A human body is made up of 100 trillion CELLS of different types. Similar cells are grouped to form tissue, and tissues form ORGAN. Using modern IMAGING technology, we can look into the body and see it work.
Humans are unique in the animal world. We are the only mammals to walk on two legs, our brains are unusually large, and our skin is almost hairless. Yet in most ways we are just like other mammals, with two pairs of limbs, two eyes and ears, and the usual mammalian internal organs.
Apart from identical twins, no two people in the world are alike. We differ in many subtle ways because we each have a unique set of genes, inherited from our mother and father. Our genes control the way we grow and develop from embryos into adults.
The microscopic units that make up all living things are called cells. In the body, there are hundreds of different types of cell, each designed to perform a specific task.
Most cells have a control center called a nucleus. The nucleus holds the DNA, which makes up the genes. Depending on the type of cell, certain genes are switched on or off. The active genes send instructions out of the nucleus, controlling all the chemical reactions that happen in the rest of the cell.
Cells of the same type are often grouped together in a pattern, to form tissue. Muscle is made up of rows of muscle cells. Skin consists of sheets of skin cells. Blood is a liquid tissue of cells suspended in a watery fluid. There are four main types: epithelial tissue, connective tissue, muscular tissue, and nervous tissue.
Tissues are grouped together into larger structures called organs, which carry out specific tasks. The heart, for instance, is an organ designed to pump blood.
Every organ contains several different tissues. The stomach, for example, consists mostly of muscle cells, which contract to churn food around. The inner lining of the stomach is made of epithelial tissue, which is continually worn away and replaced. There are also glands that secrete digestive juices; blood vessels; nerves; and connective tissue to hold it all together.
Organs work together in teams, called systems, to carry out major tasks. For instance, the stomach, intestines, and pancreas are part of the digestive system, breaking down food into molecules the body can absorb. Some systems work together—the skeletal and muscular systems combine to enable us to move.
Table 18. BODY SYSTEMS
|Integumentary system||Hair, skin, and nails protect the body from germs, injury, heat loss, and drying out.|
|Skeletal system||A framework of bones and connective tissues that supports the body and, with muscles, enables it to move.|
|Muscular system||The system of muscles (involuntary and voluntary) that contract to make the body move.|
|Nervous system||The brain, nerves, sense organs, and related tissues enable the body to detect and respond to changes.|
|Endocrine system||A system of glands that regulates body processes by secreting chemicals, called hormones, into the blood.|
|Circulatory system||The heart and blood vessels transport blood to the body’s cells to deliver nutrients and remove waste.|
|Lymphatic system||A system that returns body fluids to the bloodstream via nodes, where the fluids are screened for germs.|
|Immune system||An internal defense mechanism, consisting of cells and tissues that destroy invading germs and abnormal cells.|
|Respiratory system||The lungs and airways leading to the lungs, which take oxygen into the body and expel carbon dioxide.|
|Digestive system||The mouth and most of the abdominal organs work to break down food into molecules the blood can absorb.|
|Urinary system||The kidneys remove unwanted chemicals from the blood and expel them via the bladder and urethra.|
|Reproductive system||The organs involved in sexual reproduction—the penis and testes in men; ovaries, uterus, and vagina in women.|
Modern imaging techniques enable doctors to see inside the body without cutting it open. There are many different techniques, each suitable for looking at particular tissues or processes.
No, X-rays can also be used to look at soft tissues, such as the breast or blood vessels, to check if they are healthy. For blood vessels, a harmless dye that absorbs X-rays is first injected into the vessels—X-rays will then show their outline. This type of image is called an angiogram.
A computed tomography (CT) scan is a computer-generated image built from X-ray beams. A machine slowly moves over the area of the body, taking X-ray pictures from many angles. A computer then analyzes the X-rays to build up a detailed cross-section of the body, including its soft tissues.
Ultrasound scanning is one of the most common imaging techniques. High-frequency sound waves are bounced off internal organs and the pattern of echoes is displayed on a TV screen. Ultrasound scanners are good for studying moving liquids, such as blood or fluid in the uterus (womb).