Think for a moment about the multitude of functions taking place in your body at this very instant. A tissue is a group of cells of similar type that work together to serve a common function.
Squamous, cuboidal, and columnar epithelium can be either simple (a single layer of cells) or stratified (multiple layers of cells). The connective-tissue matrix contains three types of protein fibers in proportions that depend on the type of connective tissue.
All three types of protein fibers—collagen, elastic, and reticular—are produced by cells called fibroblasts in the connective tissue. There are two broad categories of connective tissue— connective tissue proper (loose and dense connective tissue) and specialized connective tissue (cartilage, bone, and blood).
The second type of loose connective tissue is adipose (ad-e'-pos) tissue;it contains cells that are specialized for fat storage. Dense connective tissue forms strong bands because of its large amounts of tightly woven fibers. Bone, in combination with cartilage and other components of joints, makes up the skeletal system.
Blood is a specialized connective tissue consisting of a liquid matrix, called plasma, in which so-called formed elements (cells and cell fragments called platelets) are suspended (Figure 4.2).
Muscle tissue Muscle tissue is composed of muscle cells (called muscle fibers) that contract when stimulated.
In many tissues, especially epithelial tissue, the cell membranes have structures for forming attachments between adjoining cells.
An organ is a structure composed of two or more different tissues that work together to perform a specific function. There are two main body cavities—the ventral and dorsal cavities—each of which is further subdivided. Body cavities and organ surfaces are covered with membranes—sheets of epithelium supported by connective tissue.
1 Reticular fibers have the same subunits as collagen fibers, but they are assembled into a slightly different kind of structure. In addition, the skin contains structures for detecting temperature, touch, pressure, and pain stimuli. On most parts of your body, the skin is less than 5 mm (less than a quarter of an inch) thick, yet it is one of your largest organs. The skin does not get thinner as the dead cells are shed, because they are continuously replaced from below.
Drugs that must be continuously administered are often given across the skin (transdermally) using a drug-containing patch that adheres to the skin (Figure 4.9). To speed up the movement of a drug from a transdermal patch to the patient's blood supply, the patch is prepared with a higher concentration of the drug than would be found in a pill. In transdermal patches that deliver hormones for birth control, the blood levels of hormones may also be higher than would result with birth control pills. Burns—tissue damage caused by heat, radiation, electric shock, or chemicals—can be classified according to how deep the damage penetrates. The lower layer of the dermis consists of dense connective tissue containing collagen and elastic fibers, a combination that allows the skin to stretch and then return to its original shape. Liposuction, a procedure for vacuuming fat from the hypodermis, is a way to reshape the body. Proponents claim that Lipodissolve is an effective, nonsurgical treatment for destroying unwanted pockets of fat. Two interacting factors produce skin color: (1) the quantity and distribution of pigment and (2) blood flow. The epidermis gives rise to many seemingly diverse structures: hair, nails, oil glands, sweat glands, and teeth.
Hair Hair usually grows all over the body, except on a few areas such as the lips, palms of the hands, and soles of the feet. A beach crowded with people on a sunny day is hardly a scene we would equate with disfigurement and death. The ultraviolet (UV) radiation of sunlight causes the melanocytes of the skin to increase their production of the pigment melanin, which absorbs UV radiation before it can damage the genetic information of deeper layers of cells. Acne is the inflammation that results when sebum and dead cells clog the duct where the oil gland opens onto the hair follicle (Figure 4.B). Oil (sebaceous) glands are found virtually all over the body except on the palms of the hands and soles of the feet.
As their name implies, sweat glands produce sweat, which is largely water plus some salts, lactic acid, vitamin C, and metabolic wastes such as urea.
To remain healthy, the organ systems of the body must constantly adjust their functioning in response to changes in the internal and external environment. One advantage of our body's multicellular, multiorgan-system organization is its ability to provide a controlled environment for the cells.
Homeostasis is maintained primarily through negative feedback mechanisms—corrective measures that slow or reverse a variation from the normal value of a factor, such as blood glucose level or body temperature, and return the factor to its normal value. Consider how a negative feedback mechanism controls the temperature in your home during the frigid winter months.
