The term homeostasis refers to the process of maintaining stable conditions inside living organism. For single-celled organisms, this is relatively simple, since oxygen, nutrients, waste products and other materials are exchanged directly with the environment. For multicelled organism, it is a much more complicated process. A stable internal environment must be maintained, even in the face of sudden changes in external conditions. For example, the perception of cold sets in motion a sequence of events that involve 3 different hormones, and three different types of tissues: nerve cells, muscle cells, and endocrine cells. The result is the production of heat, to maintain a constant body temperature. Homeostasis involves a sensor (in this example, heat-sensing nerve cells) and an effector (which initiates a response, such as shivering muscle cells).
I. Regulation of internal environment
Respiratory gases oxygen and carbon dioxide
pH
Concentration of nutrients and waste products
Concentration of salts and other small ions (electrolytes)
Volume and pressure of extracellular fluid and blood plasma(the acellular part of the blood).
II. Feedback loops
Homeostasic control depends on feedbackloops. There are two basic types:
Positive feedback loops bring about rapid change. This is whathappens during labor and childbirth. The contraction of the uterus causes the release of the hormone oxytocin from the hypothalamus of the brain, which causes additional contractions, which in turn causes the release of more oxytocin until the fetus is expelled from the uterus.
Most of the body's homeostatic systems rely on negative feedbackloops to resist change by sensing a stimulus and then activating mechanisms that counteract the impending change. In other words, the information provided by the feedback reverses (is negative to) the direction of the response. The process of temperature regulation given at the top of the page is an example. So is calcium regualtion-- low blood calcium levels cause the parathyroid glands to release a hormone (PTH) which causes certain cells to break down bone, increasing blood calcium.
Physiological processes, then, are internal mechanisms that allow plants and animals to function under varied environmental conditions. These mechanisms involve four basic tissues (epithelial, connective, muscle, nerve), organs (comprised of various tissues) and organ systems (comprised of various organs) to achieve this end. Organ systems are sometimes controlled internally (locally), but usually they are controlled by external (extrinsic) systems.The nervous system (nerve impulses) and the endocrine system (hormones) are the major external control systems. Over the next few lectures, we will be analyzing the major organ systems of the body and how they function.
III. Basic Animal Tissues
Epithelial -- perhaps the most functionally diverse tissue,including:
All the tissue that covers or lines the external (i.e. skin) and internal surfaces of the body. The surface of the lungs, the lining of the gastro-intestinal tract, the cells lining the blood vessels, all are included in this category.
Glands, of which there are two types, classified according to where their products are secreted. Endocrine glands secrete special chemical messengers (hormones) directly into the circulatory system. Exocrine glands, including sweat glands, mammary glands, sebaceous oil glands, secrete their products directly onto the epithelial surface outside of the circulatory system.
Connective tissues function primarily in structural roles.
Connective tissue proper includes both loose connective tissue, which serves as the "packing material" or ground substance in organs and organ systems, and it also includes denser connective tissue in the form of ligaments and tendons, structures that attach bones to the skeletal system. The basic element of these tissues is a long, fibrous protein called collagen.
Cartilage is also comprised primarily of collagen, organized in an even denser arrangement than tendons. The cells responsible for secreting the collagen are buried deep inside the connective tissue matrix, isolated from nutrients and oxygen. As a consequence, cartilage and tendons are very slow to heal from trauma. It often takes many months to recover from a ruptured tendon.
Bone, in contrast, is metabolically very active and is constantly undergoing remodeling to serve its structural function. The metabolic activity is made possible through the presence of Haversian canals which allow capillaries with oxygen and nutrients to penetrate deep within the calcium matrix of the bone. Osteoclasts secrete acid and resorb bone in the remodelling process, whereas osteoblasts are continually forming new bone. Osteoporosis and weightlessness in space can both interfere with this process, resulting in skeletal systems that are greatly weakened due to excessive loss of calcium.
Blood is also classified as connective tissue, so these tissues function in ways other than exclusively structural.
Muscle tissue shares with nervous tissue the characteristic ofexcitability, which in the case of muscle tissue results in contraction. Muscle tissue can be classified into three distinct types:
Smooth muscle--called smooth because of its appearance under a microscope. These muscle cells are generally under involuntary control and include tissues found in the walls of internal structures such as the gastro-intestinal tract, blood vessels, mammary glands, uterus, etc.
Skeletal muscle--has "striations" visible under a microscope. These are the muscles, under voluntary control, that attach by tendons and ligaments to the skeletal system.
Cardiac muscle--the heart muscle also exhibits striations. These muscle cells are "self-excitatory" and will contract spontaneously, even without nerve cell stimulation.
Nervous tissue also has the characteristic of excitability, which in the case of nerve cells results in a nerve impulse. Excitability arises from an unequal distribution of charged particles around the membrane, causing the membrane to be "polar." Excitability refers to the disruptions in polarity that occur when a nerve signal is generated.
