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The Fluid of Life


What is Blood?

The average adult has about five liters of blood coursing through their vessels. It delivers essential elements, and removing harmful wastes from each and every cell in the body. What most medical and allied health students don't realize until it is mentioned either by their instructor or they read about it in their anatomy text book is that blood falls under the category of connective tissues!


blood catheterOur bodies consist of metabolically active cells that need a continuous supply of nutrients and oxygen. Metabolic waste products need to be removed from the cells to maintain a stable cellular environment. Blood is the primary transport medium that is responsible for meeting these cellular demands. Materials transported by the blood include nutrients, waste products, gases, and hormones. The blood helps to regulate the fluid and electrolyte balance, acid base balance, and the body temperature. Protection against pathogens is provided by white blood cells, and the clotting mechanism prevents excessive loss of blood after injuries.

Functions of Blood

BLOOD FUNCTIONS AS A TRANSPORT MEDIUM. IT ALSO HAS ROLES IN TEMPERATURE REGULATION, FLUID AND ELECTROLYTE BALANCE, pH REGULATION, PREVENTION OF FLUID LOSS, NOURISHMENT AND DISEASE PREVENTION. The general functions of blood include transportation, regulation, and protection. These categories overlap and interact as the blood carries out its role in providing suitable conditions for cellular function.

  1. The following activities are under the transportation function:
    • Carries O2 and nutrients to the cells
    • Transports CO2 and wastes from the tissues to the lungs and the kidneys where wastes can be removed from the body
    • Carries hormones to the endocrine glands to the target tissue
  2. The following activities are under the regulation function:
    • Helps to regulate body temperature by removing heat from active areas, such as skeletal muscles and transporting it to other areas of the skin so the heat can be dissipate
    • Plays a significant role in fluid and electrolyte balance because salt and plasma proteins contribute to the osmotic pressure by providing weight and bulk to our blood
    • Functions in pH regulation through the action of buffers in the blood
  3. Functions of the blood that are in the protection category:
    • Clotting mechanisms prevent fluid loss through hemorrhage when blood vessels become damaged
    • Certain cells in the blood, phagocytic white blood cells help to protect the body against diseases by engulfing and destroying the agent
    • Antibodies in the plasma help protect against disease by their reactions with offending agents


Characteristics of Blood

  1. Blood has distinctive physical characteristics:

    Amount—a person has 4-6 liters of blood, depending on the body size. Of the total blood volume in the human body, 38% to 48% is composed of the various blood cells also called “formed elements”. The remaining 52% to 62% of the blood volume is the plasma or the liquid portion of the blood.

  2. Color—yeah… of course we all know it is red. Arterial blood is bright red because it contains high levels of oxygen. Venous blood has given up much of its oxygen in tissues and has a darker, dull red color. This may be important in the assessment of the source of bleeding.

  3. PH—the normal pH range of blood is 7.35-7.45, which is slightly alkaline.
    Viscosity—this means the thickness or the resistance to flow. Blood is about 3 to 5 times thicker than water. Viscosity is increased by the presence of blood cells and the plasma proteins, and this thickness contributes to normal blood pressure.


Composition of Blood

When a sample of blood is spun in a centrifuge, the cells and cell fragments are separated from the liquid. Because the formed elements are heavier than the liquid matrix, they are packed in the bottom of the tube by the centrifugal force. The straw colored liquid on the top is the plasma.

Plasma is the liquid portion of blood. It is about 90% water. The remaining portion consists of more than 100 different organic and inorganic solutes that are dissolved in water. Because plasma is a transport medium, it's solutes are continuously changing as substances are added or removed by the cells.

Plasma proteins are the most abundant solutes in the plasma. These proteins remain in the blood and interstitial fluid and are not used for energy. The three major classes of plasma proteins are: albumins, globulins, and fibrinogen. Many of the plasma proteins are synthesized in the liver, and each one has a different function.

Albumins are the most abundant plasma proteins. It is synthesized by the liver. Because they are so abundant, they contribute to the osmotic pressure of the blood and play an important role in maintaining fluid balance between blood and interstitial fluid. If the osmotic pressure of the blood decreases, fluid moves from the blood into the interstitial spaces, which results in edema. This also decreases blood volume and, in severe cases, may reduce blood pressure. When blood osmotic pressure increases, fluid moves from the interstitial spaces into the blood and increases the blood volume. This increases blood pressure and decreases the amount of water available to the cells.

Globulins—there are three types of globulins; alpha, beta, and gamma. Alpha and beta globulins are produced in the liver and function in transporting lipids and fat soluble vitamins in the blood. Gamma globulins are the antibodies that function in immunity.

Fibrinogen makes up the smallest fraction of plasma proteins. It is produced in the liver and functions in blood clotting. These types of plasma proteins are the largest of the molecules.

Solutes transported in blood plasma are nutrients, oxygen, and carbon dioxide.

Production of Blood

The production of these formed elements or blood cells is called hemopoeisis! The formed elements are cells and cell fragments that are suspended in the plasma. The three classes of formed elements are:

  • Erythrocytes or red blood cells
  • Leukocytes or white blood cells
  • Thrombocytes or platelets

When the body sustains a minor wound, the platelets cause the blood to clot and with that the bleeding usually stops. The body can easily replace the lost blood from specialized cells, mostly within the red bone marrow. However, if the human body loses a great volume of blood it is possible that this blood has to be replaced through a blood transfusion from blood that was previously donated by someone else.

However, it is important to remember that everybody's blood is not the same. Blood transfusions must be carefully matched. There are four different blood types. Also there are Rh factors which make every person's blood type even more unique. Therefore, blood from a donor must match exactly that of the recipient.

bloodPatients who are scheduled to have major surgery sometimes are offered a choice to make autologous blood donations (donations of their own blood) so that they have a perfect match.