Blood Pressure

Blood Pressure, pressure of circulating blood against the walls of the arteries (blood vessels that carry blood from the heart to the rest of the body). Blood pressure is an important indicator of the health of the circulatory system. Any condition that dilates or contracts the arteries or affects their elasticity, or any disease of the heart that interferes with its pumping power, affects blood pressure.

In a healthy human being, blood pressure remains within a certain average range. The complex nervous system mechanisms that balance and coordinate the activity of the heart and arterial muscles permit great local variation in the rate of blood flow without disturbing the general blood pressure.

Hemoglobin, the iron-protein compound that gives blood its red color, also plays a role in regulating local variation in blood pressure. Hemoglobin carries nitric oxide, a gas that relaxes the blood vessel walls. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regulating the amount of nitric oxide to which the vessels are exposed.

Two measurements are used to describe blood pressure. Systolic pressure measures blood pressure when the heart contracts to empty its blood into the circulatory system. Diastolic pressure measures blood pressure when the heart relaxes and fills with blood. Systolic and diastolic pressure are measured in millimeters of mercury (abbreviated mm Hg) using an instrument called a sphygmomanometer. This instrument consists of an inflatable rubber cuff connected to a pressure-detecting device with a dial. The cuff is wrapped around the upper arm and inflated by squeezing a rubber bulb connected to it by a tube. Meanwhile, a health-care professional listens to a stethoscope applied to an artery in the lower arm. As the cuff inflates, it gradually compresses the artery. The point at which the cuff stops the circulation and at which no pulsations can be heard through the stethoscope is read as the systolic pressure. As the cuff is slowly deflated, a spurting sound can be heard when the heart contraction forces blood through the compressed artery. The cuff is then allowed gradually to deflate further until the blood is flowing smoothly again and no further spurting sound is heard. A reading at this point shows the diastolic pressure that occurs during relaxation of the heart. Normal blood pressure in an adult is less than 120/80 mm Hg. The first number describes systolic pressure, while the second number describes diastolic pressure.

Blood pressure is influenced by a wide range of factors and varies between individuals and in the same individual at different times. For instance, blood pressure naturally increases with age because the arteries lose the elasticity that, in younger people, absorbs the force of heart contractions. Other factors, such as emotions, exercise, or stress, may temporarily raise blood pressure.

Abnormally high blood pressure, known as hypertension, that remains untreated can lead to stroke, heart attack, and kidney or heart failure. Hypertension may have no known cause or it may result from heart or blood vessel disorders or from diseases affecting other parts of the body. Abnormally low blood pressure, known as hypotension, may be caused by shock, malnutrition, or some other disease or injury.

Bone

Bone (anatomy), is a hard connective tissue, the major component of almost all skeletal systems in adult vertebrate animals. Bone appears to be nonliving—in fact, the word skeleton is derived from a Greek word meaning dried up. However, bone actually is a dynamic structure composed of both living tissues, such as bone cells, fat cells, and blood vessels, and nonliving materials, including water and minerals.

Bones are multipurpose structures that play diverse, vital roles in vertebrates. They provide a framework for the body, supporting it and giving it shape. They also provide a surface for the attachment of muscles and act as levers, permitting many complex movements. Many bones protect softer internal organs; for example, skull bones protect the brain, and rib bones form a cage around the lungs and heart. In addition to these structural and mechanical functions, bones also participate in the body’s physiology. They store calcium, a mineral essential for the activity of nerve and muscle cells. The soft core of bone, the bone marrow, is the site of formation of red blood cells, certain white blood cells, and blood platelets (see Blood).

There are two main types of bone. Compact bone, which makes up most of the bone of arms and legs, is very dense and hard on the outside. The structural units of compact bone are osteons, elongated cylinders that act as weight-bearing pillars, able to withstand any mechanical stress placed on the bone. The center of each osteon contains a hollow canal that acts as a central passageway for blood vessels and nerves.

Surrounding both compact and spongy bone is a thin membrane, the periosteum. The outer layer of this membrane contains nerves and blood vessels that branch and travel into the bone. The inner layer of the periosteum consists mainly of osteoblasts.

The point where two or more bones come together is called a joint, or articulation. Different kinds of joints enable different ranges of motion. Some joints barely move, such as those between the interlocking bones of the skull. Other bones, held together by tough connective tissues called ligaments, form joints such as the hinge joint in the elbow, which permits movement in only one direction. The pivot joint between the first and second vertebrae allows the head to turn from side to side.

Intimately associated with bone is another type of connective tissue called cartilage. Cartilage is softer, more elastic, and more compressible than bone. It is found in body parts that require both stiffness and flexibility, such as the ends of bones, the tip of the nose, and the outer part of the ear.

