9-2 shows all the categories of formed elements in a blood smear, that is, a blood sample spread thinly over the surface of a glass slide, as viewed under a microscope.
Figure 9-1 Composition of whole blood. Percentages show the relative proportions of the different components of plasma and formed elements.
Blood cells as viewed under the microscope
Composition of whole blood
Figure 9-2 Blood cells as viewed under the microscope. All three types of formed elements are visible.
The remaining 1% of the plasma consists of nutrients, electrolytes, and other materials that must be transported. With regard to the nutrients, the principal carbohydrate found in the plasma is glucose. This simple sugar is absorbed from digested foods in the intestine. It is also stored as glycogen, mainly in the liver, and released as needed into the blood to supply energy to the cells. Amino acids, the products of protein digestion, also circulate in the plasma. Lipids constitute a small percentage of blood plasma. Lipid components include fats, cholesterol, and lipoproteins, which are proteins bound to cholesterol. The electrolytes in the plasma appear primarily as chloride, carbonate, or phosphate salts of sodium, potassium, calcium, and magnesium. These salts have a variety of functions, including the formation of bone (calcium and phosphorus), the production of certain hormones (such as iodine for the production of thyroid hormones), and the maintenance of the acid-base balance (such as sodium and potassium carbonates and phosphates present in buffers). Other materials transported in plasma include vitamins, hormones, waste products, drugs, and dissolved gases, primarily oxygen and carbon dioxide.
Blood Plasma
About 55% of the total blood volume is plasma. The plasma itself is 91% water. Many different substances, dissolved or suspended in the water, make up the other 9% by weight (see Fig. 9-1). The plasma content may vary somewhat because substances are removed and added as the blood circulates to and from the tissues. However, the body tends to maintain a fairly constant level of most substances. For example, the level of glucose, a simple sugar, is maintained at a remarkably constant level of about one tenth of one percent (0.1%) in solution. After water, the next largest percentage (about 8%) of material in the plasma is protein. The plasma proteins include the following:
* Albumin, the most abundant protein in plasma, is important for maintaining the osmotic pressure of the blood. This protein is manufactured in the liver.
* Clotting factors, necessary for blood coagulation, are also manufactured in the liver.
* Antibodies combat infection. Antibodies are made by certain white blood cells.
* Complement consists of a group of enzymes that helps antibodies in their fight against pathogens.
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