5.1 An Overview of Blood

Recall that blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements—referred to as the cellular components or formed elements—include red blood cells (RBCs), white blood cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the cellular components and enables them to circulate throughout the body within the cardiovascular system.

Function of Blood

The primary function of blood is to deliver oxygen and nutrients to, and remove wastes from, body tissues and cells, but that is only the beginning of the story. The specific functions of blood also include defence, distribution of heat, and maintenance of homeostasis.

Transportation

Nutrients from the foods we eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen from the air we breathe diffuses into the blood, which moves from the lungs to the heart, which then pumps it out to the rest of the body. Moreover, endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream, which carries them to distant target cells. Blood also picks up cellular wastes and by-products and transports them to various organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various other waste products are transported to the kidneys and liver for excretion from the body in the form of urine or bile.

Defence

Many types of WBCs protect the body from external threats, such as disease-causing bacteria that have entered the tissues. Other WBCs seek out and destroy internal threats, such as cells with mutated DNA that could multiply to become cancerous, or body cells infected with viruses.

When damage to the blood vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma, the fluid portion of the blood, interact to patch the ruptured areas of the blood vessels involved. This helps to protect the body from further blood loss and initiates the healing process.

Maintenance of Homeostasis

Recall that body temperature is regulated via a classic negative-feedback loop. If you were exercising on a warm day, your rising core body temperature would trigger several homeostatic mechanisms, including increased transport of blood from your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite.

Blood also helps to maintain the chemical balance of the body. For example, proteins and other compounds in blood act as pH buffers, which thereby help to regulate the pH of body fluids and cells. Blood also helps to regulate the water content of body cells and tissues.

Composition of Blood

You may have had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of the most common blood tests—for instance, those measuring lipid or glucose levels in plasma—determine which substances are present within blood and in what concentrations. Other blood tests check for the composition of the blood itself, including the quantities and types of cellular components.

One such test, called a haematocrit, measures the percentage of RBCs, clinically known as erythrocytes, in a blood sample. It is typically performed by automated analysers but can be performed by spinning the blood sample in a specialised centrifuge, a process that causes the heavier elements suspended within the blood sample to separate from the lightweight, liquid plasma (Figure 5.1.1). Because the heaviest elements in blood are the erythrocytes, these settle at the very bottom of the haematocrit tube. Located above the erythrocytes is a pale, thin layer composed of the remaining cellular components of blood. These are the WBCs, clinically known as leukocytes, and the platelets, cell fragments also called thrombocytes. This layer is referred to as the buffy coat because of its buff colour; it normally constitutes less than 1 percent of a blood sample. Above the buffy coat is the blood plasma, normally a pale, straw-coloured fluid, which constitutes the remainder of the sample.

The volume of erythrocytes after centrifugation is also commonly referred to as packed cell volume (PCV). In normal blood, about 45 percent of a sample is erythrocytes. The haematocrit of any one sample can vary significantly, between about 36 and 50 percent, according to gender and other factors. Normal haematocrit values for females range from 37 to 47, with a mean value of 41; for males, haematocrit ranges from 42 to 52, with a mean of 47. The percentage of other cellular components, the WBCs and platelets, is extremely small so it is not normally considered with the haematocrit. The mean plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59 (or 100 minus 41), and for males, it is approximately 53 (or 100 minus 47).

Characteristics of Blood

When you think about blood, the first characteristic that probably comes to mind is its colour. Blood that has just taken up oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a darker red colour. This is because the major component of red blood cells, a protein called haemoglobin that binds oxygen, is a pigment that changes colour depending upon the degree of oxygen saturation. Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water. Viscosity is a measure of a fluid’s thickness or resistance to flow and is influenced by the presence of the plasma proteins and cellular components within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow. Consider the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow than the less viscous water. The same principle applies to blood.

The normal temperature of blood is slightly higher than normal body temperature—about 38°C (or 100.4°F), compared to 37°C (or 98.6°F) for an internal body temperature reading, although daily variations of 0.5 °C are normal. Although the surface of blood vessels is relatively smooth, as blood flows through them, it experiences some friction and resistance, especially as vessels age and lose their elasticity, thereby producing heat. This accounts for its slightly higher temperature.

The pH of blood averages about 7.4; however, it can range from 7.35 to 7.45 in a healthy person. Blood is therefore somewhat more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous buffers, including bicarbonate, haemoglobin and proteins, that help to regulate pH.

Blood constitutes approximately eight percent of adult body weight. Adult males typically average about five to six litres of blood. Adult females average four to five litres.

Blood Plasma

Like other fluids in the body, plasma is composed primarily of water. In fact, it is about 92 percent water. Dissolved or suspended within this water is a mixture of substances, most of which are proteins. Although there are hundreds of substances dissolved or suspended in the plasma, many of them are present only in very small quantities.

Plasma Proteins

About seven percent of the volume of plasma—nearly all that is not water—is made up of proteins. These include several plasma proteins (proteins that are unique to the plasma), plus a much smaller number of regulatory proteins, including enzymes and some hormones. The major components of plasma are summarised in Figure 5.1.2.

The three major groups of plasma proteins are as follows:

1. Albumin is the most abundant of the plasma proteins. Manufactured by the liver, albumin molecules serve as binding proteins—transport vehicles for fatty acids, calcium and steroid hormones. Recall that lipids are hydrophobic; however, their binding to albumin enables their transport in the watery plasma. Albumin is also the most significant contributor to the osmotic pressure of blood; that is, its presence holds water inside the blood vessels and draws water from the tissues, across blood vessel walls, and into the bloodstream. This in turn helps to maintain both blood volume and blood pressure. Albumin normally accounts for approximately 54 percent of the total plasma protein content, in clinical levels of 35–50 g/L blood.
2. The second most common plasma proteins are the globulins. A heterogeneous group, there are three main subgroups known as alpha, beta, and gamma globulins. The alpha and beta globulins transport iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells; like albumin, they also contribute to osmotic pressure. The gamma globulins are proteins involved in immunity and are better known as an antibodies or immunoglobulins. Although other plasma proteins are produced by the liver, immunoglobulins are produced by specialised leukocytes known as plasma cells. Antibodies assist in protection against infection by binding to invading microorganisms and marking them for destruction by specialised WBCs. Globulins make up approximately 38 percent of the total plasma protein volume, in clinical levels of 10–15 g/L blood.
3. The least abundant plasma protein is fibrinogen. Like albumin and the alpha and beta globulins, fibrinogen is produced by the liver. It is essential for blood clotting, a process described later in this chapter. Fibrinogen accounts for about 7 percent of the total plasma protein volume, in clinical levels of 2–4.5 g/L blood.

Other Plasma Solutes

In addition to proteins, plasma contains a wide variety of other substances. These include various electrolytes, such as sodium, potassium, and calcium ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various organic nutrients, such as vitamins, lipids, glucose, and amino acids; and metabolic wastes. All these nonprotein solutes combined contribute approximately 1 percent to the total volume of plasma.

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