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Levels of Structural Organization: The human body contains more than 100 trillion cells and more than 100 kinds of cells. The body is structurally organized into four levels: 1. Cells 2. Tissues 3. Organs 4. Organ systems

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Recall that a tissue is a group of similar cells that work together to perform a common function; such as muscle tissue. The cell types of the body are grouped into four basic kinds of tissues 1. Epithelial 2. Nervous 3. Connective 4. Muscle tissues

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Epithelial Tissue: There are many different types of epithelial tissue. Epithelial tissue lines most body surfaces and it protects other tissues from dehydration and physical damage. An epithelial tissue is usually no more than a few cells thick. These cells are usually flat and thin, and they contain only a small amount of cytoplasm. Epithelial tissue is constantly being replaced as cells die.

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N ervous tissue: The nervous system is made of nervous tissue. Nervous tissue consists of nerve cells and their supporting cells. Nervous cells carry information throughout the body.

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C onnective Tissue: Various types of connective tissue support, protect, and insulate the body. Connective tissue includes fat, cartilage, bone, tendons and blood. Some connective tissues, such as those found in bone, are densely packed. Others, such as those found in blood, are very far apart from one another.

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Three kinds of muscle tissues enable the movement of body structures by muscle contraction. The three kinds of muscle tissues are 1. Skeletal muscle 2. Smooth muscle 3. Cardiac muscle

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S keletal Muscle: Skeletal muscle is called voluntary muscle because you can consciously control its contractions. Skeletal muscles move bones in the trunks and limbs

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S mooth Muscle: Smooth muscle is called involuntary muscle because you can not consciously control its slow long- lasting contractions. Smooth muscles line the walls of blood vessels and hollow organs. Some contract only when stimulated by signal molecules, others contract spontaneously.

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C ardiac Muscle: Cardiac muscle is involuntary and is found in the heart. The powerful, rhythmic contractions of cardiac muscle pump blood to all body tissues. Groups of cardiac cells contract all at once, stimulating adjacent groups of cells to contract.

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Stem cells: Every human life starts as a single fertilized egg, which rapidly divides into a small cluster of cells. After about 5 days, a small ball of a hundred cells is formed, which encloses a mass of embryonic stem cells. These early, undifferentiated cells will give rise to all of the types of cells of the developing body. Embryonic stem cells are immortal- in that they divide indefinitely. Embryonic stem cells are not yet specialized cells. Indeed, any embryonic stem cell is capable of becoming any type of tissue found in an adult body.

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Because stem cells can develop into any tissue type, embryonic stem cells offer the possibility of repairing damaged tissues. Stem cell therapy in mice has been shown to repair heart muscle and to produce functional nerve cells in the brain. The use of human embryonic stem cells is controversial. Because obtaining embryonic stem cells destroys an early embryo, therapeutic use of stem cells raises serious ethical issues.

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Stem cells: Adults also have stem cells. Stem cells in bone marrow produce different types of blood cells. The adult brain contains stem cells that develop into raw nerve cells. Adult stem cells are not as versatile as embryonic stem cells and they are not immortal. Most stop reproducing after fewer than 100 cell divisions. Scientists are now at work on several therapeutic applications of adult stem cells.

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Organ Systems: Body organs are made of combinations of two or more types of tissues working together to perform a specific function. The heart, for example, contains cardiac muscle tissue and connective tissue, and the heart is stimulated by nervous tissue. Each organ belongs to at least one organ system, which is a group of organs that work together to carry out major activities or processes.

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The different organs in an organ system interact to perform a certain function. For example, the digestive system is composed of the mouth, throat, esophagus, stomach, intestines, liver, gallbladder, and pancreas. Some organs function in more than one organ system. The pancreas, for example, functions in both the digestive system and the endocrine system.

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The body contains four large fluid- filled spaces, or body cavities, that house and protect the major internal organs. Within the body cavities organs are suspended in fluid that supports their weight and prevents them from being deformed by body movements.

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T hese organs are also protected by bones and muscles. For example, your heart and lungs are protected by the rib cage and sternum inside the thoracic cavity. Your brain, encased within the cranial cavity, is also protected by the skull. Your digestive organs, located in your abdominal cavity, are protected by the pelvis and abdominal muscles.

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Endothermy: Like all mammals, humans are endotherms. Humans maintain a fairly constant internal temperature of about 37 degrees Celsius (98.6F) Your body uses a great deal of energy to maintain a stable internal temperature. For example, a great deal of the energy you consume in food is devoted to maintaining your body temperature. You would not survive very long f your body fell below the normal range. Very high temperatures, such as those associated with fevers, are also dangerous because they can inactivate critical enzymes.

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Your body maintains a constant temperature due to the flow of blood through blood vessels under the skin. To release heat to the air, blood flow is increased to these vessels. To retain heat, blood flow is shunted away from the skin. As an endotherm, you can remain active at external temperatures that would slow the activity of ectotherms. Endothermy enables you to maintain strenuous activity such as exercise for a long time period.

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To maintain homeostasis, the bodys organ system must function smoothly together. The nervous system and the endocrine system operate on negative feedback with other organ systems. This promotes stability throughout the body. In addition to temperature regulation, homeostasis involves adjusting metabolism, detecting and responding to environmental stimuli, and maintaining water and mineral balances.

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