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SECTION FIVE:

SCIENCE

Part Two

HUMAN BIOLOGY

HUMAN BIOLOGY

As we have already discussed in our previous section, humans belong to the class of mammals known as primates. What sets us physically apart from all other primates is our ability to walk on two legs; (bipedalism). Though most primates can achieve the task of walking on two legs, it is only for short intervals and for short distances. An important reason contributing to our ability to walk on two legs is the upright, erect posture that our bodies assume when standing. Humans have been very successful in flourishing their species on the earth. This has been accomplished due to the remarkable ability of human adaptation to the most widely diversified climates and conditions. The most prominent distinguishing feature separating humans from the rest of the animal kingdom is the highly developed brain and the ability to communicate with each other through speech. In the subject of human biology, we will study the different organs systems that constitute the body, describing the major functions these organs and systems. The major systems in the body are the skeletal system, the muscular system, the nervous system, the circulatory system, the immune system, the respiratory system, the digestive and excretory system, the endocrine system and the reproductive system. The Skeletal System The human skeleton consists of more than 200 individual bones that are bound together with connective tissues, which although somewhat flexible, do not offer any elasticity. These are called ligaments. Parts of the skeleton, where two joints meet, maintain a special connection by means of a padded layer of soft, but durable cartilage to prevent friction between those moving parts of the bones forming a joint. This prevents wear and tear on the bones through continued use. The Skull All the bones in the head compose the skull, which is made up of 22 flat and irregular shaped bones that are fused together to from the cranium. This is the part of the skull that houses the brain. There are another 14 bones, also irregular shaped, varying in size, designed to protect the face.

(By studying the Figures on page 3, you can familiarize yourself with the names of the bones that comprise the Skeletal System)

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The Vertebral Column Composed of 26 bones called vertebrae, at birth, the vertebral column consists of 33 vertebrae. Five sacral bones, located at the back of the pelvis, fuse together into one large triangular shaped bone called the sacrum. Also, four coccygeal bones, located at the end of the spine, fuse into one bone called the coccyx. The vertebral column is the backbone and serves to support the frame of the body’s skeleton and protect the spinal cord, the heart of the nervous system. The vertebrae are joined together by ligaments and cartilage so that flexibility is permitted, allowing movement of the back. There are openings in the sides of each vertebra to allow all nerves, which originate at the spinal cord to spread throughout, reaching every part of the body. Thorax This is the chest area, and in the skeletal system, primarily consists of the rib cage. There are a total of 12 pairs of ribs attached to the vertebral column encasing the area of the body in which the heart and lungs are positioned. The first ten pairs of ribs are joined at the front of the chest by the breastplate, known as the sternum. Flexible cartilage is present at these joints to allow some movement to accommodate the expansion of the lungs during respiration. The Appendicular Skeleton This is the term used to indicate the part of the skeleton to which the appendages, or limbs belong. This includes all the bones of the hands, the feet, the arms, the legs and the pelvis. These are the bones that are designed to permit the most movement and a special type of joint is used where two of these bones meet. It is called a ball and socket joint. This type of joint serves to provide rotational movement as observed when moving the arm at the shoulder and the leg at the hip. A ball and socket joint also appears at the topmost vertebra where the backbone meets the skull. This is to allow rotational movement of the head. The joints at the knees and the elbows, though similar in structure to the ball and socket joint, these joints provide limited movement to back and forth movements only. However, the joints where fingers meet the hands and where toes meet the feet are also small ball and socket joints. The term ball and socket is appropriate and accurate in describing the ball shaped end of the bone and socket-like structure of where it meets and fits within the connecting bone.

