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Intro to Anatomy 7: The Integumentary System
Intro to Anatomy 7: The Integumentary System
The Lone Ranger
Published by The Lone Ranger
Default Regulation of Body Temperature

Regulation of Body Temperature by the Integumentary System:
Thermoregulation is the process by which an animal controls its body temperature. In mammals such as ourselves, the temperature of the blood is constantly monitored by the hypothalamus of the brain. If the blood’s temperature falls too low, the hypothalamus triggers heating measures, and if the blood’s temperature rises too high, the hypothalamus triggers cooling measures.

Overheating of the blood triggers sweating. It also triggers the opening of blood vessels in the dermis, so more blood flows from the body core (where heat is being generated) to the skin, where the heat can be shed to the outside environment. This is why people who’re overheated flush bright red – it’s because of all the blood being delivered to the skin in an effort to cool it. An overheated person is especially likely to flush red in the face; this is because the brain is so sensitive to overheating, so it’s especially important that blood be cooled before entering the brain. When you’re overheated, an elaborate system of blood vessels pumps blood to the face for cooling before it goes to the brain.

Chilling of the blood triggers closing down of blood vessels in the dermis, so that blood is directed into the body core and beneath the insulative fat layer in the hypodermis. This greatly reduces heat loss to the outside environment. It also explains why fair-skinned people become noticably paler when they’re cold.

Cold also triggers contraction of the arrector pili muscles and erection of the body hair, though this has little effect on heat retention.

In severe cold, rapid, involuntary contraction of the skeletal muscles is triggered. This is known as shivering. Since muscles generate heat as they contract, the rapid muscle contractions can boost heat production significantly.

Injury and Repair of the Skin:
Because the cutaneous membrane consists largely of rapidly-growing epithelial cells, it usually heals quickly and completely when damaged. How quickly and completely depends on the wound.

An incision is a slender, straight cut made with a sharp object. An incision typically heals fairly rapidly, since there’s relatively little tissue damage. This is the sort of “wound” a skilled surgeon makes.

A laceration is a jagged cut or tear of the skin. It damages considerably more tissue than does an incision of the same length and depth, and so takes longer to heal.

A scraping wound is an abrasion, and a deep abrasion can cause damage to large amounts of epidermal and dermal tissues. Because of the large area of skin that needs to be repaired, deep abrasions tend to take quite a lot longer to heal than do incisions. A further complicating factor is that abrasions easily become infected, which slows down the healing process even more.

Puncture wounds typically don’t bleed as much as do incisions, lacerations, or abrasions. On the other hand, when the skin is punctured, bacteria and other infectious agents are often transported directly into the victim’s body. Puncture wounds can also be very difficult to properly clean. For these reasons, puncture wounds often take a long time to completely heal.
Wound Repair:
When a cut extends through the epidermis and into the dermis, it almost inevitably causes damage to dermal blood vessels. Bleeding occurs at the site of the wound, which helps to clean the wound by flushing out bacteria and debris that might otherwise cause infection or impair healing. Damaged cells and mast cells in the area of injury release chemicals such as histamine that trigger an inflammatory response and also attract macrophages and fibroblasts to the injury site.

Inflammation occurs when nearby blood vessels become enlarged and more porous. This means that more blood is delivered to the site of the injury and that cells (e.g. macrocytes) and other substances can more easily move out of the blood and into the injury site. [For a small cut, inflammation increases blood flow to the injury site, but if the cut is large- and deep-enough that major blood vessels are affected, the reverse is true. Larger blood vessels constrict when they’re cut, which reduces blood flow to the injury site, making it less likely that you’ll bleed to death before the blood can clot.]

Increased flow of blood to inflamed tissues has several effects. First, it causes the injured area to become reddened, swollen, and painful. More to the point, the increased delivery of blood causes the damaged tissue to heat up. Since many bacteria and viruses can be killed by even a slight elevation of body temperature, this is an effective anti-infection measure. Second, increased delivery of oxygen and nutrients helps to support cells that are actively repairing damage and/or fighting off infectious agents. Third, increased blood flow to the injury site makes it easier for “white blood cells” (e.g. macrophages) and fibroblasts to reach it.

Healing of a Wound: Step 1
A freshly-made wound bleeds freely, which helps to flush bacteria
and other potential pathogens out of the wound.

Within a few minutes of the injury, the blood will have begun to clot (coagulate). When blood stops flowing it clots, largely because of the actions of thrombocytes and fibroblasts. Thrombocytes are normally restricted to the blood and don’t encounter collagen fibers. But when blood flows into an open wound, thrombocytes encounter exposed collagen fibers, to which they stick. When this happens, the thrombocytes are said to be “activated.” They release various chemicals that initate a clotting cascade. Among other things, this makes the thrombocytes themselves very “sticky,” and they clump together at the injury site. This clumping together of thromocytes wherever blood vessels are ruptured forms a platelet plug, which may be enough to seal the damaged blood vessels if the cut is a small one. Meanwhile, macrophages and fibroblasts enter the injury area. The fibroblasts release long, sticky strands of the protein fibrin. These strands act like a net to trap erythrocytes and other cells and hold them in place.

