Red Blood Cells

By Regina Bailey

Biology Expert

1 of 1

Red Blood Cells

Use to navigate.

Red Blood Cells. Photo Credit: Image Credit: CDC/ Janice Haney Carr

Updated July 01, 2015.

Red Blood Cells

Red blood cells, also called erythrocytes, are the most abundant cell type in the blood. Other major blood components include plasma, white blood cells, and platelets. The primary function of red blood cells is to transport oxygen to body cells and deliver carbon dioxide to the lungs. A red blood cell has what is known as a biconcave shape. Both sides of the cell's surface curve inward like the interior of a sphere.

This shape aids in a red blood cell's ability to maneuver through tiny blood vessels to deliver oxygen to organs and tissues. Red blood cells are also important in determining human blood type. Blood type is determined by the presence or absence of certain identifiers on the surface of red blood cells. These identifiers, also called antigens, help the body's immune system to recognize it's own red blood cell type.

Red Blood Cell Structure

Red blood cells have a unique structure. Their flexible disc shape helps increase the surface area-to-volume ratio of these extremely small cells. This enables oxygen and carbon dioxide to diffuse across the red blood cell's plasma membrane more readily. Red blood cells contain enormous amounts of a protein called hemoglobin. This iron containing molecule binds oxygen as oxygen molecules enter blood vessels in the lungs. Unlike other cells of the body, mature red blood cells do not contain a nucleus, mitochondria, or ribosomes. The absence of these cell structures leaves room for the hundreds of millions of hemoglobin molecules found in red blood cells.

A mutation in the hemoglobin gene can result in the development of sickle-shaped cells and lead to sickle cell disorder.

Red Blood Cell Gas Exchange

Oxygen and carbon dioxide are transported through the body via the cardiovascular system. As the heart circulates blood, oxygen depleted blood returning to the heart is pumped to the lungs. In the lungs, pulmonary arteries form smaller blood vessels called arterioles. Arterioles direct blood flow to the capillaries surrounding lung alveoli. Alveoli are the respiratory surfaces of the lungs. Oxygen diffuses across the thin endothelium of the alveoli sacs into the blood within the surrounding capillaries. Hemoglobin molecules in red blood cells release the carbon dioxide picked up from body tissues and become saturated with oxygen. Carbon dioxide diffuses from the blood to the alveoli, where it is expelled through exhalation. The now oxygen rich blood is returned to the heart and pumped to the rest of the body. As the blood reaches systemic tissues, oxygen diffuses from the blood to surrounding cells. Carbon dioxide produced as a result of cellular respiration diffuses from the interstitial fluid surrounding body cells into the blood. Once in the blood, carbon dioxide is bound by hemoglobin and returned to the heart via the cardiac cycle.

Red Blood Cell Production

Red blood cells are derived from stem cells in red bone marrow. New red blood cell production, also called erythropoiesis, is triggered by low levels of oxygen in the blood. Low oxygen levels can occur for various reasons, including blood loss, presence in high altitude, exercise, bone marrow damage, and low hemoglobin levels. When the kidneys detect low oxygen levels, they produce and release a hormone called erythropoietin. Erythropoietin stimulates the production of red blood cells by red bone marrow. As more red blood cells enter blood circulation, oxygen levels in the blood and tissues increase. When the kidneys sense the increase in oxygen levels in the blood, they slow the release of erythropoietin. As a result, red blood cell production decreases.

Red blood cells circulate on average for about 4 months. According to the American Red Cross, adults have around 25 trillion red blood cells in circulation at any given time. Due to their lack of a nucleus and other organelles, adult red blood cells can not undergo mitosis to divide or generate new cell structures. When they become old or damaged, the vast majority of red blood cells are removed from circulation by the spleen, liver, and lymph nodes. These body structures contain white blood cells called macrophages that engulf and digest damaged or dying blood cells. Red blood cell degradation and erythropoiesis typically occur at the same rate to ensure homeostasis in red blood cell circulation.