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Why Must Blood Be Constantly Filtered By The Kidneys?


Blood is a fluid that primarily consists of cells and a substance called plasma. It contains a substance called hemoglobin, which contains a pigment that is responsible for its red color.

It is extremely important that it is kept relatively clean and chemically balanced...   In fact, this is the primary function of the kidney.


To the naked eye, blood may seem as just a red (and sometimes repulsive) fluid but it is much more than that.  Its structure and composition provide some good insights into its function and importance to the body.

It has two main components - the plasma and the formed elements.

Plasma is a clear extracellular (outside the cells) fluid in which the various cells and platelets (collectively called formed elements) are suspended.  The platelets are fragments of certain bone marrow cells. Plasma is a complex mixture of proteins, enzymes, nutrients, wastes, hormones, and gases.

Formed elements are visible structures with distinctive shapes.  They are enclosed in plasma membrane.  The two (2) main cells that comprise "formed elements" are red blood cells (erythrocytes) and white blood cells (leukocytes).

These formed elements are responsible for many important biological functions such as transporting nutrients, oxygen, and other important substances; defend the body against attacks by diseases; regulate and maintain the body's correct fluid balance.

Blood composition

Erythrocytes have two (2) main functions.  They take oxygen from the lungs and deliver it to other tissues in the body and they pick up carbon dioxide from other tissues and take it to the lungs.

The primary function of leukocytes is to defend the body against pathogens (organisms that cause disease). The main pathogens that affect humans are bacteria and viruses.


Erythrocytes [eh-RITH-ro-sites] are made in the BONE MARROW.  They look like little red disc, concave on both sides, and full of hemoglobin (or haemoglobin). The production of these cells is triggered by a hormone called erythropoietin [eh-RITH-ro-poy-EE-tin].  This hormone is secreted by the kidneys and liver, and through a series of complex biological processes, stimulates the bone marrow to produce these very important cells.

Erythrocytes are mainly responsible for transporting oxygen to tissues and taking carbon dioxide away.  If for any reason these cells are not able to perform this function effectively, tissues and organs will be deprived of oxygen and will eventually die.


Leukocytes [LOO-ko-sites] contain no hemoglobin.  They also differ from erythrocytes in that each leukocyte contains a nucleus. Leukocytes are formed in the BONE MARROW, SPLEEN, THYMUS, and LYMPH NODES.  There are three types: granulocytes, lymphocytes, and monocytes.

Leukocytes are mainly responsible for defending the body against diseases.  Granulocytes help combat bacterial and viral infections.  Lymphocytes are responsible for long term immunity and destroy foreign bodies, either directly or through production of ANTIBODIES.  Monocytes ingest bacteria and foreign bodies by a process called phagocytosis.

Different Blood Types

Basically, there are four different blood types: A, B, AB and O.  These can be further subdivided into "Positive (+)" or "Negative (-)" categories.

On the surface of each cell are combinations of proteins, glyco-proteins and glyco-lipids.  These form what is called antigens, which enable our immune system to distinguish our cells from foreign components.  The different types are determined by these antigens.  Cells with type 'A' antigens are classified as type A; cells with type 'B' antigens are classified as type B; cells with 'AB' antigens are classified as type AB, and cells without any of these antigens are classified as type O.

Types that are classified as "Positive" contain a property called Rh antigens.  If there are no Rh antigens, it is classified as "Negative".

The different classifications are very important considerations for transfusions and transplants, since they are not all compatible with each other.  Persons with type O, however, can donate to all of the different types and is sometime referred to as the universal donor.  Individuals with type AB are sometimes referred to as universal recipients, because they can receive from any of the other types.

Functions of Blood

Blood plays more roles in the body than one might expect. Its main function is to transport cells, oxygen, nutrients and other essential molecules around the body. It is involved in immune defense, nutrition, respiration, thermoregulation (regulation of body temperature), water and pH balance, waste elimination, and internal communication. Its three (3) main functions are:


  • It carries oxygen from the lungs to other organs and removes carbon dioxide.

  • It carries nutrients from the digestive system to other organs.

  • It carries waste to the liver and kidneys for detoxification or removal.

  • It carries heat to the skin for removal, which helps regulate body temperature.

  • It carries hormones from endocrine glands to target cells.


  • Leukocytes destroy microorganisms and cancer cells.

  • Contains antibodies and other proteins that neutralize or destroy pathogens (organisms that cause disease).

  • Contains platelets that initiate clotting and minimize blood-loss.



  • It transfers water to and from tissues, which helps to stabilize water balance.

  • It helps to stabilize the pH balance.

Its Concentration

Blood has two (2) very important properties - viscosity and osmolarity.  These two properties warrant special attention, since they impact directly on the blood's quality and ability to efficiently discharge its function.

  • Viscosity is the resistance of a fluid to flow due to cohesion between its particles.  A fluid is described as viscous if it is thick and gluey.  The more viscous a fluid, the more slowly it flows and vice versa.  Viscosity plays an important part in circulation.  Protein deficiency reduces the viscosity and increases circulation, whereas an excess causes sluggishly flow.  Either of these conditions put a strain on the body's organs especially the heart, and may lead to serious cardiovascular problems.

  • Osmolarity is another important factor in cardiovascular function.  In order to effectively nourish cells and remove wastes, substances must be able to pass between the bloodstream and tissue fluid through capillary walls.  This fluid transfer is known as osmolarity.  If osmolarity is too high, too much fluid is absorbed in the bloodstream, which results in hypertension.  If osmolarity drops too low, too much fluid remains in the tissues. This causes the tissues to become swollen and blood-pressure may drop to dangerously low levels.  Osmolarity is determined mainly by sodium irons, protein, and erythrocytes.

Health is directly affected by the volume and concentration (chemical balance) of the blood. Its correct volume, concentration and quality are essential for optimal health.  The kidneys work very hard to keep it free of toxins and help maintain its optimal quality.  

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