How Blood Works

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How many times have you accidentally cut your finger and, when it begins to bleed, wonder what makes up this thing we call blood? The most commonly tested part of the body, blood is truly the river of life. Every cell in the body gets its nutrients from blood. In this edition of How Stuff Works, Dr. Carl Bianco teaches you How Blood Works so that you are knowledgeable as your doctor explains the results of your blood tests. In addition, you will learn amazing things about this incredible fluid and the cells in it!

Five liters of soda to illustrate the amount of blood kept in the adult human body.

The Basics
Blood is a mixture of two things: cells and plasma. The heart pumps blood through the arteries, capillaries, and veins to provide oxygen and nutrients to every cell of the body. The blood also carries away waste products.

The adult human body contains approximately 5 liters (5.3 quarts) of blood and takes up 7-8 percent of body weight. Approximately 2.75-3 liters of blood is plasma and the rest is the cellular portion.

Plasma is the liquid portion of the blood in which the blood cells are suspended. Dissolved in plasma are electrolytes, nutrients, and vitamins (absorbed from the intestines or produced by the body), hormones, clotting factors, and proteins such as albumin and immunoglobulins (antibodies to fight infection). Plasma distributes the substances it contains as it circulates throughout the body.

The cellular portion of blood contains red blood cells (RBCs), white blood cells (WBCs), and platelets. The RBCs carry oxygen from the lungs, the WBCs help to fight infection, and platelets are parts of cells that the body uses for clotting.

Plasma
Plasma is a clear, yellowish fluid the color of straw. Plasma can sometimes appear milky after a very fatty meal or when someone has a high level of lipids in their blood. Plasma is 90 percent water, yet the 10 percent of material dissolved in it are essential for life. These dissolved substances are circulated throughout the body and diffuse into tissues and cells where they are needed. They diffuse from areas of high concentration to areas of lower concentration. The greater the difference in concentration, the greater the amount of material that diffuses. Occasionally, these substances are actively pumped from one area to another. Waste materials flow in the opposite direction, from where they are created in the cells into the bloodstream where they are removed either in the kidneys or lungs.

Hydrostatic pressure or blood pressure pushes fluid out of blood vessels. Balancing this is something called Oncotic pressure (caused by proteins dissolved in blood) which tends to keep fluid inside the blood vessels.

Proteins make up a large part of the 10 percent of material dissolved in plasma and are responsible for Oncotic pressure. Proteins are larger molecules than water and tend to stay in blood vessels because they have more difficulty fitting through the pores in capillaries and therefore are in higher concentration in blood vessels. Proteins tend to attract water to keep their relative concentration in blood vessels more in line with fluid outside the blood vessels. This is one of the ways the body maintains a constant volume of blood.

Plasma contains 6.5-8.0 grams of protein per deciliter of blood. The main proteins in plasma are albumin (60 percent), globulins (alpha-1, alpha-2, beta, and gamma globulins (immunoglobulins)), and clotting proteins (especially fibrinogen). These proteins function to maintain oncotic pressure (especially albumin), transport substances such as lipids, hormones, medications, vitamins, and other nutrients. These proteins are also part of the immune system (immunoglobulins), help blood to clot (clotting factors), maintain pH balance, and are enzymes in chemical reactions throughout the body.

Electrolytes are another large category of substances dissolved in plasma. They include:

Sodium (Na+)

Potassium (K+)

Chloride (Cl-)

Bicarbonate (HCO3-)

Calcium (Ca+2)

Magnesium (Mg+2)

These chemicals are absolutely essential in many bodily functions including fluid balance, nerve conduction, muscle contraction (including the heart), blood clotting and pH balance.

Other materials dissolved in plasma are carbohydrates (glucose), cholesterol, hormones, and vitamins. Cholesterol is normally transported attached to lipoproteins such as Low Density Lipoproteins (LDLs) and High Density Lipoproteins (HDLs). For more information on cholesterol, read How Cholesterol Works.

When plasma is allowed to clot, the fluid left behind is called serum. When blood is collected from a patient it is allowed to clot in a test tube, where the cells and clotting factors fall to the bottom and the serum is left on top. Serum is tested for all the numerous items discussed above to determine if any abnormalities exist.

In addition to blood diseases, future blood-related articles will include How Red Blood Cells Work, How White Blood Cells Work, How Blood Platelets Work, and How Blood Transfusions & Donations Work.

About the Author
Carl Bianco, M.D. is an Emergency Physician practicing at Dorchester General Hospital in Cambridge, MD, located on Maryland's Eastern Shore. Dr. Bianco attended Georgetown University School of Medicine and received his undergraduate degree from Georgetown University majoring in nursing and pre-med. He completed an internship and residency in Emergency Medicine at Akron City Hospital in Akron, Ohio. Dr. Bianco lives near Baltimore with his wonderful wife and two wonderful children. He can be reached at carl.bianco@howstuffworks.com.