Organs Of Protection And Detoxification
The Skin and The Lungs

The skin

The skin consists of two major layers. The outer layer, the epidermis, is made of four thin layers of epithelial cells. The inner layer, the dermis, is composed of connective tissue.

THE EPIDERMIS

This outer layer of protective skin is approximately 1 millimetre thick and is composed of tightly packed cells. The top layer of the epidermis is pigmented and varies in thickness in different areas of the body, determining how easily chemicals can penetrate the skin and how rapidly they are absorbed. An exception is the palm of the hand, where this layer is thicker than in other areas of the body, yet absorbs chemicals more readily.
New skin cells form in the basal cell layer, replacing the entire epidermis approximately once a month. Melanocyte cells, which form part of the epidermis, produce melanin pigment. Melanin determines skin color, and also protects against ultraviolet injury, sunburn, and skin cancer. Melanin absorbs ultraviolet and visible light, and quenches free radicals.
Epidermal cells produce lipids and a protein called keratin. Lipids, which include cholesterol and free fatty acids, help protect the skin against water loss and cracking. With the aid of sunlight, the epidermis also produces vitamin D, an essential nutrient for maintaining calcium and phosphate levels in the body (needed for the growth and repair of bones).

THE DERMIS

The thicker dermis lies under the epidermis and is composed of the proteins collagen and elastin. These proteins make the skin elastic and give it strength. Unlike the epidermis, it is well supplied with blood, lymph vessels, and nerves.
The dermis also contains the eccrine and apocrine sweat glands, sebaceous glands, and hair follicles. Eccrine sweat glands are distributed over the body's surface, helping to regulate its temperature. Apocrine sweat glands open into hair follicles and lose cells as they release secretions. Sebaceous glands are located near hair follicles. They secrete sebum, a lipid mixture that has some antibacterial and antifungal properties. Sebum also helps the body excrete lipid-soluble toxins, but only in small amounts.

HOW TOXINS ENTER THE SKIN

Toxins vary in their ability to enter the skin, and several factors affect their absorption. To be absorbed, a toxin must also be somewhat water-soluble. Toxins that are only lipid-soluble or only water-soluble are poorly absorbed. Oily solutions usually penetrate the skin easily, as it readily absorbs lipids. When the skin is wet, water-soluble chemicals penetrate more easily. At higher environmental temperatures, the skin is more absorbent. In addition, chemicals penetrate cracked or injured skin more easily than intact skin. Some toxins are absorbed directly through hair follicles in the skin.
Solvents can easily penetrate the skin because of their lipid (fat) solubility. Caustic chemicals, such as acids and alkaline solutions, can also penetrate the skin. Once a chemical has penetrated the epidermis, it moves into the dermis. The rich blood supply of the dermis readily transports the chemical into the bloodstream.

PROTECTIVE DEVICES

The normal microbial flora of the skin is a major barrier to infection, as is the sebum. Although sebum helps to prevent the invasion of substances from the external environment, such as bacteria, it cannot block the absorption of toxins through the skin. The epidermal cells are also capable of producing a variety of lipids that afford protection similar to that of sebum, but cannot stop toxins. Hair on the skin can be protective if it prevents a toxic substance from reaching the skin.
Some people use physical barriers in an attempt to protect against toxic skin exposures. Barrier creams are one method, although they cannot usually block toxin absorption. Rubber gloves may be useful, but some chemicals and microorganisms can penetrate the gloves. Thin plastic gloves prevent toxins from contacting the skin. However, if a chemical gets inside the glove, it will actually be absorbed more readily.

DETOXIFICATION

Because it contains the enzyme cytochrome P-450 the skin can metabolize drugs, steroid hormones, and some xenobiotics. It converts these chemicals into more water-soluble forms, which can then be excreted from the body. Small amounts of toxins are eliminated in the sweat excreted from the pores and through the sebaceous glands of the skin.

The lungs

The lungs are part of the respiratory tract. The upper air passage of the respiratory tract consists of the nose, the pharynx, the hypo-pharynx, and the larynx, which houses the vocal cords. The lower air passage stretches from the vocal cords through the trachea and into the lungs.

As we breathe, air enters the upper passage, then traverses the trachea. This area is the narrowest cross-section of the entire airway. The trachea branches into the right and left main stem bronchi, or bronchial tubes, behind the ribcage. One bronchi enters each lung. The bronchi then divide into two to three more branches, called bronchioles. The bronchioles lead to air sacs called alveolar sacs or alveoli. Their total surface area is estimated to be 70 square meters.

Oxygen is extracted from the air we breathe into the lungs and supplied to millions of alveoli, which pass oxygen molecules into the capillaries. Oxygen then combines with hemoglobin in the red blood cells and is carried to the rest of the body.

Exhaling diffuses carbon dioxide molecules from the capillaries into the alveoli and expels them from the body through the bronchi, trachea, and upper air passage.

