Human Biology

Introduction

Scientists believe that humans evolved from a primate that lived about 2 million years ago. The humans who have developed since that time have a number of unique characteristics. The most important is the brain. The human brain is the most complex biological structure known to exist in any organism.

Over the last 700 years, researches have learned a great deal about the human body works. Scientists have identified the dozens of types of cells that form tissues and organs and they have learned how groups of organs work together as systems to sustain life. In recent years scientists have even found ways to improve and prolong the lives of humans by using transplanted and artificial organs to replace those body parts that are diseased or injured.

Human Characteristics

Scientists classify humans as members of the kingdom Animalia, phylum Chordata, class Mammalia and order Primates. The animals we resemble most closely are the monkeys, apes, and more than 200 other types of the order Primates. The traits that make us distinctly human are mostly refinements of traits found in other primates.

Physical Characteristics

We share many physical characteristics with other primates, because humans and other primates developed from a common ancestor. This ancestor, now extinct, lived an arboreal, or tree-dwelling, existence. The evolution of some primate traits into human traits came about much later.

Primate Traits One important characteristic of all primates is a complex and highly developed brain. Compared to other animals, primates have brains that are larger in relation to their overall body size.

Primates also have sophisticated eyes that distinguish minute details – an adaptation to the ancient dim forests. The keen vision of primates is due to the position of the eyes at the front of the face. This position produces stereoscopic vision, or the ability to perceive objects in three dimensions. Special eye cells called cones also contribute to primates’ keen vision. These cells distinguish color and enable the eye to see sharp images.

A third primate characteristic is a hand with five digits. These digits include an opposable thumb—that is, a thumb that can be positioned opposite the fingers to grasp branches and objects.

Long arms with flexible shoulders and wrist joints are another feature of primates. Two bones in the forearm enable primates to rotate their hands a full semicircle; shoulder joints enable them to move their arms in many directions. Together, these structures and the grasping hand permit primates to swing from branch to branch. Some primates are able to maintain an upright sitting or standing posture during certain activities such as feeding.

Primates also share the same four types of teeth – incisors and canines for tearing, and broad premolars and molars for grinding and chewing. Together these teeth enable primates to eat both plants and other animals.

Human Traits

The earliest human possessed so many ape-like characteristics that scientists sometimes have difficulty telling whether fossil bones are those of a primitive ape or a human. As evolution continued, however, humans developed the distinctive traits that characterize them as species.

The most important human feature is a brain larger that that of any other primate

Chimpanzees, for example, have a brain capacity of about 500 cm³ (30 cu.in.). Humans, however, have an average brain capacity of about 1,400 cm³ (85.5 cu.in.). The expansion of the human brain results in the vertical forehead typical of humans.

The ability to stand and walk upright under all conditions is another distinctly human trait made possible by several specially adapted structures. The pelvis, the girdle of bone that includes the hip bone, is wide and slightly curved. This permits it to support the upper part of the body. The broad rear of the pelvis provides a large area for anchoring the walking muscles. The S-shaped spine rising from the pelvis provides support and balance. The head sits erect at the top of the spine. Even the human foot is designed for standing and walking upright. Basically flat, is contains an arch for support. The large toe is not opposable, but lies parallel to the other toes. In this way the large toe is adapted for walking instead of grasping. More than any other characteristic, upright posture with the erect head creates the distinctly “human” appearance. This posture, with the eyes at a high level, enables humans to see distant objects.

Human teeth and jaws are also distinctive in size and shape. The canine teeth of monkeys, apes, and other primates are long and sharp. These canines are useful for tearing food. Human beings have smaller. More even teeth than other primates. Human canines are only slightly longer than incisors and are used to hold food as well as to tear it. The premolars and molars, the back teeth are specialized for chewing and grinding, are broader than they are in other primates. The human jaw is shaped like an arch, while the jaw of other primates has a rectangular shape.

