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Chapter 19

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Biology 1225
Michael Butler

Optional Biology Notes. Dr. M. J. Butler. 1 Chapter 19: Animal Tissues and Organs (20) Sections 19.1, 19.6 will not be tested This chapter introduces animal anatomy and physiology. Note that the feedback mechanisms that control body parameters such as temperature, blood sugar levels etc are negative feedback mechanisms, they act to keep parameters within accepted limits. BUT, there is also such a thing as a positive feedback mechanism, and this acts to drive a parameter ever "higher". There are very few of these in the human body, an example is the positive feedback mechanism that drives ever increasing contractions of the uterus during childbirth, these only cease when the child is born and the pressure on the uterine wall is relieved and this signals contractions to stop. Positive feedback mechanisms are NOT homeostatic! The nervous and the endocrine systems are the two systems that are largely in control of homeostasis in the human body. Animal Tissues, Organs, and Homeostasis A tissue is a group of cells that assemble and interact to perform a specific function. An organ is a number of different tissues assembled together to perform a specific function. Examples are hearts, lungs, liver. Organ systems consist of a number of different organs organized to perform a specific function. Examples are the skin and the circulatory system. There are four types of animal tissue: epithelial, connective, muscle, nervous Epithelial tissues form surfaces, external and internal, they are underlain by connective tissue called basement membrane. Epithelial cells are generally flattened or cube shaped. Epithelium forms sheets of cells that provide covering surfaces inside and on the surface of the body (the skin). These include the lining of the lungs, digestive tract, blood vessels, and the reproductive and urinary tracts Tight junctions between epithelial cells prevent fluids leaking from cell to cell in tissues because they seal adjacent cells together. Tight junctions are found in stomach epithelium where they prevent acid leakage. Gap junctions between epithelial cells are protein complexes that form pores that permits flow of cytoplasm between cells (as in Optional Biology Notes. Dr. M. J. Butler. 2 cardiac cells). Adhering junctions of epithelial cells are protein “spot welds” that hold cells together (as in skin). NB, be careful of the careless terminology used in some textbooks with regards to cell to cell junctions, they often write of making seals between adjacent cell walls, but animal cells do not have walls. Glands are derived from epithelium, there are two kinds, exocrine which release substances such as mucus, tears, saliva, oil, milk etc through ducts, and endocrine glands which release hormones directly from their surfaces into the body fluids - and do not possess ducts. Connective tissues, with the exception of blood, connective tissues are generally produced by fibroblast cells that produce fibrous connective proteins called collagen and elastin. These proteins are generally embedded in secreted polysaccharides that form the ground tissue in which the fibres and cells are embedded. Tendons and elastic ligaments are connective tissues. Special connective tissues include bone, fat (so called adipose tissue), and blood. Muscle and Nervous tissues are dealt with in detail in later chapters. Specific Organ systems are discussed in detail in later, but note here that there are a number of body cavities that contain specific organ systems. The skin is an example of an organ system; it is the largest part of the integument (body covering). Skin is a complex layered multifunctional organ system which functions in protection against the sun, desiccation, entry of infectious agents, maintenance of body temperature by means of sweat glands and nervous control of blood flow in small vessels near the skin. The outermost region of the skin is the epidermis - consisting mostly of dead flattened keratinocyte cells that provide waterproofing. The underlying dermis region is live and is mostly dense connective tissue and supports the epidermal layer as well as containing nerve endings. Sweat and oil glands are of epidermal origin but are embedded in the dermis. Homeostasis. This is a KEY physiological concept. Homeostasis can be defined as the maintenance of a constant internal physiological environment by a cell and/or an organism. Homeostasis is a dynamic process that involves the interrelationship of numerous factors to maintain a given biological process within appropriate narrow limits. The nervous system and the hormonal system are the two key systems that control homeostasis, and it is the mechanism of negative feedback that operates to accomplish this control. These systems are covered in other chapters. Optional Biology Notes. Dr. M. J. Butler. 3 Feedback mechanisms maintain physiological status. Most physiological processes that require maintenance of a narrow range of some aspect, such as body fluid pH, temperature, ionic strength, etc are controlled by negative feedback mechanisms undertaken by hormones and the nervous system. In negative feedback a parameter under control is perceived by a sensor to have increased or decreased beyond a given optimal range, as soon as this is perceived the negative feedback mechanism acts to bring that property back within the acceptable range by decreasing and reversing the direction of the change. The classic example is body temperature. If the body senses that you are too hot you begin to sweat, evaporation of the sweat on the skin causes you to cool down to the acceptable range of body temperature, once this happens the body senses a normal body temperature and stops the sweating process. Thus the body has acted to decrease the change in the property that it sensed, this is negative feedback. If the body senses it is too cold it causes shivering to occur, this requires metabolic activity that produces heat (shivering of muscles), once the body senses that enough heat has been produced to bring back the body to the acceptable temperature range, shivering stops. The body reacted to a change in the property it sensed and reversed the direction of that change - thus negative feedback has occurred. There is also positive feedback, but this is not homeostatic, it drives a certain parameter out of a normal range - it accelerates the change, the rapidly increasing uterine contraction rate during childbirth is an example; release of oxytocin from the pituitary causes uterine contractions, signals generated by those contractions go to the brain which causes more oxytocin to be released, this means more contractions which in turn mean even stronger signals to go to the brain from the uterus, to cause even more oxytocin to be released and so on, the effect only ceases when the ba
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