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The Muscular System In the average human, about 40% of the total body weight is muscle. Muscles are attached to those bones of the skeleton that involve movement by elastic-like fibers called tendons. As mentioned in our BIOLOGY section, there are three different types of muscle: smooth or involuntary muscle, striated muscles often referred to as striped muscle, and cardiac muscle. Smooth or Involuntary Muscle Smooth or involuntary muscles are found in the walls of internal organs including the digestive tract, reproduction organs, the bladder, and all arteries and veins. Aptly named involuntary muscles, they function without the use of a person’s will. Many body functions must occur, whether we are conscious or not conscious of them. The most common function of these muscles is to squeeze. This is how food is passed along the digestive tract; how blood is circulated throughout the blood vessels; how urine is expelled from the body by means of the smooth muscles in the bladder; semen is discharged from the seminal vesicles by the contraction of smooth muscles and the iris in the eyes opens and closes in response to exposure from light by the means of smooth muscle contraction. Striated Muscle These are the muscles that are called voluntary and also referred to as striped or skeletal muscles. These muscles contract at will and include all the muscle groups of the limbs, neck, back, and pelvis. These muscles are made of millions of fibers that run the entire length of the muscle. An estimated 6 billion fibers compose the more than 600 muscles located throughout the body. Cardiac Muscle Unlike other muscles in the body, cardiac muscles function independently of nerve supply. In the other two groups of muscles, nerves are required to carry the message from the brain or central nervous system commanding the muscle to respond. Cardiac muscles are in use at all times in the living body, continuously pumping blood throughout the body by means of the pumping action of the heart. The figures on page 5 illustrate and identify the different muscles in the body. This will allow you to familiarize yourself with the different names given to the various parts of the human muscular system.

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The Nervous System In humans, the nervous system is composed of two main parts: The Central Nervous System and the Peripheral Nervous System. The tissues of the nervous system are designed to receive and transmit external stimuli from the environment, and then carry the impulses to other body tissues. This highly developed system includes the brain and is what makes humans so significantly different from the rest of the animals. Central Nervous System The central nervous system is composed of the brain, and the spinal cord. Connective tissue protects the central nervous system by wrapping itself around the system’s organs, which are then encased within strong bone structures. In the case of the brain, it is housed within the part of the skull called the cranium. The strong vertebrae of the spinal column, or backbone protect the spinal cord. The Brain has three main sections, though there are more parts, however, the three major parts are called the cerebrum, the cerebellum and the medulla. Cerebrum This is the largest section of the brain and is regarded as the “seat of intelligence”, where all voluntary, conscious activities of the body are controlled. It also functions as the center of interpretation of all stimuli entering the brain. The cerebrum controls all voluntary muscle movement, speech, thought and memory.

THE HUMAN BRAIN

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Cerebellum Positioned beneath the back part of the cerebrum, the cerebellum is the second largest part of the brain. Its main function is to control balance of the body, as well as regulate muscle tone. The cerebellum also controls the precision and coordination of the body’s physical activities such as walking, running, dancing, skating, writing and typing, as well as other activities involving the artistic endeavors of painting, sculpting, crafting and playing musical instruments. Medulla The medulla or medulla oblongata as it is also known, is located below the cerebellum and is attached to the spinal cord. It is through this organ that all sensory impulses and motor signals are received and transmitted. The medulla controls the function of all smooth, involuntary muscles such as the types described in the previous section on the muscular system. The Spinal Cord Positioned in the central hollow of the vertebrae, this organ is composed of masses of nerve cells and fibers. It is the conductor of impulses to and from the brain. Impulses enter and leave the spinal cord through a vast network of spinal nerves extending outwards to the other body organs. Other Parts Included within the brain are other important parts. One is the thalamus, which is the area where sensory information is integrated, and the other is the Hypothalamus. This organ controls body temperature, the content of water in the body, maturity, hunger, thirst and sex drive.

Peripheral Nervous System There are two divisions to this system: the somatic peripheral system and the autonomic nervous system. Together, these two divisions function to connect the central nervous system; (brain and spinal cord), with the rest of the body. The peripheral nervous system is the far-reaching network of nerves extending throughout the body to its outermost surface, the skin.

The somatic peripheral system serves to conduct impulses to the muscles and sensory organs within the control of will; the skeletal or striated voluntary muscles. The autonomic nervous system controls the smooth, involuntary muscles of the body. These are described in more detail in the previous section on the muscular system.