Behavioral Characteristics

Although the physical characteristics of all primates are somewhat similar, behavioral characteristics vary greatly between humans and other primates. The reason for this difference is the enlarged human brain. The brain enables human to process and remember a great deal of information. These mental abilities also enabled humans to develop a system of symbols that make spoken and written language possible. The use of language, in this sophisticated brain, humans have been able to create and use tools. With the ability to speak to one another and use tools, humans have altered their social organization from a simple agrarian structure to complex societies that depend greatly upon scientific technology.

Organization of the Body

The human body is organized in much the same way as the bodies of other vertebrates, or animals with a spinal cord. In overall structure the human body is bilaterally symmetrical, which means the body has two sides that, in most ways, are mirror images of each other. The organs of the human body are formed of specialized cells and are organized into complex systems that perform specific functions.

Plan of the Body

The human body is divided into four major parts – the head, neck, trunk, and limbs. The body is built around a jointed bony skeleton covered with layers of muscles and skin. Inside the trunk of the body is a cavity called the coelom. The coelom is divided into two smaller cavities by the diaphragm, a dome-shaped sheet of muscle. The thoracic cavity lies above the diaphragm and contains the heart, lungs, and esophagus. The abdominal cavity lies below the diaphragm and contains the organs of digestion, reproduction, and excretion. The cranial cavity is inside the skull and contains the brain.

Tissues of the body

The organs of the body are formed from four types of tissue: epithelial, connective, muscle, and nervous. Most types of tissue have several forms that perform different functions.

Epithelial Tissue

Tissue composed of one or more layers of cells protects all internal and external body surfaces. Such tissue is called epithelial tissue. Squamous epithelium is composed of flat, irregularly shaped cells. Squamous cells form the top layers of the skin, the protective covering of the heart and lungs, and the lining of blood vessels. Cuboidal epithelium is made up of cells that are basically cube shaped. They are found in many glands and in the ducts of some organs, such as the kidney, as well as in the middle ear and the brain. Columnar epithelium is composed of cells that are long, narrow, and tightly packed. They line much of the digestive system and the upper respiratory tract. Many columnar epithelial cells have tiny hairlike extensions called cilia. The wavelike motion of cilia helps move substances along these surfaces.

Connective Tissue

The most widely distributed tissue in the human body is connective tissue. It joints, supports, and protects the other types of tissue. Connective tissue is composed of relatively few cell embedded in a thick, nonliving material called the matrix. The matrix contains many tiny, living fibers.

Four kinds of connective tissue are found in the human body. Dense connective tissue makes up cartilage and bone. Cartilage is a flexible but tough material consisting of small clusters of cells embedded in the matrix. Bone consists of cells in a matrix that contains hard crystals. Loose connective tissue is found under the skin and around nerves, blood vessels, the heart and the lungs. Its matrix is semifluid. Liquid connective tissue forms blood and lymph, a clear fluid that comes from blood. The matrix in blood is a liquid called plasma. Fat tissue is composed of cells in which large droplets of fat are stored. This fat can be used for energy when needed.

Muscle Tissue

Specialized cells with the ability to contract and thereby produce movement make up muscle tissue. Muscle tissue is classified into three types. Skeletal muscles are attached to bones and move the skeleton. Smooth muscles are found in the walls of many internal organs, such as digestive organs. Cardiac muscle is found only in the heart.

Nervous Tissue

Cells that can transmit messages throughout the body make up nervous tissue. These cells are found in the brain, spinal cord, nerves, and sensory organs. Nervous tissue provides information about the environment. It also controls many body functions.

Systems of the Body

Tissues are organized into larger units called organs. Organs that work together to perform a particular function form a system. All body systems are interrelated and operate in unison.

§ The skeletal system moves, supports, and protects the body. Blood cells are manufactured inside bones, and calcium and phosphorus are stored in bone tissue.

§ The muscular system works with bones to make the body move. Muscles also protect some of the body’s organs.

§ The digestive system includes the tube running form the mouth through the trunk and several accessory organs. In this system, food is broken down into essential nutrients, nutrients are absorbed, and solid wastes are eliminated.

