Anatomy and Physiology: The Circulatory System

The Circulatory System

You might remember that blood is a form of connective tissue (widely spaced cells in a matrix, in this case a fluid matrix). In this section, you will start to understand how blood might better be called the connective tissue. Most people grow up thinking of blood as part of the “circulatory” system, but as you shall see, there are in fact two systems involved in circulation: the cardiovascular system and the lymphatic system. In terms of transport, the cardiovascular system takes the top spot, but in terms of defending against bacteria and viruses, the lymphatic system gets top billing. In this section, I explore the connection between these two parallel systems.

Blood is the giver of life, the provider of food, water, and air, the waste remover (with the help of the kidneys; see The Excretory System), but it can also be the harbinger of death, if an infection makes its way into the blood and we become septic. Adult females have an average of four to five liters of blood, and adult males average about five to six liters. How the blood manages to get from place to place is the subject of The Heart and Cardiovascular and Lymphatic Circulation. Here I discuss what blood actually does.

Function Junction

With apologies to Tina Turner, what's blood got to do with it? Blood does many wondrous things:

• Blood (plus vessels and the heart) is the primary transportation system for materials (the lymphatic system, as you will see, is the secondary transportation system): nutrients, water, wastes, O₂, and CO₂.
• Blood, which is slightly alkaline (7.35 to 7.45), regulates pH levels by absorbing acids from the interstitial fluid and neutralizing them.
• Blood regulates body temperature by transferring (via the plasma) heat generated by muscles (hemopoiesis; see The Muscles) to tissues throughout the body; heat can also be retained or lost through the constriction and dilation of vessels in the skin. As such, the average temperature of blood is higher than body temperature (38ºC, or 100.4ºF).
• Blood protects from fluid loss by clotting at the site of injuries.
• Blood protects from toxins and pathogens, through the action of white blood cells and antibodies.

Fluids Galore!

Blood makes up about 8 percent of the body's weight. Over half of its weight (about 55 percent) is the liquid matrix known as plasma. With exception of the oxygen, and a little of the carbon dioxide, all other materials in need of transport make their way via the plasma. The plasma, more so than the cells, links the two circulatory systems—cardiovascular and lymphatic—together in a continuum: plasma to interstitial fluid to lymph and back to plasma.

Plasma Before Delivery

Blood plasma is almost 92 percent water. The fact that water is such an excellent solvent makes it possible for blood to carry so many dissolved substances (solutes). The majority of these substances are plasma proteins. The smallest of these proteins (which nonetheless make up about 60 percent of all blood proteins) are albumins, which are important in terms of osmosis, helping to move more water out of the capillaries. These albumins also help to transport steroid hormones.

About 35 percent of the proteins are called globulins. These include antibodies, which play an essential role in fighting infection. Two other globulins, alpha and beta, transport fat, fat-soluble vitamins, and iron. About 7 percent of plasma protein is fibrinogen, which is made in the liver, and is an essential part of the clotting process. The last 1 percent consists of regulatory proteins, such as proenzymes, enzymes, and hormones. Given that the endocrine system is powerless without the bloodstream, it's a bit sad that so little of the plasma is made up of hormones.

The remaining 1.5 percent of the plasma is made of other solutes: electrolytes, gases, nutrients, regulatory substances, vitamins, and waste products. Once again, these are awfully small amounts, but they are, nonetheless, very important! The electrolytes alone are quite varied: Na⁺, K⁺, Ca²⁺, Mg²⁺, C^-1 , HCO₃^- , HPO₄^2-, and HSO₄^2-. Sodium and calcium ions (Na⁺ and Ca²⁺), for example, are essential for muscle contraction (see The Structure of the Muscles and Muscle Cells), and bicarbonate ions (HCO₃^-) are essential in the transport of CO² to the lungs (see The Respitory System).

When clotting occurs, fibrinogen molecules will interact to form fibrin molecules (which are fiber-like), which will also trap the blood cells. The fluid that is left is called serum, and it will have other differences (aside from no fibrinogen, in solute concentration compared to plasma), such as a lower level of calcium ions.

Fluid Between the Cracks

Why am I talking about interstitial fluid if this section is on blood? Well, blood is mainly about transporting materials to and from the body tissues; the means of actually getting the materials into the cells outside the blood vessels is by first transferring it to the fluid that surround those cells. That fluid is called interstitial fluid.

Solutes in that fluid, as well as some of the water itself, enter the cells, and solutes and water enter the interstitial fluid from the cells. As such, the concentration of the various substances in the interstitial fluid will change (for example, O₂ enters the cells, and CO₂ exits). In the same way, the concentration of the solutes in the blood are different in the blood vessels before the capillaries, than in the blood vessels after the capillaries. I discuss how the materials leave the capillaries in Cardiovascular and Lymphatic Circulation.

Draining the Cracks

Ultimately, this interstitial fluid is drained into the lymphatic capillaries (or lymphatics), where it is called lymph. It is important to get the concept that these fluids—blood, interstitial fluid, and lymph—are essentially the same, despite any differences in concentration; the main difference between the three is location. (In the same way, the only difference between magma and lava is location: magma is below the ground, and lava is above.) Lymphatic capillaries are open-ended vessels with periodic gaps in their walls, which are the entry point for the drainage of the interstitial fluid. The lymph fluid is ultimately emptied into to the plasma via the thoracic duct to the subclavian vein (see Cardiovascular and Lymphatic Circulation).

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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