Within several hours of the injury, a scab will have formed. The scab consists of blood cells held together in a more or less solid mass by protein fibers secreted by fibroblasts and other cells. The scab seals the wound until more permanent repairs can be made. Fibroblasts also secrete collagen fibers that eventually knit the wound edges together and begin to pull the wound closed. Meanwhile, macrophages engulf and destroy bacteria, viruses, dead and damaged body cells, and other potential pathogens. Epidermal cells migrate down, along the edges of the cut and attempt to replace the missing epidermal cells.

Healing of a Wound: Step 2
A scab forms over the wound, preventing further blood loss. Macrophages enter the area and engulf viruses, bacteria, and dead or damaged body cells. Granulation tissue forms as blood capillaries grow into the clot, attracted by chemicals secreted by fibroblasts.

Dermal and epidermal cells replace the lost tissue as the fibrin clot begins to disintegrate. If the wound is a large one, however, neither the epidermal nor the dermal tissues can completely cover over the injury site. If the injury is too large to be repaired by epidermal and dermal cells, protein fibers laid down by fibroblasts fill the gap. In this case, the newly laid-down tissue will consist mostly of fibrous connective tissue. This replacement tissue is much denser than was the original skin, and is much less elastic. Relatively few blood vessels or nerves penetrate into it. Severely damaged hair follicles, sebaceous or sweat glands, muscle cells, or nerves cannot be repaired and are not replaced. This replacement tissue, typically lacking in hair, sweat or sebaceous glands, and with minimal muscles, nerves, or blood vessels, is known as scar tissue, and is apparently the body’s “best attempt” at repairing serious wounds.

Healing of a Wound: Step 3
As epithelial cells attempt to repair the wound, the scab begins to disintegrate. If the wound is too large for epithelial cells
to repair, scar tissue fills the gap. Scar tissue consists largely of fibrous connective tissue laid down by fibroblasts.

Healing of a Wound: Step 4
Scar tissue fills the gap of a large wound, leaving a scar that has few or no sweat glands, hair follicles, or sensory nerves.

There are three degrees of burns, depending upon the extent of damage they cause. First-degree and second-degree burns are called partial-thickness burns, because they affect only the superficial layers of the skin.

First-degree burns affect only the epidermis of the skin. Most sunburns, for example, are first-degree burns. The skin reddens and can become quite painful, but there is no permanent damage. This painful reddening of the skin is called erythema and is caused by inflammation of the affected tissues.

In second-degree burns, damage extends completely through the epidermis and perhaps into the dermis. Blistering, pain and swelling occurs, but accessory structures such as hair follicles usually remain intact. If the blisters rupture, they can easily become infected. These burns usually heal within 1 – 2 weeks, but some scar tissue may form because if the epidermis is destroyed, it cannot be replaced.

Third-degree burns are also known as full-thickness burns, because they penetrate all the way into the subcutaneous layer, destroying the epidermis and dermis in the process. Ironically, third-degree burns are less painful than are second-degree burns, because sensory nerve endings are destroyed, along with accessory structures, blood vessels, and other dermal components. Extensive third-degree burns cannot heal themselves, because epithelial and dermal cells are destoyed and cannot cover over the injury. This means that third-degree burns pose a serious infection risk.

Burns that cover more than 20% of the skin surface represent serious threats to survival. Extensive burns can cause disruption of the fluid and electrolyte balances. Even partial-thickness burns seriously degrade the skin’s ability to prevent water and electrolyte loss, and full-thickness burns can cause fluid loss through the skin to increase to five times the normal rate.

Increased loss of fluid across the skin surface means increased cooling of the skin from evaporation. This means that burn victims must expend more energy to keep their body temperatures up.

Burned skin surfaces, damp from fluid loss and covered in dead tissue, provide nearly ideal habitats for bacteria. Second- and third-degree burns readily become infected, and widespread bacterial infection, called sepsis (from the Greek, meaning “rotting”), is the leading cause of death in burn victims.

How to estimate the percentage of the body surface affected by a burn.

There are four crucial components to the treatment of full-thickness burns. The most immediate concern is to make sure that lost fluids and electrolytes are replaced. The next concern is to ensure that the victim is provided with sufficient nutrition to meet the increased metabolic demands for thermoregulation and for healing. Burn victims must be carefully protected from infection while they are recovering. The final component to treatment of full-thickness burns is repair of the damaged tissues.

Because full-thickness burns cannot heal on their own, surgical intervention is necessary if the victim is to have any hope of recovery. A skin graft can be used to transfer intact skin from another part of the body to cover the burn site.

Nowadays, a piece of healthy skin the size of a postage stamp can be removed from a person and cultured in a laboratory setting to produce a sheet of epidermal cells several square meters in area that can then be transplanted onto body surfaces. Skin grown this way typically isn’t as strong or as flexible as the original skin was, but it’s certainly a lot better than having no skin at all or being covered in scar tissue. With modern grafting techniques, a young victim with burns over 80% of his or her body has about a 50% chance of recovery. Just a few years ago, such a person would have had almost no hope of recovery.


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Ensign Steve (12-13-2008)

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