§ The circulatory system transports nutrients, gases, and chemicals to all parts of the body. It also collects waste products from cells. Blood is circulated through blood vessels by the pumping action of the heart. The lymphatic system, part of the circulatory system, collects fluid from tissue and returns it to the blood. Both systems also help fight diseases.

§ The respiratory system takes oxygen into the body and eliminates carbon dioxide and water.

§ The excretory system removes cellular wastes from the blood. It also maintains the body’s fluid and chemical balance. Wastes leave the body through the urinary system, a part of the excretory system.

§ The nervous system monitors the outside environment and controls and coordinates body activities.

§ The integumentary system forms the body’s outer protective layer. It consists of the skin, hair and nails.

§ The endocrine system helps control body functions through chemical called hormones. Hormones regulate functions such as growth and maturation.

§ The reproductive system provides a means of producing offspring in order to maintain the species.

Technology and the Body

The human body is often compared with a complex machine. However, there is one major difference between the two. When a machine breaks down, it can be shut off until repairs are made. New parts can be ordered to replace worn-out ones. A human body cannot be shut off when repairs are needed, and new parts cannot simply be ordered.

Science, however, is finding ways to treat human disorders and replace some body parts. One solution may be an organ transplant – the replacement of a body part with an identical part from another person. Another solution may be replacement with an artificial part, or prosthesis. The design and development of artificial body parts is called biomedical engineering.

Organ Transplants

The first kidney transplant, accomplished in 1954, was a major milestone in transplant surgery. Since then about 64,000 patients have received kidney transplants. Other body parts that can be transplanted include blood, heart, lungs, cornea, liver, skin, and bone. Scientists are also studying ways to transplant the small intestine and brain tissue.

Until 1978 many transplants failed because the recipients’ bodies rejected the new organs. Rejection occurred because the body recognized a transplanted organ as a foreign substance and attacked, or rejected, the organ as it would attack invading viruses or bacteria. To prevent rejection, doctors administered drugs that suppressed all the body’s natural defenses. However, these drugs left the organ recipient susceptible to infections of all types. Today transplant recipients are given cyclosporine, an antibiotic drug that suppresses only the defenses against a transplanted organ. Since it was introduced in 1978, cyclosporine has doubled the number of transplanted organs that survive for at least a year.

Artificial Replacement Parts

Since the early 1970’s, biomedical engineers developed an amazing array of artificial parts – limbs, joints, bones, teeth, blood, hearts, and even skin. Often these prostheses involve innovative uses of modern materials and electronic equipment. For example, silicone is used in artificial skin and plastics are used in artificial joints. Researches are also designing limbs equipped with high-powered batteries and microprocessors, tiny devices that receive and channel electrical signals.

The chief aim of biomedical engineers is to design prostheses that behave like normal human parts. Some prostheses come close to achieving this goal. The Utah Arm, for example, is an artificial limb equipped with microprocessors. When attached to a person who has lost an arm, the electronic equipment picks up nerve impulses generated by the wearer’s muscles. Then the microprocessors translate the impulses into movement almost identical to those of a natural human arm.

Research is also under way on artificial organs that are part transplant and part prosthesis. One example is an artificial replacement pancreas, an important organ of digestion. Part of the artificial pancreas consists of pancreatic cells from rat that produce essential digestive juices. These cells line a system of artificial tubes in a frame of metal and plastic.

Thinking About Biology: Computers That Move Muscles

Spinal injuries have caused more than 400,000 Americans to become paralyzed. In many spinal injuries, the brain and limbs are not damaged. The problem is that the connection between these body parts has been broken because of a broken neck or back. Muscles that move limbs get their commands from the central nervous system. Generally the commands travel by way of nerves in the neck and spine. When the nerves are severed, paralysis results.

Because paralysis victims are inactive, their muscles begin to deteriorate. The process of muscle deterioration leads to other problems, such as diseases of the heart and circulatory system and weakness of the bones.