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The Endocrine System These are the ductless glands that secrete hormones, which play a vital role in various functions of the body. Glands that have ducts discharge their secretions directly on the organs to which they are connected. For example, tear ducts secrete tears directly over the eyes; salivary glands discharge saliva directly into the mouth; sweat glands perspire directly on to the skin and mammary glands secrete their milk directly through the breasts. The ductless glands of the endocrine system are not physically joined or grouped together like other systems; rather they are distributed throughout various parts of the body, but are similar in structure and function. This is why they are considered part of one system The ductless glands of the endocrine system discharge their secretions - hormones into the blood stream where they are transported to their targeted destinations. Hormones regulate several functions of cells and organs. Examples of the vitally important functions which hormones directly promote, affect, stimulate, regulate or implement, are the following: Growth, rate of metabolism, response to stress, muscle contraction, sleep and rejuvenation, immune responses, blood pressure, digestion, development and functioning of the reproductive system. The following is a list of the glands that make up the endocrine system; their location in the body; the name of the hormone they each produce, along with the function of the hormone. The pituitary gland is located at the center of the brain and produces growth hormone, which affects skeletal growth, and it also assists in protein synthesis. This gland also produces a hormone called vasopressin, which stimulates absorption of water, by the kidneys, as well as maintaining blood pressure. The thyroid gland is located in the area of the neck and controls oxidation in the cells. On the back of this gland is located the parathyroid gland which regulates the amount of calcium in the blood. The hormones released by the se two glands are called thyroxin and tri-iodotyrosine. Both hormones contain iodine and are responsible for controlling the rate of all metabolic processes in the body as well as influencing physical development. In the stomach and the intestine, certain hormones like gastrin and secretin respectively, are produced to aid in the digestive process, although there are no glands from which the hormones are secreted, instead this done from the inside walls of each of the two organs.

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The adrenal gland has two divisions; the medulla and the cortex. The adrenal glands are located above the kidneys. The adrenal medulla controls the release of sugar from the liver and contraction of the arteries by the production of the hormone adrenaline. Adrenaline increases both heart activity and muscle power. This hormone is commonly known to activate in preparation for the body to fight, or flee from an imminently dangerous or frightful situation. The adrenal cortex releases two types of hormones; one affects normal functioning of the gonads - (testes in males, ovaries in females), and helps to maintain blood sugar levels. The other stimulates the kidneys to reabsorb sodium needed by the body. The Isles of Langerhans is a gland located in the pancreas, which lies in the abdomen, between the stomach and the large intestine. The hormone produced by this gland is insulin, which regulates the storage of glycogen in the liver. You will recall from the BIOLOGY section, that glycogen is the major source of energy produced by carbohydrates. Insulin also accelerates oxidation of sugar in the cells. The gonads, as mentioned before are located in the testes in males, and in the ovaries in females. The male hormone produced by the gonads is called testosterone. The gonads in the female produce two hormones called estrogen and progesterone. All hormones produced by gonads regulate normal growth and development of sex glands, control sex characteristics and regulate reproduction. In children, there is a gland which shrinks away as the body matures in to adulthood called the thymus. It is located in the lower neck area and in front of the heart. The thymus produces the hormone thymosin, which stimulates the white blood cells into action, functioning as part of the immune system in warding off disease. The pineal gland is located at the base of the brain and produces the hormone melatonin, whose benefits and properties are still not fully known and research is being dedicated to its study at present. Also, all the functions of the pineal gland remain unknown. Among many spiritualists and eastern philosophers, the pineal gland is referred to as the “seat of the soul”. What is known about melatonin is that it influences extend to a wide variety of body functions including sleep, immunity, longevity (anti-aging), it is believed to serve as an antioxidant (cancer preventing) as well as assist in the healing and recovery process. Further research is being done, as mentioned before, and in time discoveries will be made to accurately describe the functions of this mysterious gland.