The body's temperature control center is located in a region of the brain called the hypothalamus. Let's consider what happens when we find ourselves in an environment where the temperature is above our set point, say 38°C (100.4°F).
Other mechanisms that the brain may activate to lower body temperature include dilation of blood vessels in the dermis and relaxation of the arrector pili muscles attached to the hair follicles.
Sometimes the mechanisms for lowering higher-than- normal temperatures fail, resulting in potentially deadly hyperthermia—abnormally elevated body temperature.
If the body's temperature drops too far—to 35°C (95°F) or below—a condition called hypothermia results, disrupting nervous system function and temperature-regulating mechanisms.
In this chapter we saw how cells form tissues, tissues form organs, and organs form organ systems.
In addition to lowering blood pressure, new and existing drugs that target aldosterone might be useful for treating inflammation and insulin resistance.
Elevated aldosterone is associated with raised blood pressure, cardiovascular disease and, more recently, with inflammation and obesity.
Increased production of aldosterone is mainly caused by the action of the protein angiotensin on the AT1 receptor. The mineralocorticoid receptor is found not only in the principal cells of the kidney but in epithelial, heart and other cells such as macrophages. In the past few years, researchers have found evidence for cross talk between the AT1 and mineralocorticoid receptors. Researchers are now also uncovering an important role for aldosterone in inflammation, in addition to its more traditional role in cardiovascular disease.
Human tissues come in four primary types: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. The skin, our largest organ, helps protect underlying tissues and helps to regulate body temperature. Stratified epithelium often serves a protective role, because its multiple layers provide additional thickness that makes it more difficult for molecules to pass through. Exocrine glands secrete their products into ducts leading to body surfaces, cavities, or organs. Collagen fibers are strong and ropelike and can withstand pulling because of their great tensile strength. It is found in ligaments (structures that join bone to bone), tendons (structures that join muscle to bone), and the dermis (layer of skin below the epidermis).
It contains numerous cartilage cells in a matrix of collagen fibers and a bluish white, gel-like ground substance.
To many people's surprise, bone is a living, actively metabolizing tissue with a good blood supply that promotes prompt healing.
Cardiac muscle contractions are not under conscious control; we cannot make them contract by thinking about them.
Smooth muscle is also found in the walls of organs such as the stomach, intestines, and bladder, where it aids in mixing and propelling food through the digestive tract and in eliminating wastes. The final major type of tissue, nervous tissue, makes up the nervous system: brain, spinal cord, and nerves. There are three kinds of junctions between cells: tight junctions, adhesion junctions, and gap junctions.
Organs themselves do not usually function as independent units but instead form part of an organ system—a group of organs with a common function.
The ventral (toward the abdomen) cavity is divided into the thoracic (chest) cavity and the abdominal cavity. Mucous membranes line passageways that open to the exterior of the body, such as those of the respiratory, digestive, reproductive, and urinary systems. The skin and its derivatives—hair, nails, sweat glands, and oil glands— are sometimes called the integumentary system (an integument is an outer covering).
It serves as a physical barrier that shields the contents of the body from invasion by foreign bacteria and other harmful particles, from ultraviolet (UV) radiation, and from physical and chemical insult. For example, because its outermost layer of cells contains the water- resistant protein keratin, the skin plays a vital role in preventing excessive water loss from underlying tissues. These receptors—components of the nervous system—help keep us informed about conditions in our external environment.
The thin, outer layer, the epidermis, forms a protective barrier against environmental hazards.
The outermost layer of skin, the epidermis, is itself composed of several layers of epithelial cells.
Although the epidermis is a water-resistant protective barrier, lipid-soluble materials are able to cross the lipid cell membranes of the cells of the epidermis. During the last few years, several young women using a transdermal patch for birth control have had heart attacks or strokes; some have died.
Tattoos—designs created when droplets of ink are injected into the dermis—are essentially permanent because, unlike the epidermis, the dermis is not shed.
Nutrients reach the epidermis by passing out of dermal blood vessels and diffusing through tissue fluid into the layer above.
First-degree burns are confined to the upper layers of epidermis, where they cause reddening and slight swelling.
