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	'''Red blood cells''', also called '''erythrocytes''', are blood cells that carry oxygen from the lungs to body tissues and help carry carbon dioxide back towards the lungs. They are the most numerous cells in human blood and are essential for normal oxygen delivery.

Red blood cells, also known as erythrocytes, are a type of blood cell responsible for carrying oxygen from the lungs to the body's tissues and removing carbon dioxide. They are essential for maintaining proper oxygenation and ensuring the normal functioning of various organs and systems. Red blood cells are unique in their structure and function, and their production and lifespan are tightly regulated within the body.
	Red blood cells are specialised for transport. In humans they are small, flexible, biconcave discs with no nucleus. Their shape gives them a large surface area and helps them squeeze through tiny blood vessels.

== Structure and Characteristics ==
Red blood cells have a distinctive biconcave shape, which provides them with a large surface area for efficient gas exchange. They lack a nucleus and most other organelles, allowing them to maximize oxygen-carrying capacity. The primary component of red blood cells is a protein called hemoglobin, which binds oxygen in the lungs and releases it in tissues. Hemoglobin gives red blood cells their red color.
	== Structure ==
	Mature human red blood cells do not contain a nucleus or most internal organelles. This leaves more internal space for haemoglobin, the iron-containing protein that binds oxygen. Haemoglobin also gives blood its red colour.

== Formation and Development ==
The production of red blood cells is a tightly regulated process called erythropoiesis, which occurs mainly in the bone marrow. It involves the following stages:
	The biconcave shape is important. It helps red blood cells pass through capillaries and improves gas exchange. Their membrane is flexible, but it can be damaged by inherited conditions, immune attack, infection, toxins, or mechanical stress.

# Stem Cell Differentiation: Hematopoietic stem cells in the bone marrow differentiate into erythroblasts, the precursors of red blood cells.
# Nucleus Extrusion: During the maturation process, the nucleus is expelled, and the cell becomes a reticulocyte.
# Reticulocyte Maturation: Reticulocytes migrate from the bone marrow to the bloodstream, where they mature into fully functional red blood cells within a few days.
	== Function ==
	The main function of a red blood cell is gas transport. In the lungs, oxygen binds to haemoglobin. In body tissues, where oxygen levels are lower, haemoglobin releases oxygen so cells can use it in metabolism.

The rate of red blood cell production is regulated by a hormone called erythropoietin, which is produced by the kidneys in response to low oxygen levels in the body.
	Red blood cells also help carry carbon dioxide away from tissues. Some carbon dioxide binds to haemoglobin, but much of it is carried in the blood as bicarbonate after conversion by enzymes inside red blood cells. This means red blood cells also help maintain blood acidity within a narrow range.

== Function ==
The primary function of red blood cells is to transport oxygen from the lungs to the body's tissues and remove carbon dioxide, a waste product of cellular respiration. This oxygen-carrying capacity is facilitated by hemoglobin, which binds to oxygen in the lungs and releases it in tissues with lower oxygen levels. Red blood cells also help maintain the pH balance of the blood and contribute to the regulation of blood pressure.
	== Production ==
	Red blood cell production is called erythropoiesis. It takes place mainly in the bone marrow. The process begins with blood-forming stem cells and passes through several immature stages before reticulocytes enter the bloodstream and mature.

	Erythropoietin, often shortened to EPO, is a hormone that stimulates red blood cell production. The kidneys produce more erythropoietin when oxygen delivery is low. Merck Manual states that red blood cell production also needs enough iron, vitamin B12, folate, and haem.

== Lifespan and Recycling ==	== Lifespan and Recycling ==
Red blood cells have a lifespan of approximately 120 days. As they age or become damaged, they are removed from circulation and broken down by macrophages, primarily in the spleen and liver. The components of hemoglobin are recycled, with iron being stored and reused for new red blood cell production.
	A typical human red blood cell circulates for about 120 days. Old or damaged cells are removed mainly by macrophages in the spleen, liver, and bone marrow. Useful material is recycled. Iron from haemoglobin can be stored and reused to make new red blood cells.

== Disorders and Abnormalities ==
Several disorders and abnormalities can affect red blood cells, including:
	This recycling is constant because the body makes and removes large numbers of red blood cells every day. Problems with production, destruction, or blood loss can quickly affect oxygen delivery.

* [[Anemia]]: A condition characterized by a decreased number of red blood cells or a deficiency in hemoglobin, leading to reduced oxygen-carrying capacity.
* [[Sickle Cell Disease]]: An inherited disorder causing abnormal hemoglobin production, resulting in the production of rigid, crescent-shaped red blood cells that can obstruct blood vessels and cause pain and organ damage.
* [[Thalassemia]]: A group of inherited blood disorders characterized by reduced or absent production of one of the globin chains in hemoglobin, leading to abnormal red blood cell production and anemia.
* [[Polycythemia]]: A condition characterized by an excessive production of red blood cells, leading to an increased viscosity of the blood.
* [[Hemolytic Disorders]]: Conditions where red blood cells are prematurely destroyed, leading to anemia and other complications.
	== Blood Tests ==
	Red blood cells are measured in common blood tests. A full blood count may include red blood cell count, haemoglobin concentration, haematocrit, mean cell volume, and other indices. These results help clinicians assess anaemia, blood loss, dehydration, inflammation, kidney disease, nutritional deficiency, inherited blood disorders, and bone marrow problems.

	Laboratory values need clinical context. A single abnormal result can have several possible causes, and normal ranges can differ by age, sex, pregnancy, altitude, laboratory method, and medical history.

	== Related Conditions ==
	Red blood cell problems include:

	* [[Anemia\|Anaemia]], where the blood has too little haemoglobin or too few effective red blood cells.
	* Iron-deficiency anaemia, often linked to blood loss, diet, pregnancy, absorption problems, or inflammation.
	* Sickle cell disease, where abnormal haemoglobin can make red blood cells rigid and sickle-shaped.
	* [[Thalassemia\|Thalassaemia]], a group of inherited disorders affecting haemoglobin production.
	* Haemolysis, where red blood cells are destroyed too quickly.
	* Polycythaemia, where red blood cell mass is abnormally high.

	Kidney disease can contribute to anaemia because damaged kidneys may not make enough erythropoietin. The National Heart, Lung, and Blood Institute notes this relationship in its patient information on iron-deficiency anaemia and related causes.

	== Terminology ==
	The word erythrocyte comes from Greek roots meaning red cell. In everyday language, red blood cell is clearer. In medical writing, RBC is often used as an abbreviation.

	The protein name is written '''haemoglobin''' in British English and '''hemoglobin''' in American English. Both refer to the same oxygen-binding protein.

== See Also ==	== See Also ==
	* [[Anemia]]
	* [[Hemoglobin]]
	* [[Thalassemia]]
	* [[Virus]]

* [[White Blood Cells]] - The cells of the immune system that protect the body against pathogens.
* [[Platelets]] - The cells responsible for blood clotting.
	== References ==
	* [https://www.ncbi.nlm.nih.gov/books/NBK539702/ NCBI Bookshelf: Histology, Red Blood Cell]
	* [https://www.merckmanuals.com/professional/hematology-and-oncology/approach-to-the-patient-with-anemia/red-blood-cell-production Merck Manual Professional Edition: Red Blood Cell Production]
	* [https://www.nhlbi.nih.gov/health/anemia/iron-deficiency-anemia NHLBI: Iron-Deficiency Anemia]

	[[Category:Medicine]]
	[[Category:Biology]]
	[[Category:Blood]]