Computers may soon end some of these problems. In certain experiments, researchers have enabled paralysis victims to move their legs. The researchers strap the patient's feet to the pedals of a stationary bicycle, then use a computer to produce electrical impulses that in turn trigger movement in the paralyzed muscles. This procedure allows some paralysis victims to pedal the bicycle at a rate of more than 19.2 km (12 mi.) per hour. Researchers have also used computers to help paralyzed people walk. A small portable computer provides the impulses to the muscles.

Computers may soon be used with a pedal-operated wheelchair and a special tricycle. With this equipment, paralysis victims can move around indoors.

6.1. The skeleton system

Human appearance is determined by 3 systems of the body. These systems make up the bony framework, the muscular bulk, and the outer surface covering the body. These 3 systems provide more than appearance. Bones and muscles support the body, protect vital internal organs, and allow for movement. The skin prevents harmful organisms from entering the body, and also protects internal tissues by covering the body.

The human skeleton is a remarkable system. Its materials are strong and light. Bone is as strong as strong as cast iron but several times lighter and considerable more flexible. The skeleton’s design is simple and efficient. Many bones are hollow cylinders, a shape that provides the greatest strength while using the least amount of material.

Functions of the skeleton

The skeleton serves several vital functions. Along with muscles, the skeleton makes possible a wide range of movements. It supports the body and protects internal organs. Bones store calcium and phosphate, which are taken up by the blood when needed. Also, tissue called marrow inside some bones products red and white blood cells.

Structure of the skeleton

The adult human skeleton is an endoskeleton, or internal skeleton, consisting of about 206 bones as well as connective tissues called cartilage and ligaments. The skeleton has 2 main divisions – the axial skeleton and the appendicle skeleton.

The axial skeleton forms the body’s central framework of support and protection. It consists of 80 bones in the skulls, face, vertebral column, and rib cage. The scull protects the brain. In the adult 26 irregularly shaped bone called vertebrae make up the vertebral, or spinal, column, which holds the body upright and protects the spinal cord. The vertebral column has 5 regions: cervical, thoracic, lumbar, sacral, and coccygeal. The rib cage consists of 12 sets of ribs and the sternum, or breastbone. These bones protect the heart, lungs, and other organs in the thoracic cavity. Each of the ribs is attached to the vertebral column. 7 pairs of ribs, called true ribs, are also attached to the sternum by cartilage. Other 5 pairs do not attach to the sternum and are therefore called false ribs.

The appendicular skeleton consists of 126 bones in the pectoral girdle, the pelvic girdle, and the arm and legs. The pectoral girdle – the bones of the shoulder area – provides support for the arms and allows them a wide range of movement. The pelvic girdle - bones of the hip area – attached directly to the lower part of the vertebral column.

Structure of bones

Bones are classified according to their shape. A bone’s shape is closely related to its function. For example, long bones in the arms and legs support weight and are involved in movements such as walking and lifting. Flat bones, such as the sternum and skull, have a large surface area that protects the underlying organs. The short bones of the wrists and ankles allow great flexibility and precise movements.

Bones consist of living and nonliving materials. The living cells that make up the bone are called osteocytes. Osteocytes are embedded in a network of tough protein fibers called collagen. The nonliving part, the mineral portion, consists mainly of compounds containing calcium and phosphorus that surround the osteocytes and make bones hard. A protective fibrous membrane, the periostenum covers all bones and helos commect them to muscles. The middle portion, called the shaft, is composed of a central cavity surrounded by hard bony materials – compact bone. Small channels, known as Haversian canals, run through this compact bone. The central cavity in long bones is filled with yellow marrow, which stores fat. The shaft is separated from the end of the bone by an epiphyseal line, which marks the area where growth formerly took place. Under the thin in flat bones and at the ends of long bones, the hard material is spongy bone, which consists of tough material that resists shearing forces. In certain parts the spongy bone contains red marrow that is soft and spongy.

Development of bones

During early embryonic development, the skeleton consists of only cartilage and layers of membrane. During the second month of development, the cartilage starts to be replaced by bone through the process called ossification. During ossification bone cells replace cartilage cells, and calcium compounds from the blood are deposited around the cells.

