BLOOD COMPOSITION:Formed Elements, Plasma, THE IMMUNE SYSTEM

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Health Psychology PSY408

Lecture 11

BLOOD COMPOSITION

Blood is sometimes thought of as a "liquid tissue" because it consists of cells that are suspended in a liquid.

The average adult's body contains about 5 liters of blood. Because our bodies can replace blood quickly, we

can donate half a liter of blood with no ill effects.

Blood composition can affect blood pressure. As we saw earlier, the thicker the blood, the more pressure is

needed to circulate it. What is blood made of, and how does its composition change its thickness? Blood

has two components, formed elements and plasma. We will look at formed elements first.

Formed Elements

Formed elements are the cells and cell-like structures in the blood that constitute about 45% of our blood

volume. There are three types of formed elements:

1. Red blood cells are the most abundant cells in the blood--there are about 5 million of them per cubic

millimeter of blood. They are formed in the bone marrow and have a lifetime of about 3 months. Red blood

cells are important mainly because they contain Hemoglobin, a protein substance that attaches to oxygen

and transports this element to body cells and tissues. Anemia is a condition in which the level of red blood

cells or hemoglobin is below normal (AMA, 1989).

2. Leukocytes are white blood cells. Each of several types of leukocytes serves a special protective

function--for example, some engulf or destroy bacteria. White blood cells are produced in the bone

marrow and various organs in the body. Although there normally are several thousand leukocytes per cubic

millimeter of an adult's blood, they are the least abundant type of formed element. Leukemia is a malignant

disease in which abnormal white blood cells are produced in extremely high quantities, crowding out normal

leukocytes, which fight infection, and red blood cells, which prevent anemia.

3. Platelets are granular fragments, produced by the bone marrow, that enable the body to prevent blood

loss. They do this by plugging tiny wounds or helping the blood to clot when the wound is larger.

Hemophilia is a disease in which the platelets do not function properly, thereby impairing clotting, because

the blood lacks a critical protein.

How do formed elements affect the viscosity of blood? The higher the concentration of formed elements

suspended in the plasma, the thicker the blood.

Plasma

Plasma is a liquid substance that comprises about 55% of our blood. About 90% of plasma is water, and the

remainder consists of plasma protein and various other organic and inorganic elements. Plasma protein

consists of large molecules that are needed within the blood to help other substances pass through capillary

walls. Plasma protein increases the thickness of the blood.

Although the remaining elements in plasma constitute only a small percentage of its volume, they are

extremely important substances. They include hormones, enzymes, and waste products. They also include

the nutrients we derive from digestion-- vitamins, minerals, simple sugars, amino acids, and fatty materials.

Fatty materials make up the broad class of substances in the blood called lipids. Two of these fatty materials

are triglycerides and cholesterol. Triglycerides are the material we commonly think of as fat. Made of

glycerol and fatty acids, they are the most abundant lipid in the body. Some of the fatty acids in triglycerides

are fully hydrogenated--they cannot take up any more hydrogen--and are called saturated for that reason.

They are usually solid at room temperature and are mostly derived from animal fat. Other fatty acids are

unsaturated or polyunsaturated. They can take up more hydrogen, are usually liquid at room temperature,

and are derived from plants.

Health Psychology PSY408

Cholesterol is a fatty substance that builds up in patches on artery walls over time and narrows the artery.

Although the body manufactures most of the cholesterol in the blood, the rest comes from the foods we

eat. Eating fats that are highly saturated tends to increase blood cholesterol levels. Let's see why it is a

problem.

CARDIOVASCULAR DISORDERS

The accumulation of fatty patches, or plaques, on artery walls is called atherosclerosis. These plaques tend

to harden. This is a common process by which the diameter and elasticity of arteries is reduced--a

condition called arteriosclerosis. The narrowing and hardening of arteries increase blood pressure.

Although arteriosclerosis becomes an increasing problem as adults get older, plaque begins to form early in

life. Autopsies on thousands of 15- to 34-year-old American males and females who died of other causes

showed that arteriosclerosis had begun in all subjects and worsened with age.

Of the many diseases of the heart and blood vessels, we will describe just a few. One of them is myocardial

infarction, or "heart attack." Infarction refers to the death of tissue caused by an obstruction in the supply

of blood to it. Thus, a myocardial infarction is the death of heart muscle (myocardium) tissue as a result of

arterial blockage, usually resulting from atherosclerosis.

Other diseases of the heart include congestive heart failure, aneurysm, and stroke. We will discuss them

in detail in our later lectures when we will talk about the biopsychosocial implications of the Cardiovascular

disorders.

THE IMMUNE SYSTEM

I can remember reading for the first time many years ago about a child who had to live in a large plastic

`bubble" because he was born with virtually no major immune defenses. The condition he had is very rare

and is' called severe combined immunodeficiency disease. He lived in the bubble because it was germ

free--exposure to microorganisms in the genera environment would have been fatal. Transplants of healthy

bone marrow tissue early in the child's life can cure this disorder. More common inborn immune

deficiencies involve the absence of only part of the system and can sometimes be treated with injections.

What is the immune system anyway? Let's take a detailed look.

