Building and maintaining your parts

Growing

You began life as a single cell and built yourself from there, with some help from your mom to get started. Your body developed along a plan, building a backbone with a head at the top and a tail at the bottom (somehow, you lost the tail). Now look at you: 100 trillion cells, almost every one with its own special structure and job to do. Good work! Find more about the processes of development in Chapter 15.

 

Replacing

Just like the organism they’re a part of, many kinds of cells have a life cycle: They’re born, they develop, they work, they get worn out, and they die. For an organism to continue its life cycle, these cells must be replaced continuously, either by the division of the same cell type or by the differentiation of stem cells. These relatively undifferentiated cells wait patiently until they’re called upon to divide. Some of the daughter cells differentiate into their specific programmed type while others remain stem cells and wait to be called upon the next time. Stem cells are an active area of research in physiology and in the field of regenerative medicine.

Some of the cell and tissue types that must be continuously replaced are:

• Red blood cells: The life cycle of a red blood cell is about 120 days. That means you replace all your red blood cells three times a year. New ones come from the red marrow of the bones, and old ones are scavenged for iron in the spleen and then broken down in the liver.
• Epidermis: The cells of the epidermis, the outer layer of the skin, are constantly shed from the surface and replaced from below. Your body replaces the entire epidermis about every six weeks. This process is discussed in Chapter 4.
• Intestinal lining: The epithelial cells of the intestinal lining are replaced about every week. You realize what a feat this is when you read about the intestine in Chapter 11.
• Respiratory membrane: Your body replaces the epithelial cells that line the alveolar wall and the pulmonary capillary vessels about every week. Refer to Chapter 10 for a description of the respiratory membrane.
• Sperm: The process of spermatogenesis (making sperm) is continuous, beginning in a male’s puberty and ending with his death. The quantity and quality varies with his age and health. Turn to Chapter 14 for more details.
• Bone: As you can read about in Chapter 5, bone is living tissue and very active in a number of ways. The bones bear the body’s weight and the stress of impact. Tiny cracks develop in bone all the time and are repaired quickly and constantly, a process known as remodeling. Bones serve as a storage depot for metal ions, especially calcium, which flows in and out of the bone constantly.

Some other types of tissue replace their cells at a very slow rate, such as the following:

• Brain cells: Scientists thought for many decades that brain cells that died weren’t replaced, and, in general, that no new cells were developed in the brain during adulthood. Brain researchers have now shown that this isn’t true. Still, neurons are designed to not need replacement, they’re meant to last you a lifetime. The processes whereby new cells are born in the adult brain have attracted much research interest. See Chapter 7.
• Cardiac muscle: Until recently, physiologists believed that cardiac muscle cells couldn’t regenerate, but that belief has recently been called into question. In 2009, researchers in Sweden reported evidence that, in healthy hearts, cardiac muscle cells do indeed divide, but slowly. The researchers estimated that a 20-year-old renews about 1 percent of heart muscle cells per year and that about 45 percent of the cardiac muscle cells of a 50-year-old are generated after birth. Research published in the early 2000s showed evidence that cardiac muscle cells regenerate to some extent after a heart attack.

 

Repairing parts

Your body repairs some tissues as necessary, such as after an injury:

• Skeletal muscle: Mature skeletal muscle cells, called fibers, don’t divide and aren’t replaced unless they’re damaged. After they’re formed, skeletal muscle fibers generally survive for your entire lifetime. But wait, you say that you’ve been working out and your biceps are twice as big as they were last year? Congratulations, but you didn’t add cells. The cells you had just got bigger.
• Smooth muscle: Like skeletal muscle fibers, smooth muscle fibers are replaced when they’re injured.
• Skin fibroblasts: These are different from epidermal cells. These cells proliferate rapidly to repair damage from a cut or wound and are responsible for generating scar tissue, as discussed in the next section.
• Liver cells: Normally, these cells divide only rarely. However, if large numbers of liver cells are removed — by surgical removal of part of the liver, for example — the remaining cells proliferate rapidly to replace the missing tissue. This makes it possible to transplant part of the liver of a living donor to a recipient, or to split a single liver from a nonliving donor to two recipients. In these cases, when all goes well, both parts regenerate into a complete and functioning liver.

 

Healing wounds

When you have a tiny, superficial surface wound (a little scratch), the epidermis simply replaces the damaged cells. In a few days, the scratch is gone. But when the wound is deep enough that blood vessels are damaged, the healing process is a little bit more involved. Turn to Chapter 9 for information on blood and blood vessels.

The immediate rush of blood washes debris and microbes out of the wound. Then, the vessels around the wound constrict to slow down the blood flow. A type of "formed element" in the blood called platelets sticks to the collagen fibers that make up the vessel wall, forming a natural band-aid called a platelet plug.

After the platelet plug forms, a complex chain of events results in the formation of a clot that stops blood loss altogether. This chain of events is called the clotting cascade or coagulation cascade. Enzymes called clotting factorsinitiate the cascade. Here’s a rundown of what happens, focusing on the most important steps:

• Prothrombin: This clotting factor converts to thrombin. Calcium is required for this reaction.
• Thrombin: This factor acts as an enzyme and causes the plasma protein fibrinogen to form long threads called fibrin.
• Fibrin threads: Wrapping around the platelet plug, these threads form a meshlike template for a clot.
• Clot: The meshlike structure traps the red blood cells and forms a clot. As the red blood cells that are trapped on the outside of the clot dry out (or the air oxidizes the iron in them, like rust), they turn a brownish-red color, and a scab forms.

Underneath the scab, the blood vessels regenerate and repair themselves, and in the dermis, cells called fibroblasts spur the creation of proteins to fill the space in the damaged layers. Scars are created to provide extra strength to skin areas that are deeply wounded. Scar tissue has many interwoven collagen fibers, but no hair follicles, nails, or glands. Feeling maybe lost in the area covered with scar tissue if the nerves are damaged.

 

Lasting parts

As we mention earlier in the chapter, almost all tissues and organs require replacement parts at some time. However, here are some exceptions:

• Central nervous system: For the most part, the cells and tissues of the central nervous system are incapable of self-repair and regeneration. Thus the poor prognosis in cases of spinal cord injury.
• Peripheral nerves: These are the nerve cells that transmit sensation or motor messages between the central nervous system and the skin and skeletal muscles (see Chapter 7). Many types of peripheral neurons don’t undergo regular replacement in normal functioning. They are therefore some of the oldest cells in your body. Unfortunately, they’re not regenerated when they die from injury, so some kinds of nerve damage are permanent. Because they’re not replaced when they die, the number of such nerve cells declines throughout life.
• Ova: A woman has all the eggs she’s ever going to have in her ovaries at birth. In most women, that’s about half a million more than they’ll ever need. Most eggs die before puberty. Only a few mature and participate in the monthly events of the ovarian (menstrual) cycle. And only a very, very few go on to participate in the events of reproduction described in Chapter 14.