What is Blood Composition?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 20 December 2018
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Blood composition is the chemical composition of blood. Chemically, blood is extremely complex and it varies between different species, especially between vertebrates and invertebrates. Understanding normal blood composition is an important aspect of medical care because it allows doctors to identify situations in which the blood composition is abnormal. Abnormalities in the blood can provide clues to a patient's overall health and may explain symptoms experienced by the patient.

All told, blood makes up about eight percent of body weight. It consists of plasma and cells which move in a suspension through the body. Around 55% of the blood is plasma, and plasma itself is mostly water. The cells in the blood include thrombocytes, erythrocytes, and leukocytes, all of which serve different functions in the body. The liquid in the plasma keeps these cells in a consistent suspension.

In addition to cells, blood also contains hormones, lipids, amino acids, dissolved gases, waste materials from cells, proteins, electrolytes, carbohydrates, and vitamins. This complex chemical stew delivers nutrients and gases to cells in the body so that they can function and exchanges these materials for wastes which the cells need to eliminate so that these wastes can be carried away. Blood cycles continually through the body, delivering materials and carrying others away.


The composition of the blood is constantly changing and the body uses a variety of systems to regulate it. Nutrients, for example, are taken in from the intestinal tract. The kidneys filter the blood to remove cellular wastes and to balance the levels of certain chemicals in the blood so that they stay consistent. At the lungs, blood is involved in a gas exchange where carbon dioxide is released and oxygen is taken up by the blood.

When people are sick, their blood composition can change. For example, if someone has a malfunctioning endocrine gland, the levels of hormones in the blood may be unusually high or low because the gland is not producing them in the right amount. A blood test can reveal the imbalance, providing a doctor with a tool for determining what is wrong with the patient. Likewise, when people become dehydrated, the levels of electrolytes in their blood become skewed, which can lead to cell damage.

For a blood test, blood is drawn from the body so that the blood composition can be analyzed. It is allowed to sit or spun in a centrifuge so that the plasma and cells separate to facilitate testing.


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White blood cells also increase when someone is sick. White blood cells can be considered "doctor cells". When there is a problem, more cells are produced to help fight and contain whatever foreign matter there is. Sometimes, elevated white blood cell counts are the first indicator of a health problem.

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