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For any two objects with the same shape but different sizes
Surface area is proportional to x2 Volume is proportional to x3 Surface/volume is proportional to x2/X3 = 1/x The ratio is highest for small objects |
The organ systems of the animal body "make sense only when seen as solutions to particular problems presented by the relationship between the organism and its environment.
A major problem for any living system--indeed, the major problem--is that posed by the second law of thermodynamics: to maintain the high level of organization characteristic of such systems in the face of the universal trend toward disorder.
To do this, organisms need sources of energy and raw materials to maintain and operate their energy-extracting machinery.
In broad outline, the complexities of the human digestive, respiratory, and circulatory systems can be viewed as particular ways of meeting these requirements that evolved over the long period of animal evolution.
A second problem is also imposed by the laws of physics and chemistry. The molecular structures and chemical reactions characteristic of living systems can take place only within certain quite stringent limits of temperature and pH.
Elements needed in micro amounts are disruptive-- even lethal--except within a very narrow range of concentration.
Living systems are characterized by a capacity for controlling their internal environment. This characteristic is called homeostasis.
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Even a single cell, as we have seen, is capable of maintaining an internal composition distinctly different from its external environment. However, it is extremely vulnerable to changes in temperature or chemical composition of the medium in which it lives and is, of course, unable to function unless it is surrounded by liquid water. It is likely that the strongest evolutionary pressures toward larger size and multicellularity were related to the maintenance of homeostasis. |
The larger the organism, the smaller the surface-to-volume ratio.
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For any two objects with the same shape but different sizes
Surface area is proportional to x2 Volume is proportional to x3 Surface/volume is proportional to x2/X3 = 1/x The ratio is highest for small objects |
Thus larger animals can resist external change better than smaller ones. A multicellular animal maintains a tightly controlled inner environment protected from drastic changes in temperature and chemical balance and shielded from invaders.
The world in which your individual cells function and flourish is decidedly different from the world around you (but not so very different from the warm soup in which we all began).
A third problem faced by the multicellular organism is posed by multicellularity itself: coordinating the activities of each of the enormous complex of living cells, making tissues and organs responsive to the overall needs of the organism, needs that change with fluctuations in the external environment.
There are two major control systems in the animals, the
Speaking very generally, the endocrine system is responsible for changes that take place over a relatively long time period--minutes to months--whereas the nervous system is involved with more rapid responses--milliseconds to minutes.
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Although they have different structures these two control systems seen to be closely interrelated. For instance, the production of sex and other hormones was once believed to be under the control of the "master" pituitary gland; more recently it has been discovered that the pituitary, though perhaps a master in some aspects of the endocrine world, is actually an executive secretary of the hypothalamus, a major brain center. And within little more than the last decade, it has been discovered that the embryonic development of certain major brain centers is profoundly influenced by hormones regulated by the pituitary. |
Even anatomically, the endocrine and nervous systems are not distinct. One of the body's most important glands, the adrenal medulla, the source of adrenaline (also called epinephrine), is not, strictly speaking, a gland. That is, it is not modified epithelial tissue but is rather a large ganglion--a collection of nerve cell bodies--whose nerve endings secrete the hormone.
In addition to the interplay between the endocrine and nervous systems, the nervous system itself has subdivisions that interact with one another in a finely tuned system of checks and balances.
One subdivision, the somatic system, innervates skeletal muscle and is by and large under our conscious control. Another, the autonomic ("involuntary") nervous system, innervates smooth muscle, cardiac muscle, and glands.
The autonomic system is further subdivided into the sympathetic and the parasympathetic divisions. The sympathetic division is most active in times of stress or danger. (To remember which is which, it is helpful to associate sympathy with emotions.)
Among the major effectors of the sympathetic division is the adrenal medulla, and the overall effects of general sympathetic stimulation are those we associate with a "rush of adrenaline."
The parasympathetic division plays its major role in supporting everyday activities such as digestion and excretion.
The body's integration and control systems characteristically act through negative feedback loops.
The simplest example from everyday life of such a system is the thermostat that regulates your furnace. When the temperature in your house drops below the preset thermostat level, the thermostat turns the furnace on. When the temperature rises above the preset level, the thermostat turns the heat off.
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In a living organism, systems are seldom completely on or off, and homeostatic control is much more finely modulated. The principle, however, is the same. For instance, endocrine cells in the pancreas produce two hormones vital in carbohydrate metabolism: the alpha cells produce glucagon, and the beta cells produce insulin. When the glucose concentration of the blood falls, glucagon is produced, which causes release of glucose from the liver. When glucose levels in the blood rise, the beta cells release insulin, which causes uptake of glucose by body cells. Thus, in effect, glucose concentration is controlled by two separate negative feedback systems, one stimulatory and one inhibitory. |
Negative feedback loops may involve both the nervous and endocrine systems. One important homeostatic function of the body, for example, is keeping the blood volume constant. A hormone known as antidiuretic hormone (ADH), which is produced by the pituitary gland, alters the permeability of tubules of the kidney to water and so decreases water excretion. The production of ADH is controlled by sensory receptors in the circulatory system, particularly in the heart, that measure blood pressure. When blood pressure goes up, firing of these receptors inhibits the release of ADH; as blood pressure goes down, the stimulus from the receptors decreases, ADH production increases, water is retained by the kidney, and blood pressure rises.
Some negative feedback systems involve additional relay loops. The thyroid gland, for instance, produces the thyroid hormone, thyroxine, which, among other effects, steps up cellular metabolism. The amount of thyroxine produced depends on the production of another hormone, thyroid-stimulating hormone (TSH) from the pituitary gland. A major factor regulating the production of TSH is the concentration of thyroid hormone in the circulating blood. Thus, though one more step is involved, the principle is the same: the concentration of the hormone itself or the response to the hormone by a target tissue inhibits the synthesis or secretion of the hormone in question.
Many functions are controlled by a number of separate feedback control pathways. An example is shown in the diagram below, which illustrates the pathways important in temperature regulation. Core body temperature is controlled by a thermostat located in the hypothalamus, an area of the brain. The thermostat collects information from a number of thermoreceptors, integrates it, compares the results with its thermostat setting, and marshalls appropriate responses.
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Body temperature in mammals is regulated by a complex network of activities involving both the nervous and endocrine systems. The chief controlling center is the hypothalamus, a region in the brain. In some animals (although not, apparently, in humans), the hormonal pathway-indicated by dashed lines-is of major importance. TSH is thyroid-stimulating hormone, which is produced by the pituitary and which stimulates the production of thyroid hormone. Thyroid hormone increases cellular metabolism, apparently by acting directly on the mitochondria. Many behavioral responses, such as seeking sun or shelter, are also involved. |
The fourth problem an organism faces, following the dictates of its genes, is to multiply.
Reproduction may be carried out in a variety of ways, but in mammals it is always sexual and always involves formation of gametes and their union to form a zygote, or fertilized egg, and the development of the zygote into an adult.