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The Respiratory System Breathing, which is what is meant be respiration, occurs both internally and externally. External breathing involves the lungs, which are used for breathing in air (inspiration), and breathing out carbon dioxide and water vapor (expiration). Internal respiration concerns the cells. In our BIOLOGY section, on the subject of cells, you will recall the area of cellular respiration, which concerns biochemical reactions by which energy is released from carbohydrate molecules. In this section, we will instead focus on external respiration. The respiratory tract consists of specialized organs and smaller parts of the lungs, which must be understood as to their function in the respiratory system. As air comes into the body through inspiration, it first passes through the nasal cavity (nostrils and sinuses) where it is humidified and filtered for dust particles. It passes into the pharynx (throat) and the larynx (voice box), where it then leads into the trachea (windpipe). The trachea, shaped in the form of a tube, then divides into two smaller tubes at the area directly behind the sternum (breast bone). These are called the bronchi, (singular, bronchus). Each bronchus subdivides into many smaller tubules called bronchioles, which spread throughout the lungs. At the end of each bronchiole is a tiny air sac called an alveolus, which is surrounded by blood capillaries. Oxygen diffuses from the alveoli in the lungs and enters into the blood stream through the capillaries, where it is transported by red blood cells and distributed throughout the body. In exactly the reverse manner, carbon dioxide is diffused from the blood stream into the lungs and out through the remainder of the respiratory tract to be exhaled through the mouth. The two lungs in the body are encased within two double-membrane, air tight sacs called pleural sacs. Another organ involved with respiration is the diaphragm; a muscular wall that separates the chest area (thorax), from the abdomen. As the diaphragm contracts, it increases the area of the chest cavity by pulling downwards, allowing room for the lungs to expand as they fill up with air. Upon expiration, the diaphragm relaxes, decreasing the size of the chest cavity in conjunction with the air being pushed out of the lungs as it exits the body.

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The Circulatory System The Heart At the center if the circulatory system is the heart. Consisting of a bundle of cardiac muscles that specialize in contracting and relaxing rhythmically to produce the heartbeat, it lies directly behind the sternum in the chest cavity. A large number of arteries cover the surface of the heart, functioning to provide oxygenated blood (blood laden with oxygen and nutrients) to the muscle fibers of the heart. Structure and Function of the Heart The heart has four chambers, two atria (singular, atrium), in the upper third of the heart, and two ventricles in the lower two-thirds of the heart. The atrium receives blood and the ventricle pumps blood out of the heart. Large arteries and veins lead into the top of the heart, which carry blood to and from the heart. The heart is a double pump with the four chambers functioning as two pairs of pumps. Specialized valves within the walls separating the chambers ensure that blood flow is maintained in one direction, and prevent backflow. Fig. 25 illustrates a cross-section of the heart, identifying these valves, the main vessels leading to the heart, as well as its chambers. The valves are held in place by miniature tendons called chordae tendinae, and they are attached to the inside walls of the heart by papillary muscles. The Path of Blood As was explained in the previous section on the Respiratory System, blood becomes oxygenated in the lungs. Arteries function to carry the nutrient rich oxygenated blood from the heart to the rest of the body, and veins serve to bring deoxygenated blood back to the heart, after supplying the rest of the body. The exceptions to this rule are the pulmonary arteries and veins. These are use to send the deoxygenated blood to the lungs via the pulmonary artery where they receive the fresh supply of oxygen and necessary nutrients, and then returned to the heart via the pulmonary vein. Deoxygenated blood is received by the heart from the rest of the body through the main vein called the vena cava. It always arrives through the right atrium and is then pumped by the ventricle through the pulmonary artery to the lungs where it receives oxygen. Once returned to the heart, the oxygenated blood is pumped out of the heart to the rest of the body by means of the main artery called the aorta.

