The human digestive system

The system used in the human body for the process of digestion. The human digestive system consists primarily of the digestive tract, or the series of structures and organs through whichfood and liquids pass during their processing into forms absorbable into the bloodstream. The system also consists of the structures through which wastes pass in the process of elimination and other organs that contribute juices necessary for the digestive process.

The digestive tract begins at the lips and ends at the anus. It consists of the mouth, or oral cavity, with its teeth, for grinding the food, and its tongue, which serves to knead food and mix it withsaliva; the throat, or pharynx; the esophagus; the stomach; the small intestine, consisting of theduodenum, the jejunum, and

the ileum; and the large intestine, consisting of the cecum, a closed-end sac connecting with the ileum, the ascending colon, the transverse colon, the descending colon, and the sigmoid colon, which terminates in the rectum. Glands contributing digestive juices include the salivary glands, the gastric glands in the stomach lining, the pancreas, and the liver and its adjuncts—the gallbladder and bile ducts. All of these organs and glands contribute to the physical and chemical

breaking down of ingested food and to the eventual elimination of nondigestible wastes. Their structures and functions are described step by step in this section.

The tissues

A tissue are a group of cells with a similar structure that do the same type of activity. Tissues are present in complex animal and plants. Sponges, jellyfish, algae, fungi do not have tissues.

ANIMAL TISSUES.
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covering>>Epithelial >>Protection>> Skin, blood vessels,interior of organs.

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Glandular>> Secretion>> Glands, glandular epithelium of theintestine.
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Adipose>>Reserve/insulation>> All the body
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Cartilaginous>> Skeleton >>ears, inter-vertebrate disc, tracheal rings...

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Bone Skeleton Skeleton
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Blood>> Transport>> All the body
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Muscular >>Smooth>>Movement>>Internal organs
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Striated>> Movement>> Skeletal Muscles
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Cardiac>> Movement>> Heart
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Nerve>> Information/responses>> Nervous system
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Respiratory system

A respiratory system's function is to allow gas exchange. The space between the alveoli and the capillaries, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on the organism. Inhumans and other mammals, for example, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs.

Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skinplays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leavesknown as stomata.

The nervous system

The nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells,(plural form glia), that aid in the function of the neurons. The nervous system is divided broadly into two categories: the peripheral nervous system and the central nervous system. Neurons generate and conduct impulses between and within the two systems. The peripheral nervous system is composed of sensory neurons and the neurons that connect them to the nerve cord, spinal cord and brain, which make up the central nervous system. In response to stimuli, sensory neurons generate and propagate signals to the central nervous system which then processes and conducts signals back to the muscles and glands. The neurons of the nervous systems of animals are interconnected in complex arrangements and use electrochemical signals andneurotransmitters to transmit impulses from one neuron to the next. The interaction of the different neurons form neural circuits regulate an organism's perception of the world and what is going on with its body, thus regulating its behavior. Nervous systems are found in many multicellular animals but differ greatly in complexity between species.

The human senses

The human body is a beautifully engineered structure. The body has a general plan. We all have arms, legs, a head, ears, and eyes on the outside of our bodies. We also have similar parts on the inside of our bodies. We all have one heart, two lungs, one brain, two kidneys, and one stomach.

Understanding and learning about the human body is a complicated subject. If you just take an organ or system and emphasize its function, you must tell students how the system fits into the rest of the body's functions.

You can introduce the parts of the body, both internal and external, by using the organs that allow humans to operate their senses. Thenervous system controls the actions and sensations of all the parts of your body by using a complex network of nerves, which carry electrical signals to and from the brain. The brain and spinal cord compose the central nervous system, acting as the central clearing house. One part of your nervous system controls your body's relationship to the external environment (somatic) and another part controls your body's internal organs (autonomic). The human body has five major senses which operate to gather information from the world around us, sight, hearing, smell, taste, and touch. Any stimulus to one of the sense areas is detected by sensory nerves and is sent to the brain for interpretation.

The eye

The eye is basically a mechanism for receiving light and transforming it into electrical energy. The receptors in the eye transform it into electrical signals, which are passed on to the brain. The eye has a lot of components. The cornea and lens at the front of the eye focus the light into a sharp image on the back of the eye, the retina. The retina is made up of rods and cones.

Rods are highly sensitive to light and allow us to see under low levels of illumination. However, they are unable to resolve fine detail and are subject to saturation. This is the reason for the temporary blindness that is caused when we move from a dark room into sunlight. There are approximately 120 million rods per eye which are mainly situated towards the edges of the retina.

Cones are less sensitive to light than rods and can tolerate more light. There are three types of cones, each of which is sensitive to a different wavelength of light. These cones which number about 6 million are mainly concentrated on the fovea, a small area of the retina on which images are formed.

The human ear

The human ear has three main sections, which consist of the outer ear, the middle ear, and the inner ear. Sound waves enter your outer ear and travel through your ear canal to the middle ear. The ear canal channels the waves to your eardrum, a thin, sensitive membrane stretched tightly over the entrance to your middle ear. The waves cause your eardrum to vibrate. It passes these vibrations on to the hammer, one of three tiny bones in your ear. The hammer vibrating causes the anvil, the small bone touching the hammer, to vibrate. The anvil passes these vibrations to the stirrup, another small bone which touches the anvil. From the stirrup, the vibrations pass into the inner ear. The stirrup touches a liquid filled sack and the vibrations travel into the cochlea, which is shaped like a shell. Inside the cochlea, there are hundreds of special cells attached to nerve fibers, which can transmit information to the brain. The brain processes the information from the ear and lets us distinguish between different types of sounds.