What is Blood?

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Red blood cells and platelets

Red blood cells and platelets are among the formed elements that make up blood.

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1. Blood is made up of 55% plasma and 45% formed elements—red blood cells, white blood cells, and platelets.

Blood is fluid connective tissue that circulates throughout the body. Why is it considered to be fluid connective tissue instead of just a fluid? It is composed of living cells suspended in plasma, the liquid that makes up around 55% of the blood. Plasma transports blood cells, proteins, electrolytes, hormones, and nutrients throughout the body. It also brings waste products from the body tissues to the urinary system, where the kidneys filter them out of the blood.

Blood is fluid connective tissue made up of living cells suspended in plasma.

There are three broad categories of blood cells that have important functions. Red blood cells, or erythrocytes, carry oxygen from the lungs out to the rest of the body. White blood cells, or leukocytes, help protect the body from pathogens. There are five different types of leukocytes that combat infection in different ways. Platelets, or thrombocytes, clump together and form clots to repair torn blood vessels.

2. Blood has five main functions.

Blood has five principal functions that make it essential for a person’s survival:

  • Blood transports oxygen from the lungs to body cells and brings carbon dioxide from body cells to the lungs.Oxygen is an essential ingredient in the aerobic cellular respiration carried out by the cells of the human body. We get oxygen from the air we inhale, and red blood cells transport this oxygen from the lungs to the rest of the body. Carbon dioxide is produced as a waste product of cellular respiration, and it is deposited into the blood, so it can be brought to the lungs and exhaled.
  • Blood transports hormones and nutrients throughout the body.Endocrine glands, such as the pituitary gland and the adrenal glands, secrete hormones into the bloodstream, which carries them to the body’s organs. Hormones are chemical messengers that regulate many of the body’s functions. As we digest food, the villi of the small intestines absorb nutrients into the blood. These important molecules—glucose, amino acids, vitamins, minerals, and fatty acids—help the body’s cells survive and carry out their functions. Blood also transports waste substances to the kidneys and liver, which remove them and process them for elimination.
  • Blood regulates body temperature.When the body needs to warm up or cool down, the circulatory system plays an important role. Blood vessels in the skin can expand or contract to control how much blood comes to the skin’s surface. The expansion of blood vessels brings the blood closer to the skin’s surface, so heat can be released to help cool the body. This is called vasodilation. Vasoconstriction is when blood vessels contract, keeping blood further away from the skin’s surface to prevent heat loss.
  • Blood protects the body from pathogens.White blood cells are key players in the body’s innate and adaptive immune responses. Some white blood cells are specialized to engulf bacteria and other pathogens through a process called phagocytosis. Others have adapted to detect and “tag” particular pathogens for removal.
  • Blood clots to prevent blood loss at sites of injury.When a blood vessel tears, platelets in the area activate, connecting with other platelets to form a plug to prevent further blood loss. These platelets release enzymes that help a blood clot form.

3. Blood cells are produced within red bone marrow.

Red bone marrow contains hematopoietic stem cells, or hemocytoblasts, which divide and differentiate into myeloid and lymphoid stem cells.

Myeloid stem cells give rise to red blood cells, platelets, and myeloblasts—cells that differentiate into myeloid white blood cells: neutrophils, basophils, eosinophils, and monocytes.

Lymphoid stem cells give rise to lymphoblasts, which differentiate into white blood cells classified as lymphocytes: B cells, T cells, and natural killer (NK) cells.

4. There are five types of white blood cells: neutrophils, basophils, eosinophils, monocytes, and lymphocytes.

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Granular myeloid white blood cells

Neutrophils, basophils, and eosinophils are granular myeloid white blood cells.

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Myeloid white blood cells are the mature, differentiated forms of myeloblasts in the red bone marrow. There are two varieties of myeloid white blood cells: granular and agranular.

Granular myeloid white blood cells include neutrophils (the most numerous type of white blood cell), basophils, and eosinophils. These types of white blood cells have granules in their cytoplasm and nuclei with multiple lobes.

In contrast, monocytes are agranular myeloid white blood cells. They do not have cytoplasmic granules, and their nuclei are not lobed.

Lymphoid white blood cells are the mature, differentiated forms of lymphoblasts, which are descended from lymphoid stem cells in the red bone marrow. Lymphoid white blood cells are called lymphocytes, a category which includes B cells, T cells, and natural killer cells. Like monocytes, B and T lymphocytes are agranular, and their nuclei are not lobed. NK cells are granular—in fact, they are often referred to as large granular lymphocytes.

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External Sources

An article from the Leukaemia Foundation describing blood formation.

Definition of hematopoietic stem cell from the National Cancer Institute.

Lecture notes about blood and body defenses from Eastern Kentucky University.

What is plasma?” from the University of Rochester Medical Center Health Encyclopedia.