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The Digestive and Excretory Systems The human digestive system begins with the mouth and ends with the anus. Its proper term is the alimentary canal, or gastrointestinal tract; however, it is very commonly called the gut, or digestive tract. There are essentially five separate functions carried out by the digestive system. Firstly, it governs the intake or ingestion of food; secondly, it conducts the transportation of the food to organs where the third function is carried out. This function is that of chemically breaking down the food into a state, which it can then be easily absorbed. This then, is the fourth function; that of absorbing, or assimilating the food. The fifth function is to temporarily store the undigested food, to be later expelled from the body through the excretory process. Beginning in the mouth where ingestion takes place, food is chewed by the grinding action of the teeth. Salivary glands in the mouth release saliva containing the enzyme ptyalin, which begins the process of breaking down carbohydrates. Once food is sufficiently ground in the mouth into a thick paste, it is swallowed, where it passes the throat and enters the esophagus (gullet or food pipe). The esophagus does not act to digest food, but simply transports it down to the stomach. It does this through a series of muscular contractions known as peristalsis. The border between the esophagus and the stomach is guarded by a sphincter, which opens to allow food to enter the stomach and closes to keep it in. This is called the cardiac sphincter due to its physical proximity to the heart. While in the stomach, the food remains for about 2 hours while it is churned and further chemical breakdown takes place. You will recall from our section on the endocrine system, that there are ductless glands lining the inside wall of the stomach. It is through these glands that two types of enzymes are released to do their job in chemically breaking down the food even further. Rennin is one of the se enzymes, and it serves to break down protein in milk. Pepsin is the other of the two enzymes released in the stomach, which also works to break down other forms of protein found in animals and vegetables. By the time the stomach has completed its job, the broken down food, now called chyme, it passes on to the next portion of the alimentary canal; the small intestine. Separating the lower end of the stomach from the small intestine is another sphincter, which works in much the same way as the cardiac sphincter. This one, the pyloric sphincter, allows small amounts of chyme to pass through a little at a time.

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Once chyme enters the small intestine, further breakdown through the action of enzymes occurs. The small intestine is divided into three sections according to function. The first part, following the stomach, is called the duodenum. The major work of digestion now takes place at this stage, where chyme enters the duodenum. It is here that glands in two organs, not part of the alimentary canal, act in the digestive process. The liver, one of these organs, is the largest internal organ in the human body The liver synthesizes a chemical called bile, which acts to emulsify fats so that lipase, an enzyme secreted by the duodenum and the pancreas, works to break down fats in the chyme. Bile manufactured by the liver, is stored in a small pouch that is adjacent to he liver called the gall bladder. The bile reaches the duodenum through bile ducts, which lead to it from the gall bladder. The other organ not part of the alimentary canal involved in the process at this stage is the pancreas. The pancreas releases pancreatic juice; a combination of water and several digestive enzymes into the duodenum. These enzymes, each specific in its function, work to breakdown fats, carbohydrates and protein. Also, within the walls of the small intestine are intestinal glands, which also secrete a combination of digestive enzymes. These work to break down starches, sugars and proteins. The result of the digestion taken place so far is that food is broken down into a soluble form that can be readily absorbed by cells. Carbohydrates are converted in to glucose or fructose; proteins are broken down into amino acids and fats into fatty acids and glycerol. Now, the digested food continues on its journey through the small intestine, into the second and third sections called the jejunum and ileum, respectively. In these sections of the small intestine, the inside walls are aligned with thousands of tiny finger-like projections called villi, (singular, villus). It is through the villi that the now digested soluble food molecules are absorbed into the blood stream through the capillaries found in the villi. This is the process of assimilation. The remainder of undigested food, not assimilated into the body, now enters the large intestine, also known as the colon. The colon serves as temporary storage of the undigested food as it travels through this final part of the alimentary canal, where it will finally be expelled, or excreted pass the rectum; a muscular terminal of the canal, and out of the body through the anus. The anus also has a sphincter; the anal sphincter, which functions to push the waste out by the muscular contractions of the rectum. The anal sphincter opens to allow the exit of the waste, or feces, and closes after they are pushed through.

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The Reproductive System In human reproduction, the cycle begins with the union of the male sperm and the female ovum. Each of these are considered reproductive cells, or germ cells. The organs that contain germ cells, which later develop into male gametes, or spermatozoa, are known as testes, or male gonads. The organs that contain germ cells, which later develop into female gametes; eggs or ova, are called ovaries. Ovulation, Copulation and Insemination The union of the male and female gametes results in the female egg becoming fertilized. Occurring internally, the process of the male sperm successfully fertilizing the female egg is called insemination.. During a period called ovulation, occurring in the female approximately every twenty-eight days, an egg is released from the ovary and travels down towards the uterus. At this stage the egg is “ripe” for fertilization. It is during the act of sexual intercourse also known as copulation, or coitus that the male and female are locked in close embrace, and the male inserts the male genital organ; the penis into the female genital organ; the vagina. The eventuality of coitus, or copulation is the male discharging semen the fluid containing sperm into the female vagina. Typically, the male ejaculates more than 250 million sperm into the vagina, from which some make their way to the uterus. It takes just one sperm cell to successfully inseminate the egg, for pregnancy to result. Gestation In the female, the egg released by the ovary travels through an organ called the fallopian tube, before it reaches the uterus. The egg cannot travel by itself and is instead pushed along by several thousand cilia lining the inside walls of the female reproductive tract. Meanwhile, the sperm released by the male and deposited in the vagina travel on their own steam by means of flagella; tail-like projections that propels the sperm forward. Insemination actually occurs in the female’s fallopian tube where the egg is making its way toward the uterus. As the one successful sperm cell, out of the nearly 10,000 that reach the uterus unites with the egg, it actually penetrates the wall, or egg cell membrane. Their fusion results in the creation of a new cell called the zygote.