A controversial and popular treatment for these wrinkles is to inject Botox, the toxin from the bacterium that causes botulism.
The hypodermis, a layer of loose connective tissue just below the epidermis and dermis, is not usually considered part of the skin. The physician makes a small incision in the skin above the area of unwanted fat, inserts a fine tube, and moves the tube back and forth to loosen the fat cells, which are then sucked into a container.
The treatment consists of a series of injections containing two main ingredients: one that chemically digests fat molecules and another that causes fat cells to burst.
The pigment, called melanin, is produced by cells called melanocytes at the base of the epidermis. Differences in skin color are due to differences in the form of melanin produced and the size and number of pigment granules. When well-oxygenated blood flows through vessels in the dermis, the skin has a pinkish or reddish tint that is most easily seen in lightskinned people.
Melanocytes respond to the UV radiation in sunlight by increasing the production of melanin.
We will now consider the first four of these in view of their structure and roles in everyday life. The shaft projects above the surface of the skin, and the root extends below the surface into the dermis or hypodermis, where it is embedded in a structure called the hair follicle. Nonetheless, that is a connection we should make, because skin cancers are increasing at an alarming rate, largely due to our exposure to the sun. In skin cancer, the genetic material in skin cells is altered by UV radiation so that the cells grow and divide uncontrollably, forming a tumor. Melanomas usually have a mottled appearance and contain colors such as brown, black, red, white, and blue. Most block the higher-energy portion of the sun's UV radiation, known as UV-B, while providing only limited protection against the lower-energy portion, called UV-A.
What steps might you take to balance the risk of developing skin cancer with the risk of vitamin D deficiency? In fact, about four out of five teenagers have acne, a skin condition that will probably annoy, if not distress, them well into their twenties and possibly beyond.
During the teenage years, oil glands increase in size and produce larger amounts of oily sebum. Although some wastes are eliminated through sweating, the principal function of sweat is to help regulate body temperature by evaporating from the skin surface. We have already learned that the body's organ systems are interdependent, working together to provide the basic needs of all cells—water, nutrients, oxygen, and a normal body temperature. Although conditions outside the body sometimes vary dramatically, our organ systems interact to maintain relatively stable conditions within.
We see this in diabetes, a condition in which either the pancreas does not produce enough of the hormone insulin or the body cells are unable to use insulin. When the normal value is reached, the corrective measures cease; the normal value is the feedback that turns off the response. A control center determines the factor's set point—the level or range that is normal for the factor in question.
In response, a control center in the brain would stimulate the thyroid gland to increase production of calcitonin, which lowers blood calcium level. A thermostat in the heating system serves as both the temperature-sensing receptor and the control center.
Its set point is approximately 37°C (98.6°F), although it differs slightly from one person to the next.

Thermoreceptors in the skin detect heat and activate nerve cells that send a message to the hypothalamus, which then sends nerve impulses to the sweat glands to increase their secretions. The former response releases more heat to the surrounding air and explains the flushed appearance we get during strenuous exercise.
How does this explain why public authorities are less concerned about children than they are about the elderly developing a heat-related illness during a heat wave? Subtle drops in body temperature are detected largely by thermoreceptors in the skin, which send a message to the hypothalamus in the brain.
Some marathon runners have died as a result of elevated core temperatures, as have people sitting in a hot tub heated to too high a temperature (say, up around 114°F). Given what you know about the body's response to cold temperature, why are fingers and toes particularly susceptible to frostbite?
There are four main types of tissue in the human body: epithelial (covers body surfaces, forms glands, and lines internal cavities and organs), connective (acts as storage site for fat, plays a role in immunity and in transport, and provides protection and support), muscle (generates movement), and nervous tissue (coordinates body activities through initiation and transmission of nerve impulses). The types of connective tissue are connective tissue proper (loose and dense connective tissue) and specialized connective tissue (cartilage, bone, and blood).
Neurons convert stimuli into nerve impulses that they conduct to glands, muscles, or other neurons.
Cells produced in the deepest layer are pushed toward the skin surface, flattening and dying as they move away from the blood supply of the dermis and replace their cytoplasmic contents with keratin. They protect the tips of our fingers and toes and help us grasp and manipulate small objects.
