Growth is a synthetic process involving the formation of molecules and compounds.

How do we grow and develop? The body is made of cells and of the tissues and organs that cells form. Growth occurs because of an increase in cell size or by cell division. It involves building materials into living protoplasm. It occurs throughout the protoplasmic system (intussusception) and is not restricted to accretionary growth (adding materials externally). In growth, new cells form to replace old cells. Growth is rapid early in life; later it slows down. After birth, there is no increase in the number of cells in the muscles and nerve tissue, but these cells increase in size and become more specialized. New cells form throughout life in some tissues such as skin and connective tissue.

Protoplasm is the basic chemical substance of the body. There are about a hundred different elements known to chemists but only eighteen seem absolutely necessary in composing protoplasm. Protoplasm is composed largely of elements that are relatively scarce outside living matter: hydrogen, nitrogen, carbon, and oxygen. Protoplasm forms units that constitute the cellular organization of your body. To perform different tasks, the body composes billions of cells in groups. Cells vary in form and size, but they are all built the same. Each cell is a mass of protoplasm differentiated into a main mass of cytoplasm and a smaller spherical nucleus near the cell's center. The cell membrane surrounds the cytoplasm. Parts of the cell serve as centers for enzymatic action in energy conversion. There is a chromatin network, which divides into a specific number of chromosomes bearing the genes of heredity.

There are two main divisions of cells in the body: the somatic cells and the germ cells. The somatic cells make up the body cells, carry on the basic functions of living, and live only for the generation. The germ cells perform only one function: reproduction. They are immortal and are stable in their constitution, but they can be influenced by environmental factors.

In the body, there is extracellular material, which is composed of fluid, fibers, and matrix (a jelly-like, cement substance). Fibers outside the cells are common and compose much of the body's connective tissue, which is one of five types of body tissue.

The tissue is an assemblage of similar cells with the same function. Vascular tissue forms blood and carries oxygen, food waste, etc.; epithelial tissue forms protective sheets to line cavities and cover surfaces (skin, mucous membrane); connective tissue binds, supports, and pads (bone, cartilage, adipose); muscular tissue makes movements possible by contraction (striped, smooth, cardiac muscle); and nervous tissue transmits impulses and co-ordinates bodily activities (brain, spinal cord, nerves).

Tissues form into organs to form structural and functional units of tissues working together. There are some simple organs (a single muscle or a gland), and they perform simple functions. Others are complicated, such as the liver and brain, and do many things. Usually only one tissue carries the burden of the organ's functions; the others are merely auxiliary. For an example, in the heart, muscle is the chief tissue, but connective tissue binds the parts of the organ together and epithelial tissue forms an inner lining for the heart's chambers and a protective sheath around its exterior.

In your body, many organs group together in a system to carry on a particular bodily function. To use an example, the digestive system is composed of many organs (mouth, esophagus, stomach, intestines), and these coordinate to digest your food. There is no system in the body that is independent of any other system; that is, they all work together to make you an integrated functional organism.

Growth is a means of increase in the size of existing cells or the creation of new ones by cell division. This continues throughout life. The surface area of the individual cell limits how much it can absorb. Some cells adjust to the size restriction by altering their shape to elongated form (like a nerve cell), to a flattened shape (like a skin cell), or by using hairlike projections to increase absorption the way an intestinal cell does. This adaptation allows cells to increase surface area without an increase in volume.

If cell division did not occur, tissues could not grow as each new cell eventually grows to the size of the parent cell. Old cells self-destruct by releasing an enzyme that digests the cell from within. Sometimes it takes a cell as little as a few hours to move through the cell cycle, or it can live as long as the body lives.

Some parts of the body grow faster than others. For an example, a newborn baby has a smaller head than when he/she reaches maturity. It is one-quarter of the length of its body at birth, but the brain is relatively large and well developed. When he/she is an adult at maturity, the head is less than one-seventh of the total body length. The baby at birth has legs one-third of its length, and an adult has legs about half the length of the body.

The rate of growth is not constant because the most rapid rate occurs before birth when in nine months the fetus increases weight by 2.4 billion times. There are spurts of growth in the first two years and then again at puberty. These years are separated by a slower growth rate. Sometimes height increases by two or three inches in a year and weight only increases by about six pounds.

Every child goes through the same basic patterns of physical growth, but there are wide ranges of normality. When a toddler, the child has a relatively large head and a short torso and legs. By the time he/she reaches the age of three, the head growth slows down, and the torso and legs catch up. Proportions become more like an adult. All children do not proceed at the same normal rate of growth as this depends on a few factors. These factors include: glandular functioning, health, prenatal care, environment, socioeconomic status, ethnic background, male, female, nutrition, emotional state, heredity, and environment.

The sequence of growth from head to toe can be seen in the gradual change in proportions from the prenatal period to adulthood. This is called directional growth. An infant learns to sit and then to stand and walk as his/her torso-to-toe development matures to the point that his/her legs and feet can hold him/her up and he/she can coordinate the muscles necessary for walking. Gradually, the child learns the coordination of arms with legs, etc.

Until maturity, the elongated bones in the fingers, arms, legs, and hips grow quickly by expanding at each end. These growth centers contain gristlelike cartilage cells that create layer upon layer of new bone tissue. When the cartilage cells stop dividing, the growth centers harden into bone, marking the end of growth in that region. Most growth centers, as those in the femur and tibia of the leg, have ossified by the age of 17 to 20 years. The breastbone stops growing around the age of 25.

You have 350 or so bones in the body as an infant, and these bones gradually fuse into the approximate 206 bones in the adult skeleton. Some people have an extra pair of ribs or fewer vertebrae in the spine and have fewer bones. Disease plays a factor in growth also as it can stunt the growth of the skeleton by preventing the bones from ossifying completely.

The interplay of environment and heredity controls growth. This is due to climate, disease, nutrition, and pollutants. Genes determine the amount of growth hormone secreted by the pituitary gland, but malnutrition prevents the pituitary from functioning at its full potential. Tall parents tend to have tall children while short parents have short children. The genes regulate the variation in growth.

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