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The zygote begins its development at first in the fallopian tube and then later in the uterus, which is then referred to as the womb. This is called the embryonic stage, and it is at this point that the cell differentiates into a new entity made up of the combination of both male and female gametes. The period of time for development of the embryo is approximately 280 days, or nine and a half months. During this period of time, called gestation, the new human embryo is supplied with all the nutrients facilitating its growth and development by the placenta; a temporary organ formed only during pregnancy for this specific purpose. It is after about eight weeks that what began as a single fertilized egg, divides and divides again and again, multiplying itself over and over until there are billions of cells organized into tissues, organs and organ systems. At this point, the embryo starts to resemble a human infant and is called a fetus rather than an embryo. Maintaining its link with its mother’s placenta is conducted through a literal “life-line” called the umbilical cord, leading from the placenta to the fetus. The placenta continues growing until the fifth month of pregnancy. At the end of the gestation period, the fetus passes through the birth canal from the uterus, through the cervix, and finally exits its mother’s womb through the vagina in the process referred to as labor, becoming a newborn baby. The Male Reproductive System Some organs of the male reproductive system are located outside of the body, while others are positioned internally. The scrotum is one of these and is essentially a sac-like organ that houses the testes, the glands that produce sperm and the male hormone testosterone.

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The penis is also positioned outside of the body. During sexual excitement, this protruding erectile organ becomes fully engorged with blood, enabling the now enlarged and erect penis to penetrate the vagina. The penis is the organ that delivers the sperm into the body of the female. Each testis contains thousands of tiny tubes called seminiferous tubules, and it is here that sperm is manufactured. Leading from these tubes are coiled, slightly larger tubes called epididymis, which provide storage for the sperm. In its pathway, sperm ejected from the seminiferous tubules travel through the epididymis to a duct called the vas deferens, passing the seminal vesicles on their way, where they pick up nutrients. From the vas deferens, the sperm travel to the urethra, a single tube leading from the bladder through the penis. Before leaving the male body through ejaculation, the prostate gland and the Cowper’s glands also contribute to the mixture of seminal fluid. The prostate gland, about the size of a chestnut located at the base of the urethra where it leaves the bladder, secretes a thin milky liquid constituting the greater part of semen. The Cowper’s glands; a pair of small glands the size of a pea and located on each side of the base of the penis, secrete a thick, clear fluid believed to protect the sperm against excess vaginal acidity. The Female Reproductive System The specialized organs of the female reproductive system serve to carry out three major functions: The production of egg cells, the disintegration and expulsion of non-fertilized egg cells through the process of menstruation, and the protection of the developing embryo. Lying in the lower region of the abdomen on either side of the mid-line of the body are the ovaries. These oval-shaped organs produce eggs on an alternating basis each month. As an egg matures, it bursts out of its housing within the ovary called a follicle, and is released into the fallopian tube, which leads to the receptacle shaped uterus. Here, the development of the embryo takes place. The vagina, being an orifice, or opening in the female body, is the main reproductive organ and in the body of a virgin, is sealed by a thin membrane called a hymen. At the opening of the vagina, structures exist to keep it closed except for when in use. These structures are shaped in the form of two pairs of vertically positioned lips. The inner pair, which is smaller in size and thinner in shape, is called the labia minora. The outer larger and thicker pair is called the labia majora. During sexual excitement, the inside walls of the vagina secrete a lubricant so as to minimize the friction caused during the process of coitus.

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