Sebum, the oily substance secreted by oil glands, lubricates the skin and hair, prevents desiccation, and inhibits the growth of certain bacteria. It is a dynamic state, with small fluctuations occurring around a set point, and is sustained primarily through negative feedback mechanisms. Ehlers-Danlos syndrome (EDS) is a group of disorders caused by defects in genes that disrupt the production of collagen, which is a chief component of connective tissue. Most aldosterone production occurs in the adrenal gland, where another steroid hormone, cortisol, is also biosynthesized via a similar pathway. Studies show that aldosterone stimulates the release of reactive oxygen species and pro-inflammatory factors in diverse cell types and organs including the kidney, retina, endothelium and cells of the immune system. These cells form linings—in the blood vessels or lungs, for instance—where their flattened shape allows oxygen and carbon dioxide to diffuse across the lining easily. Examples of exocrine glands include the glands that produce digestive enzymes, milk glands, and the oil and sweat glands of the skin.
Sometimes described as the body's glue, its most common role is to bind (tendons and ligaments) and support the other tissues (cartilage and bone). The ground substance may be solid (as in bone), fluid (as in blood), or gelatinous (as in cartilage). For example, when skin is cut, fibroblasts move to the area of the wound and produce collagen fibers that help close the wound, cover the damage, and provide a surface upon which the outer layer of skin can grow back.
The characteristics of any specific connective tissue are determined more by its matrix than by its cells.
Loose connective tissue contains many cells but has fewer and more loosely woven fibers than are seen in the matrix of dense connective tissue (Figure 4.2). Known commonly as gristle, hyaline cartilage is found at the ends of long bones (look carefully at your next drumstick), where it allows one bone to slide easily over another. Fibrocartilage forms a cushioning layer in the knee joint as well as the outer part of the shockabsorbing disks between the vertebrae of the spine. An important function of blood is to transport various substances, many of which are dissolved in the plasma. The cells of smooth muscle tissue taper at each end, contain a single nucleus, and lack striations. Nervous tissue consists of two general cell types, neurons and accessory cells called neuroglia (Figure 4.4). For example, organs such as the trachea, bronchi, and lungs constitute the respiratory system. The thoracic cavity is subdivided again into the pleural cavities, which house the lungs, and the pericardial cavity, which holds the heart.
Some mucous membranes, including the mucous membrane of the small intestine, are specialized for absorption. Details about the structure and functions of the other organ systems, shown and described in Figure 4.7, are presented in subsequent chapters. The internal organs are suspended in body cavities that protect the organs and allow organs to slide past one another as the body moves.
It is considered an organ system because the skin and its derivatives function together to provide many services for the body. Besides offering this somewhat passive form of protection, skin contains cells called macrophages that have a more active way of fighting infection, as we will see in Chapter 13.
Keep these many functions of the integumentary system in mind as you read on about the structure of the skin and its derivatives.
The inner layer, the dermis, contains blood vessels, nerves, sweat and oil glands, and hair follicles.
Thus, if you dissolve a drug in a lipid solvent, it can cross the epidermis, diffuse into the underlying connective tissue, and be absorbed into the blood.
There are warning labels with contraceptive patches informing women of these increased risks. In second-degree burns, damage extends through the epidermis into the upper regions of the dermis. The most pronounced effects begin in the late forties, when collagen fibers begin to stiffen and decrease in number and elastic fibers thicken and lose elasticity. It does, however, share some of the skin's functions, including cushioning blows and helping to prevent extreme changes in body temperature, because it contains about half of the body's fat stores. Liposuction is not a way of losing a lot of weight, because only a small amount of fat—not more than a few pounds—can be removed.
Although FDA has approved both drugs for other purposes, it has not approved this procedure combining the drugs for the purpose of eliminating unwanted fat.
These cells, with their spiderlike extensions, produce two kinds of melanin: a yellow-to-red form and the more common black-to-brown form. A person's genetic makeup determines the combination of the yellowish red or the brown form of melanin produced. Intense embarrassment can increase the blood flow, causing the rosy color to heighten, particularly in the face and neck. This is a protective response, because melanin absorbs some UV radiation, preventing it from reaching the lower epidermis and the dermis. Nerve endings surround the follicle and are so sensitive to touch that we are aware of even slight movements of the hair shaft. Some experts fear that the rates of skin cancer will increase dramatically if the ozone layer, which blocks some of the UV radiation before it reaches Earth, continues to become thinner (the ozone layer is discussed in Chapter 24).
