Red blood cells, also known as erythrocytes, are a major component of blood and are continuously produced in the bone marrow. These cells are responsible for a primary function in the body: transporting oxygen. They gather oxygen from the lungs and deliver it to various tissues throughout the body, which then use this oxygen to produce energy. Red blood cells make up a significant portion of blood volume, approximately 40% to 45%, and are what give blood its distinctive red color.
Unique Physical Structure
Red blood cells possess a distinct biconcave disc shape, resembling a doughnut with an indented center rather than a hole. This specific shape offers several advantages, including an increased surface area that facilitates more efficient gas exchange. The biconcave form also provides the cells with flexibility, enabling them to bend and squeeze through the body’s narrowest capillaries, which can be as small as 3 to 5 micrometers in diameter, despite the red blood cell itself being about 6 to 8 micrometers in diameter.
Mature red blood cells are unique among human cells because they lack a nucleus and most other organelles. This absence of internal structures maximizes the space available within the cell. The lack of a nucleus means red blood cells cannot divide or synthesize new proteins, as their function is focused on gas transport.
The loss of the nucleus occurs during the final stages of red blood cell development, which contributes to their flexibility. This adaptability helps them navigate the circulatory system and reach all tissues. The cell’s membrane allows it to deform without rupturing, then return to its original shape.
The Hemoglobin Protein
Within each red blood cell, there is a high concentration of hemoglobin, an iron-rich protein that gives red blood cells their characteristic color. Hemoglobin’s primary function is to bind oxygen reversibly, picking up oxygen in the lungs and releasing it in the body’s tissues. Each human red blood cell contains approximately 270 million hemoglobin molecules.
Each hemoglobin molecule is a complex structure containing four heme groups, and the iron atom within each heme group directly binds to oxygen. One hemoglobin molecule can bind up to four oxygen molecules at a time. This reversible binding allows hemoglobin to efficiently load oxygen in oxygen-rich environments like the lungs and unload it in oxygen-depleted tissues.
Normal hemoglobin levels typically range from 14 to 18 grams per deciliter (g/dL) for males and 12 to 16 g/dL for females. The iron component in hemoglobin also plays a role in transporting a small amount of carbon dioxide, though most carbon dioxide is carried in the blood plasma as bicarbonate ions.
Essential Role in Oxygen Transport
The combined characteristics of red blood cells—their biconcave shape, lack of organelles, presence of hemoglobin, and flexibility—work together to facilitate efficient oxygen delivery throughout the body. As blood flows through the lungs, oxygen diffuses across the thin walls of the air sacs into the capillaries, where it then enters the red blood cells and binds to hemoglobin.
These oxygen-rich red blood cells travel from the lungs to the left side of the heart, which then pumps the oxygenated blood to tissues and organs throughout the body. Upon reaching tissues with lower oxygen concentrations, hemoglobin releases its oxygen cargo, which then diffuses into the cells for metabolic processes. Simultaneously, red blood cells pick up waste carbon dioxide from the tissues.
The red blood cells, now carrying less oxygen and some carbon dioxide, travel back to the right side of the heart and then to the lungs. In the lungs, carbon dioxide is released from the blood into the air sacs to be exhaled, completing the cycle. A typical red blood cell circulates for approximately 100 to 120 days before being removed from circulation and recycled, with about 2 million new red blood cells produced per second in adults to maintain this continuous transport system.