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Circulatory System IV: Red Blood Cells

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  1. 0:06 Functions of Red Blood Cells
  2. 0:41 Anatomy of a Red Blood Cell
  3. 1:28 Sickle Cell Anemia
  4. 2:02 Further Red Blood Cell Anatomy
  5. 2:45 Life of a Red Blood Cell
  6. 4:24 Lesson Summary
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Taught by

Sarah Wright

Why don't mature red blood cells have nuclei or mitochondria, and how do these guys squeeze through capillaries? While learning about the brief but glorious lives of red blood cells, you'll also see which characteristics help them transport oxygen and carbon dioxide to other cells.

Circulatory System IV: Red Blood Cells

In the human body, the blood serves many purposes, but one of the most important purposes of blood is to transport oxygen and carbon dioxide between the lungs and the rest of the tissues of the body. Oxygen and carbon dioxide transport are so important that about one quarter of all the cells in the human body are red blood cells, whose only job is to transport oxygen and carbon dioxide between the lungs and the tissues of the body. Every aspect of red blood cell biology, including their size, shape, and contents, is adapted to maximize oxygen and carbon dioxide carrying capacity and transfer.

The size and shape of a red blood cell allow it to carry and transfer oxygen and carbon dioxide
Red blood cell functionality

Anatomy of a Red blood Cell

Let's look at the anatomy of a red blood cell. Even though red blood cells are the most numerous type of cell in the human body, they are also one of the smallest. Being small allows red blood cells to flow through very small capillaries and increases their surface area to volume ratio, which in turn increases the speed with which they can release or absorb dissolved gases. In addition, the distinctive biconcave disc shape of the red blood cell (like the one shown here) also increases surface area without creating projections of the cell surface that might cause the cells to get stuck in the blood vessel. Red blood cells are also quite flexible, which helps them squeeze through tight capillaries without getting stuck. This is a critical issue for red blood cells that need to be able to easily flow through capillaries that are barely wider than they are.

Sickle Cell Anemia

In fact, there is a very dangerous disease called Sickle Cell Anemia where some of the red blood cells are very stiff and crescent-shaped with pointed ends. The pointed shape and stiffness of the sickled red blood cells makes them much more likely to get stuck in the blood vessels and cause blockages which can result in severe pain and organ damage. Sickle-shaped cells only circulate for about 10-20 days. The body can't make enough red blood cells to keep pace with such a rapid turnover rate, and the result is anemia, which is any condition where the number of red blood cells in the blood is much lower than normal.

Sickled red blood cells are stiff and crescent-shaped and only circulate for about 10-20 days
Sickle cell anemia

Further Red Blood Cell Anatomy

Each red blood cell is packed with about 250 million molecules of hemoglobin, an iron-containing protein that can bind to either oxygen or carbon dioxide. Other than hemoglobin, there isn't much else inside a mature red blood cell because in mammals, red blood cells do not have a nucleus or mitochondria. Yep, you heard me right; mature red blood cells do not have a nucleus or mitochondria.

You're probably wondering right now how this is possible. After all, red blood cells have to come from somewhere, and cells do need mitochondria and a nucleus with DNA to go through mitosis, make new cells, and make all of that hemoglobin protein. As it turns out, immature red blood cells do have mitochondria and a nucleus for just those reasons.

Life of a Red Blood Cell

Inside our bones is a tissue called bone marrow, and its function is to produce blood cells. The bone marrow contains lots of rapidly dividing cells, which generate about 6 to 7 trillion new red blood cells every month. These immature red blood cells start making massive amounts of hemoglobin. When they are almost completely full of hemoglobin, they lose their nucleus, mitochondria, and other organelles, which are broken down into nucleic acids, amino acids, carbohydrates, and lipids that are recycled for other uses.

Red blood cells are not considered fully mature until they have lost their nucleus, ribosomes, and mitochondria. This may seem strange, and it is very odd for a cell not to have a nucleus, ribosomes, or mitochondria, but in the case of red blood cells, they don't really need them. You see, mature red blood cells don't grow, they don't divide, and they don't make any new proteins. If a cell doesn't make new proteins or divide, it also doesn't need ribosomes to translate RNA into proteins, and it doesn't need DNA for transcription or replication, which makes DNA and the nucleus unnecessary. Red blood cells also use very little energy, which makes the mitochondria unnecessary. Besides, if red blood cells used mitochondria, they would also use the oxygen they're supposed to be transporting, and they'd be less efficient.

Mature red blood cells do not have a nucleus, ribosomes, or mitochondria
Mature red blood cells

The downside to not having a nucleus or mitochondria is that these cells cannot make any new proteins or use a lot of energy, so they cannot repair themselves, or do much in the way of cell maintenance. As a result, red blood cells only circulate in the blood for about 4 months before they are degraded and their molecules are recycled for use in other cells.

Lesson Summary

So to review: Red blood cells have only one job, and that is to transport oxygen and carbon dioxide between the lungs and the rest of the cells of the body. Red blood cells are perfectly suited to transporting these gases because of their small size, high surface area shape, and their overwhelming amount of hemoglobin, which is an iron-containing protein that can bind to either oxygen or carbon dioxide.

In people who have Sickle Cell Anemia, some of the red blood cells are very stiff and crescent-shaped with pointed ends. The pointy shape and stiffness of the sickled red blood cells makes them much more likely to get stuck in the blood vessels and cause blockages which can result in severe pain and organ damage.

Red blood cells are produced in the bone marrow where they mature and lose their nucleus, mitochondria, and other organelles. Since red blood cells don't divide, or make any new proteins, or use much energy, they don't need these organelles or their nucleus, which would take up valuable space in a red blood cell, so they get rid of them. But this also means that they cannot repair damage, or do much cell maintenance, so their lifespan is limited and they only circulate for about 4 months before they are degraded and their components are recycled to make new cells.

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