Three diseases affect the bronchial tube system: asthma, bronchitis, and emphysema. Asthma is characterized by attacks of breathing difficulty. Bronchitis is an inflammation of the bronchial tubes. Toxins can trigger both asthma and bronchitis, which are reversible in their early stages. The toxins in cigarette smoke can cause both chronic bronchitis and emphysema. Emphysema destroys lung elasticity by damaging the walls separating the alveoli from one another, creating tiny craters. Other alveoli become permanently enlarged. Emphysema is irreversible.

HOW TOXINS ENTER THE LUNGS

The lungs have the greatest exposure of any organ to the environment. The air we breathe contains microorganisms, chemicals, dust, and pollution. Small solid particles and liquid aerosols can easily enter the lungs and be deposited in three ways: impaction, sedimentation, and diffusion. Gases are absorbed directly through the cells lining the respiratory tract.

IMPACTION

In impaction, large particles continue in straight paths through the airway passages. Most larger particles land on the surface of the nose and throat area (nasopharynx) or at the branching of the bronchi. These particles become embedded in mucus or trapped by nasal hairs and are eliminated by sneezing, swallowing, or blowing the nose. The nasopharynx removes 95 percent of particles 5 microns or larger.

SEDIMENTATION

Medium-sized particles, 1 micron (the size of a cell) to 5 microns in diameter, are deposited in the lungs by sedimentation. Most of these land in the mucus layer of the bronchioles, and are eventually either moved up in the mucus and exhaled, or swallowed. If the particles do reach the alveoli, they can become trapped permanently and may damage the lungs.

DIFFUSION

The smallest aerosol particles, less than 0.1 micron in diameter, are deposited in the lungs by diffusion. Many of these particles are exhaled immediately, but those that become trapped can eventually cause lung disease, known as pneumoconiosis. Two types of pneumoconiosis are asbestosis, caused by asbestos fibers, and silicosis, caused by silica dust.

ABSORPTION

Gases are absorbed differently in the respiratory tract, depending on their solubility and flow rate, and the duration of exposure. Most absorption of gases takes place in the upper air passages. Some gases dissolve in the fluid that lines the epithelium (the cell layer lining the respiratory tract).
The nose absorbs gases more readily when air flow is increased, which may account for increased absorption by physically active people. The gas may also alter the lining fluid, so that the rate of absorption is increased.

PROTECTIVE DEVICES

The lungs protect themselves against environmental pollutants with filters, epithelial barriers, enzyme systems, and immune responses. Filters include mucus and cilia.
Mucus is produced by glands located beneath the epithelium. Certain cells contain cilia, which are hair like projections that beat in a synchronized fashion at about a thousand times per minute. Together, mucus traps particles and cilia help to move them out of the lungs. A person can then sneeze and cough out the irritants. However, cilia cannot transport particles if there is insufficient mucus. Influenza virus can paralyze the cilia, leading to secondary bacterial infections. Some people have a condition known as immotile cilia syndrome, which means their cilia do not move, and they are prone to sinus and respiratory tract infections.
Epithelial barriers consist of special cells in the epithelium. Alveolar macrophages, a type of white cell, ingest particles, and kill bacteria and viruses, which they then present to lymphocytes. The lymphocytes, another type of white cell, destroy them. Alveolar macrophages also contain aryl hydrocarbon hydroxylase, a type of enzyme that detoxifies chemicals.
In addition, an enzyme system helps to protect the lungs. When particles are inhaled, inflammatory enzymes, known as proteases, are released. These proteases can damage the lung cells or the connective tissue in the lungs. Specific proteins known as antiproteases protect the alveoli by combining with proteases to inactivate them. Cigarette smoke destroys the balance between proteases and antiproteases, increasing the activity of the proteases. The most common antiprotease is alpha-I-anti-trypsin. People with a deficiency of this antiprotease are more prone to emphysema.

DETOXIFICATION

The lungs contain enzymes from the mixed function oxidase family, enabling them to metabolize drugs and xenobiotics to more water-soluble chemicals, which can then be excreted by the kidneys.
The lungs also have antioxidant enzymes to counteract free radicals, including superoxide dismutase, glutathione enzymes, and catalase. In addition, alveolar lining fluid, containing transferrin, ceruloplasmin, and glutathione, protects the lungs from oxidant stress. Vitamin E, an antioxidant found in cell membranes, protects the lungs against toxic lipid peroxides produced by the cell membranes of the lungs when attacked by organisms. In patients who smoke cigarettes, the fluid lining the alveloi can be deficient in vitamin E.
Finally, the lungs have immune responses to protect them against inhaled organisms. Lymphocytes in the lungs produce immunoglobulins (antibodies), while other immunoglobulins cross from the blood into the lungs. Immunoglobulins IgA, IgG, and IgE have all been found in the respiratory tract. IgA neutralizes many viruses, and it seems to prevent antigen absorption across the lung cells. T-lymphocytes (white blood cells that help fight infection) help protect the lungs against microbes and tumor cells. T-lymphocytes also release lymphokines, which are molecules that activate and stimulate macrophages (white blood cells that ingest foreign material).

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