Portions of the skull ossify after birth. The spaces between bones, called fontanels, are covered by a tough membrane. This membrane ossifies over 2-year period after a child is born.

A person grows as bones lengthen. In long bones, growth takes place at both ends of the bone in regions called epophyseal plates. An epiphyseal plate is a layer of cartilage that contains cells that undergo mitosis. Divisions of these cells increase the amount of cartilage, and the length of the born increases.

Joints

Because bones do not bend, movement can occur only when bones meet. The point, where 2 or more bones meet, is called a joint. There are 2 kinds of bones: movable and immovable. A joint that permits movement is a movable joint. Some of them are full movable, some are partly movable. Immovable joints exist in bones that are fused together, as in the skull. The body has 4 major types of movable points. Hinge points, such as knees and knuckles, allow forward and backward movement. Pivot joints, such as where the skull joins the vertebral column, permit a rotating movement. Ball-and-socket joints, such as a hip, allow the widest possible movement. Gliding joints in the wrist and ankle allow sliding movement.

Cartilage and a special lubricant called synovial fluid keep joints moving smoothly. Bones are held together at a movable joint by ligaments, which are strong bands of connective tissue.

Athletes and other active people frequently dislocate or sprain joints. Stretching or learning ligaments causes a sprain. Joints may also become swollen and painful in a condition called arthritis. The most painful and crippling type is rheumatoid arthritis, in which cartilage becomes inflamed and enlarged. Eventually it is replaced by bone, which fuses and prevents movement. In osteoarthritis, which is common among elderly people, cartilage wears away, and the bones rub together.

6.2. The muscular system

Bones would be virtually useless if there were no muscles. However, only some muscles move bones. Others assist in circulating blood and in digesting food. The body has more than 600 muscles, accounting for about 40% of the body weight of a healthy person.

Functions of muscles

A muscle is an organ made up of many muscle cells. Muscles attached to bones cause movement at joints. Some muscles are always working in the body whether a person is conscious of this effort or not. For example, the hearts beats and the eyelids open and close. Though movement is the chief function of muscles, they also protect some internal organs. Additionally, sitting and standing require some muscles to be active.

Types of muscles

Muscle issue is made of special cells that have the ability to contract and relax. Three types of muscle tissue make up the muscular system: skeletal, smooth, and cardiac. Each differs in structure and task.

Skeletal muscle

Muscles that move bones are called skeletal muscles. They attach to bones either directly or by means of strong bands of no elastic connective tissue called tendons. Because skeletal muscles are generally under a person’s conscious control, they are also called voluntary muscles. However, they sometimes move without conscious control, such as when responding to danger.

Muscle cells are called muscle fibers. Skeletal muscle fibers have a long tapering shape. Each fiber contains many nuclei and 1000 to 2000 full-length protein threads called myofibrils. Tiny units called sarcomeres can be seen forming bands across the myofibrils. These units lie in single file in a way that gives myofibrils a striped, or striated, appearance. For this reason skeletal muscle is also called striated muscle.

Smooth muscle

Smooth muscle is made up of spindle-shaped cells with one nucleus each. Most smooth muscles protect organs of the digestive, respiratory, and circulatory systems. Smooth muscles are not under conscious control, so they are called involuntary muscles. They do not respond as quickly as voluntary muscles, but they do not tire as easily.

Cardiac muscles

Cardiac muscle is involuntary, and found only in the heart. Unlike other types of muscle, cardiac muscle does not receive impulses from the nervous system. Instead, the heart has its own regulator – the sinoatrial node that cause the muscle cells to contract.

How muscles cause movement

When a skeletal muscle contracts, there is creating a pulling action that results in movement. Muscles can only pull, they cannot push. For this reason, muscles work in opposing pairs. A muscle pair is termed antagonistic if, for example, the contraction of one muscle bends a joint and the contraction of the other straightens the joint. A muscle that bends a joint is called a flexor, and a muscle that straightens a joint is called an extensor.