Unlike basal or squamous cell carcinomas, melanomas, when left untreated, often metastasize (spread rapidly) throughout the body, first infiltrating the lymph nodes and later the vital organs. Whereas UV-B causes skin to burn, recent research suggests that exposure to UV-A weakens the body's immune system, possibly impairing its ability to fight melanoma.
Sometimes the sebum in plugged follicles mixes with the skin pigment melanin, forming a blackhead. Although all three types develop from epidermal cells, they differ in their locations, structures, and functions. Just as city dwellers breathe the same air and drink the same city water, the cells of the body are surrounded by the same extracellular fluid.
Normally, as a meal is digested and nutrients are absorbed into the bloodstream, the rising level of glucose in the blood stimulates the pancreas to release insulin. The nervous system can bring about quick responses to changes in internal and external conditions. The control center integrates information coming from all the pertinent receptors and selects an appropriate response.
Parathyroid hormone from the parathyroid glands is a hormone that raises blood calcium levels. If the calcium level dropped too low, receptors would signal the control center in the brain.
If it senses that the temperature inside your home has fallen below a certain programmed set point, the thermostat turns on the heating system (the effector). Body temperature must not vary too far from this mark, because even small temperature changes have dramatic effects on metabolism. As the secretions (perspiration) evaporate and body temperature drops below 37°C (98.6°F), the signals from the brain to the sweat glands are discontinued. The brain then sends nerve impulses to sweat glands, ordering a decrease in their activity, as well as messages to vessels in the dermis, telling them to constrict. In this homeostatic control system, thermoreceptors are the sensors, the hypothalamus is the control center, and sweat glands, blood vessels in the skin, and skeletal muscles are the effectors. In both situations, perspiration could bring no relief; its evaporation would be prevented by high humidity in the case of the runners or by the surrounding water in the case of the hot-tubbers. Skeletal muscle tissue is usually attached to bone, is voluntary, has cross-striations visible under a microscope, and has several nuclei in each cell. Neuroglia increase the rate at which impulses are conducted by neurons and provide neurons with nutrients from nearby blood vessels.
Adhesion junctions link cells by intercellular filaments attached to thickenings in the plasma membrane. The thoracic cavity is further divided into the pleural cavities, which contain the lungs, and the pericardial cavity, which contains the heart. The dermis, just below, is a much thicker, nondividing layer composed of connective tissue and containing nerves, blood vessels, and glands. How are differences in the matrix of different types of connective tissue related to their functions? Explain why symptoms of EDS include joints that extend beyond their normal range and stretchy, saggy skin. Aldosterone elevation increases the reabsorption of Na+ and H2O in the kidney, which raises blood pressure.
Aldosterone concentration correlates with body mass index and appears to contribute to insulin resistance in models of obese mice. It looks at the functions of the skin as an organ system and discusses how all the body’s systems interact to maintain relatively constant internal conditions, when they can, at every organizational level. Connective tissue serves as a storage site for fat, plays an important role in immunity, and provides the body and its organs with protection and support.
The basement membrane is a noncellular layer that binds the epithelial cells to underlying connective tissue and helps the epithelial tissue resist stretching. Endocrine glands (covered in more depth in Chapter 10) lack ducts and secrete their products, hormones, into spaces just outside the cells. However, certain connective tissues specialize in transport (blood) and energy storage (adipose tissue). They are common in structures where great elasticity is needed, including the skin, lungs, and blood vessels.
One type of loose connective tissue, areolar (ah-ree'-o-lar) connective tissue, functions as a universal packing material between other tissues. It serves as cushioning between certain bones and helps maintain the structure of certain body parts, including the ears and the nose. The matrix secreted by bone cells is hardened by calcium, enabling bones to provide rigid support. One kind of formed element, the red blood cell, transports oxygen to cells and also carries some of the carbon dioxide away from cells.
