The most numerous of the cellular elements in the blood are the erythrocytes (red blood cells). On average, there are 5 million red blood cells per microliter (ml) of blood, or a total of about 25 to 30 trillion red blood cells in the adult human body. The percentage of the blood made up of red blood cells is referred to as hematocrit. An average hematocrit is about 45% (42% females, 47% males). As such, the viscosity of the blood is determined primarily by these elements.

Red blood cells are small biconcave discs. Each cell is approximately 7.5 mm in diameter and 2 mm thick. This shape maximizes the surface area of the cell and facilitates the diffusion of oxygen across the cell membrane. Furthermore, red blood cells are very flexible and easily change their shape. This feature allows them to squeeze through capillaries as narrow as 3 mm in diameter. However, as the red blood cells age, their membranes become quite fragile and the cells are prone to rupture. Aged cells are removed by the spleen. The average life span of a red blood cell is about 120 days. As such, red blood cells must be replaced at a rate of 2 to 3 million cells per second. Erythrocyte production is regulated by the hormone erythropoietin. Low levels of oxygen stimulate release of erythropoietin from the kidneys into the blood.

The primary function of red blood cells is to transport oxygen to the tissues. The red, oxygen-carrying molecule within the red blood cell is hemoglobin. This molecule has two components: the globin portion and the heme portion. There are one globin portion and four heme groups per molecule. Each heme group contains an iron atom that binds reversibly with oxygen. The average hemoglobin content in the blood is about 15 g/100 ml of blood, all of it within the red blood cells. In fact, because of their high hemoglobin content, each red blood cell has the capacity to transport more than 1 billion oxygen molecules. This hemoglobin/oxygen-carrying capacity of the red blood cell is facilitated by lack of a nucleus and any other membranous organelles within these cells.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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