Most skeletal muscles are attached to 2 bones. During contraction one bone serves as an anchor. The point at which the muscle is attached to the anchoring bone is the origin. The point at which the muscle is attached to the moving bone is the insertion. Between these 2 points is a joint. Contraction of a muscle thus causes movement at the joint.

Two muscles of the upper arm – the biceps and the triceps – illustrate how antagonistic muscles produce movement. The biceps has its origin at the shoulder and its insertion on the radius, a bone of the forearm. When the biceps contracts, then the forearm is drawn toward the front of the shoulder. Of no antagonistic muscle opposed the biceps, the arm would remain bent. However, the triceps on the back of the upper arm has its origin on the humerus of the upper arm and its insertion on the ulna. When the triceps contracts, then it straightens the arm.

6.3. The integumentary system

Bones, muscles, and body organs are covered by the largest single organ, the skin. The skin is also called the integument. Along with the hair and nails, it makes up the integumentary system.

Functions of the integumentary system

The skin performs many functions for the body, the most important of which is protection. The unbroken skin prevents harmful organisms from entering the body. It also cushions the body against physical injury. The skin is a sense organ containing receptors for touch, heat, cold, pressure, and pain. It is also an organ of elimination because it reds the body of certain waste and so helps control body temperature. Blood vessels near the skin’s surface also allow heat to escape. When exposed to direct sunlight, components of the skin produce vitamin D. In addition, skin acts as a waterproof covering that keeps fluids inside the body.

Structure of the integumentary system

Skin consists of all 4 types of body tissue: nervous, muscle, connective, and epithelial. As a result, it is elastic, flexible, and responsive. Its thickness depends upon its function. For example, an extremely thin layer of skin covers the eardrums, which must be sensitive to sound waves. In contrast, thick skin covers the soles of the feet.

Layers of Skin

The skin consists of 2 layers. The skin outer layer is called the epidermis. The thick inner layer is called the dermis.

The epidermis itself has 2 layers. The top one is actually about 20 layers of dead, scale like, flattened cells. These cells die quickly because they are cut off from their food supply. They contain a protein called keratin, which makes them waterproof. The body loses several thousand of these cells each day, and new cells are produced by mitosis in the lower epidermal layer. As the surface cells disappear, those in the lower layer become the outer surface. It takes about 27 days for all of the outer skin cells to be replaced. In addition to these skin-generating cells, the lower layer has cells that contain melanin, the pigment that makes skin dark. Every person has approximately the same number of these cells. Therefore, skin color differences result from variations in the amount of pigment produced by these cells.

The dermis is composed mainly of connective tissue, which gives the skin its strength and elasticity. Blood vessels, nerves, hair roots, and oil and sweat glands are all located in the dermis.

Subcutaneous layer

A protective layer of loose fatty tissue and dense connective tissue called the subcutaneous layer attached the dermis to the bones and muscles. Although not technically part of the skin, the subcutaneous layer, like skin, helps protect the body against injury and heat loss.

Glands

Sebaceous glands, or oil glands, secrete sebum, an oil that reaches the skin’s surface through the places where hair emerges from the skin. The oil prevents hair and skin from drying out and helps waterproof the skin. A condition called acne commonly occurs during adolescence. Acne occurs when oil mixes with dead cells and plugs up pores in the skin, causing blackheads. In addition, inflammation of oil glands causes pimples. Acne may be related to hormonal changes that take place during adolescence.

More than 2,5 million sweat glands exist in the dermis. Most consist of a tiny duct that opens to the skin’s surface and rids the body of excess water and certain wastes. The evaporation of sweat also acts to cool the body when it becomes overheated.

Hair and nails

Hair is present on the skin over the entire body, except on the soles of the feet, the palms of the hands, and the lips. Hair is manufactured in hair follicles, which are small folds of epidermis that extend into the dermis. Tiny blood vessels at the base of the follicle nourish the hair root. A group of actively dividing cells near the base produces new hair. The hair shaft, which extends above the skin’s surface is composed of dead epidermis.

