6. 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.