Blood Is Thicker Than Water
When most people think of blood, they think of red blood cells. In fact, these cells and other formed elements of the blood make up a smaller percentage of total blood volume compared to the liquid portion of blood: the plasma. Blood also contains another component, which remains inactive until needed: platelets.
Plasma
For an average adult, the plasma makes up 55 percent of total blood volume. It is primarily composed of water but also contains dissolved gases, charged ions such as sodium and potassium, fats, carbohydrates, vitamins, minerals, and proteins.
Albumin
The most abundant materials dissolved in plasma are the plasma proteins. Albumin, which is produced in the liver, makes up the bulk of plasma proteins and serves primarily as the “stuff” in the plasma. The presence of albumin creates an imbalance that causes water to be drawn out of the tissue of the body and into the blood.
Anatomy of a Word
colloid osmotic pressure
When proteins, rather than ions, salts, or other materials, function as solutes—a substance dissolved in another substance, in this case a liquid—the water-drawing force is referred to as colloid osmotic (oncotic) pressure.
Without this vast supply of albumin, much of the fluid that leaks from your capillaries would remain in the tissues of the body and result in severe swelling (edema). Clinical edema can occur when protein production is decreased in cases of liver damage or disease.
Albumin also acts as a carrier substance. Many materials that must be transported in the blood stream are insoluble (that is, they do not dissolve) in water, which is a huge obstacle, since blood is mostly water. Because albumin is soluble in water, it can bind to and completely coat other nonsoluble materials and carry them off.
Globulins
The second most abundant group of proteins in plasma is the globulins, which are divided into three subcategories labeled alpha-, beta-, and gamma-globulins. The alpha- and beta-globulins are water-soluble proteins that function as carrier molecules, transporting water-insoluble materials such as lipids and certain vitamins. A more recognizable name for gamma-globulin is antibody. Thus, gamma-globulins function as part of the immune system to fight infectious agents in the body and provide long-lasting immunity.
Fibrinogen
Fibrinogen acts as an emergency repair protein in the event of damage to a blood vessel. In other words, it is essential in forming a blood clot and preventing blood loss. Fibrinogen is the most abundant of the clotting factors in the plasma.
Platelets
You’ve already heard about plasma and the formed elements of the blood such as white blood cells and red blood cells. But we can’t overlook another essential component of blood: platelets. They are cellular fragments and not complete cells, and their main function is to stop bleeding.
Platelet Formation
Huge multinucleated cells called megakaryocytes located in the bone marrow shed fragments of their cytoplasm and membrane as small packages, known as platelets. In times of low platelet numbers, the body secretes the hormone thrombopoietin, which simulates the development of new megakaryocytes and more platelets.
Platelet Structure and Activation
Platelets are packages of enzymes and other materials wrapped in a membrane. They are about half the size of the RBCs. A microliter of blood can contain as many as half a million platelets. They remain inactive until a vascular injury occurs. Then they are activated, secrete their contents onto their surface, and become extremely sticky.
Endothelial cells lining the blood vessel keep the platelets inactive by secreting materials such as nitric oxide and prostacyclin. However, these cells also enrich the underlying connective tissue with a molecule called von Willebrand factor (vWF), a potent platelet activator. As long as the endothelium remains intact and contiguous, the platelets never encounter vWF and remain inactive. When an injury occurs, receptors on the platelet surface bind to vWF, which signals the rapid release of the platelet contents, causing vascular constriction (to prevent blood loss) and activating other platelets. The platelets stick to the wound site, to each other, and to RBCs and WBCs, forming a platelet plug and slowing the loss of blood from the vessel. The platelet plug is the first in a series of steps that end in the formation of a blood clot.