Nails are mainly dead cells composed of keratin that protect the tips of fingers and toes. At the base of the hard nail plate is a whitish, semicircular area called the lunula. Cell division takes place in the root of the nailbed.

6.4. The respiratory system

All living things get their energy through a biochemical process that takes place within their cells. Some organisms can produce energy without oxygen. Humans, however, do require oxygen to produce energy. Human bodies are adapted to carry oxygen from the atmosphere to body cells and to eliminate the waste products resulting from the energy-producing process.

The respiratory system and the excretory system are involved in these function s. Through the respiratory system, oxygen is inhaled and diffused into the blood. In addition, carbon dioxide is diffused from the blood into the lungs and is exhaled. Carbon dioxide is the chief waste product of cellular activity. Other cellular wastes are eliminated by the excretory system.

The respiratory system consists of the organs of breathing. However, breathing is only one part of respiration. Respiration is the process by which the body takes in oxygen, uses it to produce energy, and then eliminates some waste products of the cellular activity. Three subprocesses are involved in respiration. They are external respiration, internal respiration, cellular respiration. In external respiration, or breathing, the body exchanges gases between the atmosphere and the blood. Internal respiration is the diffusion of gases between the blood or tissuefluid and body cells. Cellular respiration is the process by which cells break down glucose molecules in the presence of oxygen to form the energy molecule ATP.

The Lungs and Breathing

The major breathing organs are two lungs, located in the tho­racic cavity. The lungs are spongy, cone-shaped, saclike or­gans. Each lung weighs about 600 g. The right lung has three main divisions, or lobes, and is slightly larger than the left lung, which has two lobes. Both lungs are encased in a tough membrane that also lines the thoracic cavity. This double mem­brane, the pleura, secretes a lubricating fluid that allows the lungs to move smoothly. Inflammation of the pleura can lead to fluid buildup in the thoracic cavity. This condition is called pleurisy.

Breathing begins when the diaphragm, the dome-shaped muscle below the chest cavity, contracts and moves downward. The intercostal muscles between the ribs also contract, causing the rib cage to move up and out. Together, these muscle con­tractions cause the chest cavity to enlarge. When the chest ex­pands, the air pressure in the chest cavity drops. Air pressure outside the body is then greater than that inside the chest cavity. Air then flows into the lungs from outside the body, equalizing the pressure. This part of the breathing process is called inspira­tion or inhalation.

When the air pressure has been equalized, it causes the dia­phragm and intercostal muscles to relax and return to their nor­mal positions. This in turn reduces the size of the chest cavity. As the size of the chest decreases, the air pressure inside the chest cavity gradually becomes greater than the air pressure out­side the body. Air then leaves the lungs, again equalizing the pressure. This part of the breathing process is called expiration or exhalation.

The Pathway of Air

Air enters the body through two openings in the nose called nostrils. From there the air flows into the nasal cavities, two spaces in the nose. The cavities are separated by a cartilage and bone partition called the septum. The cavities are lined with mucous tissue that contains many blood vessels. The mucous tissue warms and moistens the incoming air. Moisture must be present for diffusion of gases to take place within the lungs. Cilia and hairs also line the cavities and filter foreign particles from the air. The cilia move constantly, carrying these particles outward toward the nostrils.

Air travels from the nasal cavities into the back side of the pharynx, a tube at the rear of the nasal cavities and mouth. The pharynx is a common passageway for both food and air. While air must get into the cartilage-ringed trachea, or windpipe, at the front of pharynx, food must get to the esophagus at the back side of the pharynx. Therefore, food and air cross each other's paths. If food entered the air passageway, the person would choke. To ensure that food does not enter the air passageway, the body makes involuntary adjustments. During the process of swallowing, a flap of tissue called the epiglottis closes over the glottis, or the upper part of the trachea. At the same time, the soft palate closes off the nasal cavities. During inhalation, the glottis is open to allow air to enter the trachea.

