Circulation and Gas Exchange

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Among its solutes are inorganic salts in the form of dissolved ions, sometimes called electrolytes.

Another important class of solutes is the plasma proteins, which influence blood pH, osmotic pressure, and viscosity. Various plasma proteins function in lipid transport, immunity, and blood clotting.

Plasma transports nutrients, gases, and cell waste.

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Cellular Elements

Suspended in blood plasma are two types of cells:

Red blood cells rbc = erythrocytes, transport oxygen.

White blood cells wbc = leukocytes, function in defense.

Platelets are fragments of cells that are involved in blood clotting.

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Red blood cells, or erythrocytes, are by far the most numerous blood cells.

They transport oxygen throughout the body.

They contain hemoglobin, the iron-containing protein that transports oxygen.

Erythrocytes - Oxygen Transport

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Leukocytes - Defense

There are five major types of white blood cells, or leukocytes: monocytes, neutrophils, basophils, eosinophils, and lymphocytes.

They function in defense by phagocytizing bacteria and debris or by producing antibodies.

They are found both in and outside of the circulatory system.

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Platelets - Blood Clotting

Platelets are fragments of cells and function in blood clotting.

When the endothelium of a blood vessel is damaged, the clotting mechanism begins.

A cascade of complex reactions converts fibrinogen to fibrin, forming a clot.

A blood clot formed within a blood vessel is called a thrombus and can block blood flow.

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Collagen fibers

Platelet plug

Platelet releases chemicals

that make nearby platelets sticky

Clotting factors from:

Platelets

Damaged cells

Plasma (factors include calcium, vitamin K)

Prothrombin

Thrombin

Fibrinogen

Fibrin

5 µm

Fibrin clot

Red blood cell

Blood clotting

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Stem Cells and the Replacement of Cellular Elements

The cellular elements of blood wear out and are replaced constantly throughout a person’s life.

Erythrocytes, leukocytes, and platelets all develop from a common source of stem cells in the red marrow of bones.

The hormone erythropoietin (EPO) stimulates erythrocyte production when oxygen delivery is low.

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Differentiation of Blood Cells