Cardiac muscle cells resemble branching cylinders and have striations and typically only one nucleus. Neurons generate and conduct nerve impulses, which they conduct to other neurons, muscle cells, or glands.
In tight junctions (Figure 4.5a), the membranes of neighboring cells are attached so securely that they form a leakproof seal. The common function of these organs is to bring oxygen into the body and remove carbon dioxide.
The abdominal cavity contains the digestive system, the urinary system, and the reproductive system. Others, like those of the respiratory system, for instance, secrete mucus that traps bacteria and viruses that could cause illness. Although we perspire (imperceptively) through our skin almost constantly, during times of strenuous exercise or high environmental temperatures our sweat glands become active and increase their output of perspiration dramatically. Beneath the skin is a layer of loose connective tissue called the hypodermis or subcutaneous layer, which anchors the skin to the tissues of other organ systems that lie beneath. On their way, they flatten and die because they no longer receive nourishment from the dermis.
Today, transdermal drug administration is commonly used to provide hormones for birth control or the treatment of menopausal symptoms, nicotine to ease the urge to smoke while trying to quit, and antiemetics to stop nausea from motion sickness. Do you think that women or their families should be able to sue the patch manufacturer for their losses? In addition, it contains blood vessels, hair follicles, oil glands, the ducts of sweat glands, sensory structures, and nerve endings. Where skin is traumatized by, for example, a burn or an ill-fitting shoe rubbing against your heel, this fluid accumulates between the epidermis and dermis, separating the layers and forming blisters. These changes, combined with reductions in moisture and the amount of fat in the hypodermis, lead to wrinkles and sagging skin. In infants and toddlers, this layer of fat that lies under the skin—often called baby fat—covers the entire body, but as we mature, some of the fat stores are redistributed. There is no evidence that Lipodissolve is unsafe, and there is no scientific evidence that it is effective. The melanin is then taken up by surrounding epidermal cells, thus coloring the entire epidermis.

The skin requires some UV radiation for the production of vitamin D, but too much can be harmful. The survival rate in persons whose melanoma is found before it has metastasized is about 90% but drops to about 14% if the cancerous cells have spread throughout the body. Apply your sunscreen about 45 minutes before going out into the sun, allowing time for it to be absorbed by the skin so that it is less likely to wash away with perspiration.
Ironically, by providing protection from sunburn, sunscreens have had the potentially devastating effect of enabling people to spend more time in the sun, possibly increasing their risk of developing melanoma. Although the nail itself is dead and lacks sensory receptors, it is embedded in tissue so sensitive that we detect even the slightest pressure of an object on the nail. The changes thus induced in the activity and structure of oil glands set the stage for acne. Sebum lubricates hair and skin and contains substances that inhibit growth of certain bacteria.
As their name implies, they produce wax, which protects the ear by trapping small particles. The general effect of insulin is to lower the blood level of glucose, returning it to a more desirable value. The endocrine system produces hormones, which bring about slower and longer-lasting responses to change. When the receptor detects a change in some factor or event—some variable—it sends that information to the control center, the second of the three components. Describe a negative feedback relationship involving these hormones as effectors that would maintain homeostasis by keeping blood calcium levels stable. In response, the control center would stimulate the parathyroid glands to increase production of parathyroid hormone, which would raise blood calcium levels. When the thermostat senses that the internal temperature has returned to the set point, it turns the heating system off. Temperature is sensed at the body's outer surface by skin receptors and deep inside the body by receptors that sense the temperature of the blood.
In this homeostatic system, the thermoreceptors in the skin are the receptors, the hypothalamus is the control center, and the sweat glands are the effectors.
The brain may also initiate behavioral responses to a high body temperature, such as seeking shade or removing a sweatshirt. Finally, at a body temperature of 30°C (86°F), blood vessels are completely constricted and temperature-regulating mechanisms are fully shut down. Cardiac muscle tissue is found in the walls of the heart, is involuntary, has cross-striations, and usually contains only one nucleus in each cell.
Gap junctions have small pores that allow physical and chemical communication between cells. Below the dermis is the hypodermis, a layer of loose connective tissue that anchors the skin to underlying tissues.