At the top of the trachea is the larynx, or voice box. Two ligaments called vocal cords are stretched across the larynx. The larynx is called the voice box because sound is produced when air is forced between the cords. The amount of tension in the cords determines the pitch of a sound. Nine cartilage rings connected by ligaments hold the mucus-lined larynx open dur­ing inhalation and against the pressure from food passing through the adjacent esophagus. The largest of the cartilage rings appears as the Adam's apple in the throat.

The trachea descends to a point near the middle of the breastbone. There it divides into two branches called bronchi. Bronchi walls consist of muscle supported by carti­lage and are lined with mucus and cilia. The bronchi reach deep into the lungs, subdividing about 25 times into smaller and smaller passageways. The first 10 subdivisions are called sec­ondary bronchi. The remaining subdivisions are microscopic-sized tubes called bronchioles. Bronchiole walls consist of smooth muscle and are lined with mucus and cilia. The continuous beating of the cilia in the bronchi and bronchioles carries foreign particles and excess mucus into the pharynx. This material may then be expelled by being swal­lowed or coughed out.

The smallest bronchioles branch into tiny ducts, which end in clusters of tiny bulges. These bulges are air sacs called alve­oli. Each lung has more than 300 million alveoli. Each alveolus measures from 0.1 to 0.2 mm in diameter. The total surface area provided by the alveoli is estimated at about 70 m².

Exchange of Gases

Alveoli are completely surrounded by capillaries. The actual exchange of gases occurs when oxygen in the air of the alveoli diffuses into the blood in the capillaries. In turn, the carbon dioxide in the blood diffuses into the air of the alveoli. The epithelial tissue forming the walls of both the alveoli and capil­laries is only one cell thick. Together, the walls of an alveolus and an adjacent capillary measure only 0.0004 mm. The oxygen in inhaled air dissolves in the mucus on the lining of the alveoli.

In the blood, most oxygen combines with hemoglobin to form oxyhemoglobin. Oxygen from the oxyhemoglobin diffuses into body cells and is used in metabolism, the chemical and physical activities within cells. Metabolism includes the build­ing up and breaking down of complex molecules and the releas­ing of energy during the breakdown. As a result of metabolism, oxygen concentration in the body cells is low, but carbon diox­ide concentration is high.

Carbon dioxide, a metabolic byproduct, diffuses from body cells into the blood. Carbon dioxide is transported in the blood in three ways. About 5 percent dissolves in the plasma. About 25 percent enters the red blood cells and combines with hemo­globin. With help from a special enzyme, the remainder—or about 70 percent—combines with water in the red blood cells to form carbonic acid:

CO₂ + H₂O H₂CO₃

(carbon dioxide) (water) (carbonic acid)

Almost immediately, carbonic acid separates into hydrogen ions (H⁺), which combine with hemoglobin, and bicarbonate ions (HCO₃^-), which diffuse into the plasma.

H2CO3 H+ +HCO3-

As a result of this chemical process, most carbon dioxide is transported in the plasma as bicarbonate ions.

When blood reaches the lungs, chemical reactions occur that reverse the process, releasing carbon dioxide:

H⁺ +HCO₃^- H₂CO₃

CO₂+H₂O

The carbon dioxide diffuses from the blood into the lungs. The carbon dioxide is exhaled along with water vapor.

Regulation of Breathing

Many factors influence the control of breathing, in­cluding carbon dioxide and oxygen levels in the blood. The level of carbon dioxide in the blood plays a vital role in regu­lating breathing. Carbon dioxide affects blood acidity. Certain nerve cells are sensitive to changes in blood acidity. These nerves send messages to the breathing center at the base of the brain. hen the carbon dioxide level in the blood is high, the messages cause the breathing center to trigger speedup in breathing rate. Conversely, a low carbon dioxide level reduces the stretch receptors in the lungs. When the lungs expand sufficiently, the stretch receptors send messages to the breathing center. The breathing center then sends messages that make the muscles relax. Stretch receptors thus operate as another kind of breathing control mechanism.

Date: 2014-12-22; view: 1107