Body Chemistry

When the body's chemical reactions are in balance, every chemical is maintained within its normal range for example, the normal range for potassium in the blood is 15-20 mg per 100 ml blood. Levels of a chemical may continually rise and fall in the blood and other organs, but the result is the equilibrium condition called homeostasis. Most of the body's chemical processes affect a variety of substances through interactions with other processes. When the level of phosphorus in the blood builds...

Integumentary System

Many of us try to achieve a healthy tan by staying out in the sun too long, not using sunscreen, and otherwise making ourselves vulnerable to skin cancer. Skin cancer is the most common form of carcinoma (cancer) in the United States. The three major types are basal cell carcinoma, squa-mous cell carcinoma, and malignant melanoma. Although the first two are easily curable, the third is extremely dangerous and can be fatal. Even basal cell and squa-mous cell carcinomas can cause great damage and...

Respiratory System

Many athletes prepare for competition by training in the mountains of Colorado. Because the atmospheric pressure is lower there, the athletes get less of the oxygen their bodies need. Their bodies adapt to the conditions in Colorado by increasing the number of red blood cells (which carry oxygen), and the adaptation continues for a while afterward, even when the athletes are at lower altitudes. This presumably increases their endurance. All animals need oxygen for metabolizing food, as...

Genus Homo

The genus Homo somehow evolved from earlier hominids that also gave rise to the australopithecines. The earliest known hominid having the generic name Homo was Homo habilis, a short, small-brained creature that walked upright and lived in Africa between 2 and 2.5 million years ago, while the australopithecines were still in existence. Famed anthropologists Louis Leakey (1903-1972) and Mary Leakey found a series of African fossils they placed in the genus Homo. One they named Homo habilis (handy...

Plants

Among the first organisms to colonize dry land were plants, which are generally thought to have arrived on land about 450 million years ago. (Recent evidence from DNA analysis suggests they may have done so as long as 700 million years ago and that they were accompanied by symbiotic fungi.8) The bacteria and protoctists still are generally restricted to moist or wet surroundings, but plants evolved in ways that enabled them to move onto dry land during the Paleozoic era or even earlier. For any...

Muscular System

In recent years, Americans have become more muscle-conscious, with many of us working out in gyms, running, and swimming to build up muscles as well as to derive general health benefits. Even for people with no interest in body-building, it is important to use the muscles daily. Inactive people can develop stiff muscles, setting up a cycle of even less activity, weight gain, and other health problems. All muscle cells are specialized for contraction. Those in striated muscle tissue make up the...

Cell Division

Though a parent organism has two genes for each trait, the sperm or egg cells produced by the organism have only one. That difference results from a special form of cell division occurring during formation of the gametes (sperm or eggs), called meiosis. Meiosis is a modification of mitosis, the process used by most cells to form new cells. Mitosis is not a simple splitting in two, but a complicated series of steps. The five major steps are called interphase, prophase, metaphase, and telophase....

Epithelial Tissue

Epithelial tissue makes up skin, nails, and hair. It also lines the digestive tract, urinary tract, and other tracts that have external openings. Skin color depends on pigments in the epithelial cells, such as melanin. Melanin is produced when the skin receives the ultraviolet (UV) rays of sunlight, providing a tanned or freckled barrier against damage from UV light. The health of the skin reflects the conditions of other body systems. Although melanoma is the most threatening skin condition,...

The Human Reproductive System

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In humans, fertilization usually takes place in one of the oviducts, or Fallopian tubes, where the egg travels from an ovary. Seminal fluid that has been released during the male's ejaculation flow into the vagina, through the cervix, into the uterus, and into the oviducts. (Because the vagina's muscular contractions help draw the seminal fluid into the reproductive tract, this can occur even if the penis is not inside the vagina.) Sexual intercourse often ends in an orgasm for one or both...

Precambrian

For more than a billion years more, Earth was lifeless. The rocky surface was still cooling, and the atmosphere containing ammonia, methane, and other gases would have been poisonous to the organisms of today. There was as yet no oxygen. Some time between 3.8 and 2.5 billion years ago, the first life appeared on Earth. The oldest fossils found so far microfossils of archaeans and other bacteria have been dated as 3.5 billion years old. (For more details about the archaeans, see Chapter 5.) For...

Skeletal System

The bones of early humans are in many cases all that is left for us to study. After centuries or longer, maggots and other decomposers remove all traces of living tissue, leaving only the mineral-like bone. Perhaps for that reason, we tend to think of the skeletal system as a nonliving structure only. However, bone in a living person is more than that. Bone is made up of microscopic living cells embedded in an abundant, hard, nonliving material. The living bone cells continually form new bone...

Immune System

Acquired Immune Deficiency Syndrome (AIDS) attacks the body's natural defenses against other diseases, leaving it vulnerable to damage or destruction by them. AIDS is caused by the human immunodeficiency virus (HIV), a retrovirus that enters lymphocytes and merges with their DNA. The DNA is then transformed by the HIV so that it makes copies of the HIV rather than of itself the HIV copies burst out of the cell and attack new host cells. Some success in controlling AIDS has been achieved with a...

Circulatory System

For about 40 years, medical researchers have linked high levels of cholesterol in the blood to increased risk of cardiovascular disorders, such as heart attack and stroke. Though cholesterol is found normally in the body and plays an important role in some structures, a high level of it is a danger signal. Now a new suspect is emerging A high level of homocysteine, a substance that builds up when the amino acid methionine is not sufficiently metabolized, is also linked to cardiovascular...

Anthophyta Flowering Plants

Ranging from grasses to flowering shrubs to trees, the angiosperms are a vast array of species. The two main divisions are the monocots (monocotyledonous plants) and the dicots (dicotyledonous plants). Monocots have seeds that contain just one seed leaf, or cotyledon parallel veins in their leaves and other characteristics in common. Dicots have two seed leaves, branching leaf veins, and other shared characteristics. The angiosperms first appear in the fossil record near the end of the...

Cool Desert

Just east of the Sierra Nevada Mountains, where clouds have lost precipitation as snow or rain, the state of Nevada is quite dry. Though the desert there looks bare, like the hot desert farther south, the climate is different. This is a cool desert. Cool deserts are characterized by cold winters with much snow and rain falling during the winter and occasionally over the summer. In the United States they are found along the western edge of the Great Plains. Cool deserts have short, moist,...

Preludes to Darwin

These English and Swedish thinkers contributed to the changing ideas about fixed species of organisms, but French philosophers went further. Count GeorgesLouis de Buffon (1707-1788) wrote about the struggle for existence, the tendency for life to multiply at a faster rate than its food supply, variations within species, similarities of structure in different organisms, a very long time scale, the extinction of some earlier animals, geographic distribution of plants and animals, and other...

Modern Homo sapiens

The Homo sapiens who displaced the Neanderthals were anatomically modern. Like the Neanderthals, they had large brains. Their skeletons, including the skulls, were like ours. The tools they left in eastern and southern Africa include a bola for throwing at small game, flake tools, a long flake blade, and the core from which blades were struck. We have more evidence about how they lived in Europe. The cave paintings in France especially seem to be evidence of people who used art and perhaps...

Endocrine System

Unlike most body systems, the endocrine system is scattered throughout the body. The glands are defined as a system because they act in the same way and affect each other. Hormones control the events of the menstrual cycle and many other body processes, such as changes in blood chemistry and responses to external stimuli. The hormone insulin, for example, is necessary for the cells to absorb sugar normally for use in cell respiration. In diabetic patients, too little insulin is made by the...