The Respiratory System is responsible for supplying oxygen to the blood and expelling waste gases, of which carbon dioxide is the primary constituent, from the body. The upper structures of the respiratory system are combined with the sensory organs of smell and taste (in the nasal cavity and the mouth) and the digestive system (from the oral cavity to the pharynx). At the pharynx, the specialized respiratory organs diverge into the airway. The larynx, or voice box, is located at the head of the trachea, or windpipe. The trachea extends down to the bronchi which branch off at the tracheal bifurcation to enter the hilus of the left or right lung. The lungs contain the narrower passageways, or bronchioles, which carry air to the functional unit of the lungs, the alveoli. There, in the thousands of tiny alveolar chambers, oxygen is transferred through the membrane of the alveolar walls to the blood cells in the capillaries within. Likewise, waste gases diffuse out of the blood cells into the air in the alveoli, to be expelled upon exhalation. The Diaphragm, a large, thin muscle below the lungs, and the intercostals and abdominal muscles are responsible for contracting and expanding the thoracic cavity to effect respiration. The ribs serve as a structural support for the whole thoracic arrangement, and pleural membranes help provide lubrication for the respiratory organs so that they are not chafed during respiration.

The Digestive System is responsible for processing food, breaking it down into usable proteins, carbohydrates, minerals, fats, and other substances, and introducing these into the bloodstream so that the body can use them. The digestive, or alimentary, tract begins at the mouth, where the teeth and tongue begin the breakdown of food, aided by saliva secreted by the salivary glands. The chewed food, combined with saliva, is swallowed, carrying it in peristaltic (contractile) waves down the esophagus to the stomach. In the stomach, the food combines with hydrochloric acid, which further assists in breaking it down. When the food is thoroughly digested, the fluid remaining, called chyme, is passed through the pylorus sphincter to the small intestine and large intestines. Within the long, convoluted intestinal canals, the nutrients are absorbed from the chyme into the bloodstream, leaving the unusable residue. This residue passes through the colon (where most of the water is absorbed into the bloodstream) and into the rectum where it is stored prior to excretion. This solid waste, called feces, is compacted together and, upon excretion, passes through the anal canal and the anus. Along the way through the digestive tract, the pancreas, spleen, liver, and gallbladder secrete enzymes, which aid in the digestive process.

Sensory Organs: The integument system is the name given for the skin, hair, nails, and glands covering the body. It also includes the eyes, the ears, the nose, and the mouth, all of which are a part of the body's sensory system. The world is perceived by means of coded messages (electrical impulses) sent to the brain by the sensory organs. Our perception is predominantly developed by means of the pattern of sound pressure entering the ears and the pattern of light entering the eyes. The senses of touch, taste, and smell, however, are also important to our perception of the world around us.

The Ear is divided into three parts: the outer ear, the middle ear, and the inner ear. Each section performs its own separate function in a process that converts sound waves into nerve impulses, which are then transmitted to the brain. The outer ear has two parts; the pinna and the external auditory canal. The outer ear collects and channels sound. The middle ear, or tympanic cavity, is a tiny cavity hollowed out of the temporal bone. It is an intermediary in the processing of sound energy. It is responsible for increasing the intensity of incoming sound waves and transforming them into mechanical vibrations that can easily travel through the inner ear. The inner ear has two parts. One is made of bone, the other of a membrane that lies inside the bone. Both have complicated shapes, and for this reason they are called labyrinths. Each labyrinth has three parts: vestibule, semicircular canals, and cochlea. The inner ear contains the receptor cells, which receive the mechanical vibrations and transmit them to the brain.


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