Which of these hormones dominates receptor activation depends on several factors, including to what extent cortisol gets deactivated by its conversion to cortisone. While most of the inflammatory effects of aldosterone are dependent on the mineralocorticoid receptor, and may therefore be reduced by receptor antagonists, others are not. Your body can carry out these functions and more because its cells are specialized to perform specific tasks.
Muscle tissue is responsible for body movement and for movement of substances through the body. Whereas all other types of tissue consist primarily of cells, connective tissue is made up mostly of its matrix. Reticular fibers are thin strands of collagen1 that branch extensively, forming interconnecting networks suitable for supporting soft tissues (for example, they support the liver and spleen). The other two kinds of formed elements are white blood cells, which help fight infection, and platelets, which help with clotting.
In addition, skeletal muscle cells have striations, which are alternating light and dark bands visible under a light microscope. Special junctions at the plasma membranes of these cells strengthen cardiac tissue and promote rapid conduction of impulses throughout the heart. Although neurons come in many shapes and sizes, most have three parts—the cell body, dendrites, and an axon.
Tight junctions are found in the linings of the urinary tract and intestines, where secure seals between cells prevent urine or digestive juices from passing through the epithelium. Sliding and changing shape are important when the lungs fill with air, the stomach fills with food, the urinary bladder fills with urine, or when our bodies bend or stretch. The evaporation of this perspiration from the skin's surface helps rid the body of excess heat.
These dead cells are constantly being shed, at a rate of about 30,000 to 40,000 each minute. Along this death route, keratin—a tough, fibrous protein—gradually replaces the cytoplasmic contents of the cells. The muscles regain the ability to contract over the next several months, however, and the injection has to be repeated (Figure 4.11). Furthermore, because liposuction removes the cells that store the fat, fat does not usually return to those areas.
In the dermis, a tiny smooth muscle called the arrector pili is attached to the hair follicle.
It should come as no surprise, then, that acne occurs most often on areas of the body where oil glands are largest and most numerous: the face, chest, upper back, and shoulders. The next stage of acne is pimple formation, beginning with the formation of a red, raised bump, often with a white dot of pus at the center. Sometimes, however, the duct of an oil gland becomes blocked, causing sebum to accumulate and disrupt the gland's proper function. As internal conditions at any level vary, the body's processes must shift to counteract the variation. Without insulin, blood glucose can rise to a point that causes damage to the eyes, kidney, nerves, and blood vessels. The system is a negative feedback mechanism because it produces an effect (cooling of the skin) that is relayed (feeds back) to the control center, shutting off the corrective mechanism when the desired change has been produced. This response, known as piloerection, is less effective in humans than in more heavily furred animals. If the core temperature reaches about 42°C (107°F), the heartbeat becomes irregular, oxygen levels in the blood drop, the liver ceases to function, and unconsciousness and death soon follow. These effects, which require more research, may instead be interrupted by new drugs, currently in development.
Cuboidal cells are found in many glands and in the lining of kidney tubules, where they provide some protection and are specialized for secretion and absorption. The cells are distributed in the matrix much like pieces of fruit suspended in a gelatin dessert. Areolar connective tissue is found, for example, between muscles, where it permits one muscle to move freely over another.
The protein fibers and somewhat gelatinous ground substance of cartilage are responsible for the tissue's resilience and strength. The striations are caused by the orderly arrangement of the contractile proteins actin and myosin, which interact to cause muscle contraction. The dorsal (toward the back) cavity is subdivided into the cranial cavity, which encloses the brain, and the spinal cavity, which houses the spinal cord.
Later we will see how changes in the flow of blood to the skin help regulate body temperature. This durability explains why tattoos—designs created when tiny droplets of ink are injected into the dermal layer—are permanent (Figure 4.10).
A sudden fright, for example, can cause a rapid drop in blood supply, making a person pale. Likewise, eyebrows and eyelashes help keep unwanted particles and glare (and perspiration and rain) out of the eyes. Contraction of this muscle—which pulls on the follicle, causing the hair to stand up—is associated with fear and with cold.