See BONE DISEASE

Chromosome

Chromosome is a microscopic structure within cells that carries the molecule deoxyribonucleic acid (DNA)—the hereditary material that influences the development and characteristics of each organism. In bacteria and bacteria-like organisms called archaebacteria, chromosomes consist of simple circles of DNA floating freely in the organism. In all other life forms, collectively called eukaryotes, chromosomes reside within a well-defined nucleus. In eukaryotes, chromosomes are highly complex structures in which the shape of the DNA molecules is linear, rather than circular.

Chromosomes consist chiefly of proteins and DNA. Tiny chemical subunits called nucleotide bases form the structure of DNA. A sequence of bases along a DNA strand that codes for the production of a protein is known as a gene (see Genetics). Genes occupy precise locations on the chromosome.

Each cell contains enough DNA to form a thread extending about 2 m (about 7 ft). Proteins called histones play a key role in packaging DNA within chromosomes. Sections of the DNA molecule wind around clusters of histones to form units called nucleosomes, which resemble spools encircled with thread. Another type of protein, called nonhistone chromosomal protein, further compresses nucleosomes into a compact, narrow coil. Chromosomes become most condensed when a cell is preparing to divide.

The chromosome structure ensures that even when the DNA is highly confined, it is free to carry out transcription, or the production of messenger ribonucleic acid (mRNA). Messenger ribonucleic acid is the molecule that carries the DNA instructions that determine the types of proteins a cell will reproduce to the sites where proteins are constructed.

Connective Tissue

Connective Tissue is the tissue that supports and connects the various parts of the body. Originating primarily in the cells of the mesodermal (middle tissue) layer of the embryo (see Embryology), it forms such varied types of tissue as bone, cartilage, fat, ligaments, and tendons. Connective tissues are composed of a variable structure of cells and fibers surrounded by an intercellular matrix that may be a fluid, solid, or gel, depending on the function of the particular connective tissue. White fibrous connective tissue forms most of the tendons and ligaments. Yellow elastic connective tissue forms such structures as the pads between the vertebrae and the elastic elements of the arterial walls and the trachea. Among other types of connective tissue, cartilage takes part in the formation of joints and the development of bone, and fat tissue provides a cushion for the support of such vital organs as the kidneys and stores excess food for use when needed. Lymphatic tissue (see Lymph) and blood are clearly related in embryonic development to the connective tissues.

Diaphragm

Diaphragm in anatomy, is a wide muscular partition separating the thoracic, or chest cavity, from the abdominal cavity. It is a characteristic of all mammals and is rudimentary in some birds. In humans the diaphragm is attached to the lumbar vertebrae, the lower ribs, and the sternum or breastbone. Three major openings in the diaphragm allow passage of the esophagus, the aorta, the veins, the nerves, and the lymphatic and thoracic ducts.

The diaphragm is roughly elliptical in humans. It slants upward, higher in front than in the rear, and is dome-shaped when relaxed. Contraction and expansion of the diaphragm are significant in breathing. During inhalation the diaphragm contracts, becoming flattened and increasing the capacity of the thorax. Air rushes into the lungs to fill the partial vacuum thus formed. Air is exhaled when the diaphragm relaxes. When the diaphragm contracts, it exerts pressure on the abdomen, stimulating the stomach, and thus aiding in the process of digestion. A hiccup is caused by a spasmodic, involuntary contraction of the diaphragm.

Digestive System

Digestive System, series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. The simple molecules travel through the bloodstream to all of the body's cells, which use them for growth, repair, and energy.

All animals have a digestive system, a feature that distinguishes them from plants. Plants produce their own food in a process called photosynthesis, during which they use sunlight to convert water and carbon dioxide into simple sugars. But animals, including humans, must take in food in the form of organic matter, such as plants or other animals.

Digestion generally involves two phases: a mechanical phase and a chemical phase. In the mechanical phase, teeth or other structures physically break down large pieces of food into smaller pieces. In the chemical phase, digestive chemicals called enzymes break apart individual molecules of food to yield molecules that can be absorbed and distributed throughout the body. These enzymes are secreted (produced and released) by glands in the body.

The digestive system of most animals consists mainly of a long, continuous tube called the alimentary canal, or digestive tract. This canal has a mouth at one end, through which food is taken in, and an anus at the other end, through which digestive wastes are excreted. Muscles in the walls of the alimentary canal move the food along. Most digestive organs are part of the alimentary canal. However, two accessory digestive organs, the liver and pancreas, are located outside the alimentary canal. These organs contribute to chemical digestion by releasing digestive juices into the canal through tubes called ducts.

Topics:

• The Human Digestive System
• Regulation of the Digestive Process
• Ailments of the Digestive System

Related topic: Digestive System of Animals

Emotion

Emotion, term frequently and familiarly used as synonymous with feeling. In psychology it signifies a reaction involving certain physiological changes, such as an accelerated or retarded pulse rate, the diminished or increased activities of certain glands, or a change in body temperature, which stimulate the individual, or some component part of his or her body, to further activity. The three primary reactions of this type are anger, love, and fear, which occur either as an immediate response to external stimuli or are the result of an indirect subjective process, such as memory, association, or introspection.