The Immune System

You may not realize it, but wars are raging inside your body. They happen continuously, every day. Most of

the time they are minor skirmishes, and you are unaware of them. When they become major battles,

however, you are usually aware something's going on. The "good guys" are the organs and cells that make

up your immune system. This system fights to defend the body against foreign" invaders, such as bacteria

and viruses.

The immune system is quite remarkable. Scientists knew little about this intricate and enormously important

system until the 1970s. But it is now the subject of major research efforts, and new information about how

the immune system functions is emerging rapidly. We know, for instance, that this system is highly sensitive

to invasions by foreign matter and is able to distinguish between "self," or normal body constituents, and

"not self--friend and foe.

Antigens

When the body recognizes something as a "not self" invader, the immune system mobilizes body resources

and attacks. Any substance that can trigger an immune response is called an antigen. Bacteria and viruses are

recognized as invaders by tell-tale aspects of their protein coats and DNA.

Health Psychology PSY408

What triggers an immune response? Some of the first antigens that come to mind are bacteria, fungi,

protozoa, and viruses. Bacteria are microorganisms that exist in vast numbers throughout the

environment--in rivers and oceans, in the air, on and in plants and animals, and in decaying organic matter.

Billions of them may populate just one pound of rotting garbage. Because they help in breaking down

organic matter into simpler units, their activities are essential to the life and growth of all living things. Some

bacteria cause illnesses, such as tuberculosis, scarlet fever, and food poisoning. They do this by growing

rapidly and competing with our cells for nutrients and by excreting toxic, or poisonous, substances that

destroy our cells or impair their metabolic processes.

Although treatment with antibiotics kills bacteria, these drugs are becoming less effective because they have

been overused and bacteria are developing drug-resistant strains.

Fungi are organisms, such as molds and yeasts, which attach to an organic host and absorb nutrients from

the host. Some of them can cause skin diseases through direct contact, as occurs in ringworm and athlete's

foot, and internal diseases through inhalation of contaminated air. Other fungi are very beneficial-- for

example& penicillin is derived from molds (AMA, 1989). Protozoa are one-celled organisms, such as

amoebas, that live primarily in water and insects. Drinking water contaminated with protozoa can cause

amoebic dysentery, an intestinal illness, and being bitten by an infected mosquito can cause malaria.

The tiniest antigens are viruses, particles of protein and nucleic acid that are smaller than cells and, strictly

speaking, may not even be alive. They consist of genetic information that allows them to reproduce. A virus

functions by attaching to a cell, slipping inside, and taking over by issuing its own genetic instructions. The

invaded cell abandons its own metabolic activities and becomes a "factory" for making viruses. In short

order, enough viruses can be produced to rupture the cell and spread to infect other cells. Viruses can be

quite devious, too, developing new strains and lying dormant in the body for periods of time before

becoming infectious. They are responsible for a variety of diseases, including flu, herpes, measles, and polio.

The immune system also tends to recognize the tissue of an organ transplant as "not self" and treat it as an

antigen. This is what physicians mean when they say that the body "rejected" a transplant. There are two

basic ways to encourage transplant acceptance. The first is to select the transplant carefully so that the

tissues of the donor and the recipient are closely matched. The closer the genetic relationship between the

two people, the better the match is likely to be. Identical twins provide the best match, of course. The

second approach uses drugs to suppress the immune system so it won't mobilize and reject the organ. A

drawback to this approach is that long-term suppression of immune function leaves the patient susceptible

to disease.

For many people, the immune system mounts an attack against normally harmless substances, such as

pollen, tree molds, poison ivy, animal dander, and particular foods. These people suffer from allergies; the

specific substances that trigger their allergic reactions, such as sneezing and skin rashes, are called allergens.

Most allergic people react to some, but not all, of the known allergens--someone with hay fever may not be

allergic to poison ivy, for instance. Being allergic is partly determined by heredity. Some allergies can be

reduced by administering regular, small doses of the allergen, usually by injection.

The Organs of the Immune System

The organs of the immune system are located throughout the body. These organs are generally referred to

as lymphatic or lymphoid organs because they have a primary role in the development and deployment of

lymphocytes, specific white blood cells that are the key functionaries or "soldiers" in our body's defense

against invasion by foreign matter. The main lymphatic organs include the bone marrow, thymus, lymph

nodes and vessels, and spleen.

Health Psychology PSY408

Lymphocytes originate in bone marrow, the soft tissue in the core of all bones in the body. Some of these

cells migrate to one of two organs where they mature. One of these organs is the thymus, which, as we saw

earlier, is a gland that lies in the chest. The other organ is not known for certain, but it is thought to have

the same function in maturing human lymphocytes that a structure called the "bursa" has in birds. Most of

this processing of lymphocytes occurs before birth and in infancy.

The lymph nodes are bean-shaped masses of spongy tissue that are distributed throughout the body. Large

clusters of them are found in the neck, armpits, abdomen, and groin. What do they do? Each lymph node

contains filters that capture antigens and compartments that provide a home base for lymphocytes and

other white blood cells.

Although the lymph nodes and vessels play an important role in cleansing body cells of antigens, they can

become a liability in some forms of cancer either by becoming infected with cancer or by distributing cancer

cells to other parts of the body through the lymph and blood.

Lymphocytes and antigens that enter the blood are carried to the spleen, an organ in the upper left side of

the person's abdomen.

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