The bump occurs when obstructed follicles rupture and spew their contents into the surrounding epidermis. Then, bacteria can invade the gland and hair follicle, resulting in a condition called acne.
Homeostatic mechanisms do not maintain absolute internal constancy, but they do dampen fluctuations around a set point to keep internal conditions within a certain range. A healthy diet, exercise, medication, and sometimes insulin injections can help people with diabetes regulate their blood glucose level. Now, let's apply these principles to see how homeostatic mechanisms regulate body temperature. If the input indicates that body temperature is below the set point, the brain initiates mechanisms to increase heat production and conserve heat.
The body also responds to cooling by increasing metabolic activity to generate heat and by the repeated contraction of skeletal muscles, known as shivering. As her body temperature returns to normal, she throws off the blankets, looks flushed, and perspires. These drugs target aldosterone directly, by inhibiting the main aldosterone synthesis enzyme.
Columnar epithelium, consisting of elongated, column-shaped cells, is specialized for absorption and secretion. Cartilage lacks blood vessels and nerves, so nutrients reach cartilage cells by diffusion from nearby capillaries.
Dendrites are highly branched processes that provide a large surface area for the reception of signals from other neurons. The plasma membranes of adjacent cells do not actually touch but are instead bound together by intercellular filaments attached to a thickening in the membrane.
When you go swimming or soak in the bathtub for a long time, the dead cells on your skin's surface absorb water and swell, causing the skin to wrinkle. Given your knowledge of skin functions, what would you predict the immediate medical concerns to be when a patient has third-degree burns? Hair also has a sensory role: receptors associated with hair follicles are sensitive to touch. The tiny mound of flesh that forms at the base of the erect hair is sometimes called a goose bump. Such ruptures may occur naturally by the general buildup of sebum and cells or may be induced by squeezing the area. When the input indicates that body temperature is above the set point, the brain initiates mechanisms that promote heat loss. Finally, behavioral responses, such as folding one's arms across one's chest, may be called upon to help combat a drop in body temperature.
In severe cases, dialysis machines may be used to artificially warm the blood and pump it back into the body. Use the mechanisms of body temperature control to explain what is happening as Hannah's fever rises and falls. Because this process is fairly slow, cartilage heals more slowly than bone, which is a tissue with a rich blood supply. A neuron generally has one axon, a long extension that usually conducts impulses away from the cell body. Modified cholesterol molecules in the skin's outer layer are converted to vitamin D by UV radiation. This is particularly noticeable where the layer of dead cells is thickest, such as on the palms of the hands and soles of the feet.
Compared to well-oxygenated blood, which is ruby red, poorly oxygenated blood is a much deeper red that gives the skin a bluish appearance.
The sebum, dead cells, and bacteria that thrive on them then cause a small infection—a pimple or pustule—that will usually heal within a week or two without leaving a scar. These, like many examples of columnar cells, have numerous small, fingerlike folds on their exposed surfaces, greatly increasing the surface area for absorption. Far more numerous than neurons, the neuroglia (or more simply, glial cells) support, insulate, and protect neurons.
Adhesion junctions are common in tissues that must withstand stretching, such as the skin and heart muscle. The vitamin D then travels in the bloodstream to the liver and kidneys, where it is chemically modified to assume its role in stimulating the absorption of calcium and phosphorus from the food we eat. Today, laser treatments that destroy the pigments of the tattoo can often be used for tattoo removal.
The goblet cells of this lining produce mucus to ease the passage of food and protect the cells of the lining.
They increase the rate at which impulses are conducted by neurons and provide neurons with nutrients from nearby blood vessels.
Gap junctions (Figure 4.5c) connect the cytoplasm of adjacent cells through small holes, allowing certain small molecules and ions to flow directly from one cell into the next. When it is cold, your body shunts blood away from the skin to the body's core, which conserves heat and keeps vital organs warm. In heart and smooth muscle cells, gap junctions help synchronize electrical activity and thus contraction.
This shunting reduces the oxygen supply to the blood in the small vessels near the surface of the skin.
When you do not get enough sleep, the amount of oxygen in your blood may be slightly lower than usual, causing the color to darken. In some people, the darker color of blood is visible through the thin skin under their eyes as dark circles.

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