Plasma is a pale yellow fluid that consists of about 91% water, 7% proteins, and 2% other components, such as ions, nutrients, gases, waste products, and regulatory substances (figure 11.1 and table 11.1). Unlike the fibrous proteins found in other connective tissues, such as loose connective tissue, plasma contains dis-solved proteins. Plasma proteins include albumin, globulins, and fibrinogen.Albumin (al-bū′ min) makes up 58% of the plasma proteins. Although the osmotic pressure of blood
Figure 11.1 Composition of Blood
results primarily from sodium chloride, albumin also makes an important contribution. The water balance between the blood and the tissues is determined by the movement of water into and out of the blood by osmosis. Globulins (glob′ū-linz; globule) account for 38% of the plasma proteins. Some globulins, such as antibodies and complement, are part of the immune system . Other globulins and albumin function as transport molecules because they bind to molecules, such as hormones , and carry them in the blood throughout the body. Some globulins are clotting factors, which are necessary for the formation of blood clots. Fibrinogen (f ı̄-brin′ ō-jen) is a clotting factor that constitutes 4% of plasma proteins. Activation of clotting factors results in the conversion of fibrinogen to fibrin(fi′brin), a threadlike protein that forms blood clots (see“Blood Clotting” later). Serum (ser′um) is plasma without the clotting factors.
Plasma volume and composition remains relatively constant. Normally, water intake through the digestive tract closely matches water loss through the kidneys, lungs, digestive tract, and skin. Oxygen enters the blood in the lungs, and carbon dioxide enters the blood from tissues. Other suspended or dissolved substances in the blood come from the liver, kidneys, intestines, endocrine glands, and immune tissues, such as the lymph nodes and spleen. The concentration of these substances in the blood is also regu-lated and maintained within narrow limits.
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