Blood is a red, viscous, liquid connective tissue. It is composed of plasma and formed elements (red blood cells, white blood cells and blood platelets).
Blood is a tissue because it contains specialized cells. More specifically, blood is the only liquid connective tissue in the human body. It is distributed throughout the body by the circulatory system.
Blood is also a colloid. Centrifugation of a blood sample separates its main constituents based on their density. Plasma accounts for about 55% of the blood volume, while the formed elements make up the remaining 45%.
Plasma is the liquid blood constituent.
Formed elements, sometimes called cellular elements, are cells and cell fragments found in the blood. They are sometimes referred to as the solid component of the blood.
Plasma is a yellow aqueous solution (|\approx90\%| water). It is high in proteins and nutrients. It also contains all the other substances that are transported by blood, such as hormones, antibodies and cellular waste.
Red blood cells, white blood cells and platelets are immersed in plasma.
The main functions of plasma are to contain and transport formed elements, nutrients, hormones, antibodies and waste products throughout the body.
In this short video by Héma-Québec, a scientist, a donor and a recipient answer frequently asked questions about plasma donation.
Red blood cells, also called erythrocytes, are the most abundant cells in the blood. They are soft and have a biconcave disc shape that resembles a doughnut. They have no nucleus and very few organelles. This structure allows them to have a large surface area relative to their size.
The main functions of red blood cells are to bind and transport respiratory gases: oxygen |(\text{O}_2)| and carbon dioxide |(\text{CO}_2).|
In order to be transported with the rest of the blood, |\text{O}_2| and some |\text{CO}_2| temporarily bind to the red blood cells. The cell membrane of each red blood cell contains about 250 million[1] hemoglobin molecules.
Hemoglobin is an iron-rich protein that can bind |\text{O}_2| and |\text{CO}_2.|
Blood is red because of the large amount of hemoglobin it contains. In addition, the colour of blood varies depending on the gases that are bound to the hemoglobin. When bound to |\text{O}_2,| hemoglobin becomes bright red. When bound to |\text{CO}_2,| hemoglobin becomes dark red.
Hemoglobin is responsible for binding respiratory gases to red blood cells and transporting them throughout the body. It is therefore important to always have a sufficient amount of it. The normal concentration of hemoglobin in the blood varies between 120 g/L and 180 g/L.
The term anemia refers to a low hemoglobin level or a low red blood cell count. Malformed red blood cells, iron and/or vitamin B12 deficiency, as well as certain diseases can lead to anemia.
Anemia can cause symptoms such as paleness, fatigue, lack of concentration, shortness of breath, dizziness and fainting.
White blood cells, also known as leukocytes, are cells that specialize in defending the organism. They have a nucleus, which can be round, kidney bean-shaped or multi-lobed. The shape of the nucleus is the main way to distinguish between the different types of white blood cells.
The function of white blood cells is the defence against infections.
White blood cells work to protect the body in several ways. Some are used to detect infections. Others enable the release of antibodies. Finally, some white blood cells are capable of enclosing and eliminating bacteria through the process of phagocytosis.
Platelets, also known as thrombocytes, are cell fragments without a nucleus.
The function of blood platelets is to seal tears in the blood vessel walls to prevent blood loss.
Platelets play an important role in blood clotting (coagulation) and wound healing. When a blood vessel is ruptured, blood platelets are quickly activated. They pile up and collect in the wound to form a temporary plug, which seals the tear.
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Urry, L. A., Cain M. L., Wasserman, S. A., Minorsky, P.V., Orr, R. (2021). Campbell Biology (12th edition). ERPI