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Biology

Chapter 25: All

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Lecture

Internal Regulation

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Mechanisms for Internal Control

All body systems function optimally in a given set of conditions. The brain and nervous system work together to monitor and adjust thermal controls and concentrations of gases, salts, nutrients, and immune system agents against constantly changing internal and external conditions as an organism eats, works, grows and ages, or becomes injured.

Osmoregulation

All terrestrial animals are constructed to regulate the amount of water in their systems, since water dissolves and carries many nutrients, ions, and salts. These molecules supply the fuel for or regulate many metabolic processes, and are often the useless (waste) result of such processes. By controlling the water level, the body controls the concentration of all chemicals dissolved in the water.

Metabolic processes such as cell growth and muscle activity produce chemical wastes; so does the respiratory process. These wastes, mostly water, carbon dioxide, and molecules containing nitrogen (usually ammonia, converted to a less harmful substance, either urea or uric acid), must be eliminated from the body. The type of nitrogenous waste depends on the animal. Reptiles and birds tend to produce uric acid; mammals and amphibians produce urea.

Simple marine animals can excrete their wastes by diffusing them directly through their outer-surface cell walls, since the concentration of salts and certain molecules inside the organism will be greater than it is outside the organism. Fresh water animals have to control their salt and ion levels more actively in order to prevent vital nutrients from leaching out.

Many invertebrates have have specialized excretory structures which process wastes before releasing them to the environment. In roundworms and flatworms, wastes pass through the nephridial organs, which filter out and retain useful compounds (glucose, salts, and water), before allowing the remaining concentrated wastes to diffuse directly through the skin. Insects have Malphigian tubes, which collect wastes from the open circulatory system blood supply and deposit them in the gut to be filtered for water and nutrients before final elimination.

Vertebrates, which have a wider range of habitats, also have more specialized structures for retaining nutrients and water from what would otherwise be simple metabolic wastes. Marine animals must replace water lost from their low-salt internal cells to the high-salt environment; they "drink" water through their gills, which filter out and excrete the salts. Animals which get much of their water from marine sources must also be able to filter out and excrete salts. Freshwater vertebrates must force water out of their high-salt cells to the lower-salt environment; many simple secrete the water or sweat. In all three cases, the amount of salt or water excreted has to be carefully controlled as the environment around the organism changes.

Feedback Mechanisms

In the human organism, feedback mechanisms allow the body to detect and digest its reaction to changes in its internal environment. One of the glands that will study later when we look at the endocrine system is the hypothalamus.The hypothalamus filters blood, and when it detects too little water, it signals the pituitary gland to secrete a hormone called ADH (antidiuretic hormone). The presence of ADH in blood signals the kidney to increase the amount of water it reabsorbed as part of the excretory function. The increase in water leads to a reduction in the amount of ADH that the hypothalamus requests from the pituitary gland.

This kind of process is called a negative feedback mechanism. A condition causes some function to begin. The end product of the function reverses the condition, and the function stops. We have seen similar mechanisms with Gene expression, such as the lac operon in bacteria. Here, we see how presence and concentration of triggering chemicals in the blood can trigger the release of hormones that affect the concentration of the triggering chemicals.

For an example of how the pancreas controls the sugar level in the bloodstream by negative feedback, take a look at the How stuff works site on homeostasis.

Water conservation

All terrestrial animals are constructed to regulate the amount of water in their systems, since water dissolves and carries many nutrients, ions, and salts. These molecules supply the fuel for or regulate many metabolic processes, and are often the useless (waste) result of such processes. By controlling the water level, the body controls the concentration of all chemicals dissolved in the water.

Metabolic processes such as cell growth and muscle activity produce chemical wastes; so does the respiratory process. These wastes, mostly water, carbon dioxide, and molecules containing nitrogen (usually ammonia, converted to a less harmful substance, either urea or uric acid), must be eliminated from the body. The type of nitrogenous waste depends on the animal. Reptiles and birds tend to produce uric acid; mammals and amphibians produce urea.

Simple marine animals can excrete their wastes by diffusing them directly through their outer-surface cell walls, since the concentration of salts and certain molecules inside the organism will be greater than it is outside the organism. Fresh water animals have to control their salt and ion levels more actively in order to prevent vital nutrients from leaching out.

Many invertebrates have have specialized excretory structures which process wastes before releasing them to the environment. In roundworms and flatworms, wastes pass through the nephridial organs, which filter out and retain useful compounds (glucose, salts, and water), before allowing the remaining concentrated wastes to diffuse directly through the skin. Insects have Malphigian tubes, which collect wastes from the open circulatory system blood supply and deposit them in the gut to be filtered for water and nutrients before final elimination.

Vertebrates, which have a wider range of habitats, also have more specialized structures for retaining nutrients and water from what would otherwise be simple metabolic wastes. Marine animals must replace water lost from their low-salt internal cells to the high-salt environment; they "drink" water through their gills, which filter out and excrete the salts. Animals which get much of their water from marine sources must also be able to filter out and excrete salts. Freshwater vertebrates must force water out of their high-salt cells to the lower-salt environment; many simple secrete the water or sweat. In all three cases, the amount of salt or water excreted has to be carefully controlled as the environment around the organism changes.

Kidney Function

The main organ for water redemption and nitrogenous waste processing in mammals is the kidney; together with the urinary bladder (and their ducts), it makes up the urinary system. Water containing nitrogenous wastes (urine) flows into the urne through the renal cortex, or surface, and into the renal medula, where it is collected in tubes and deposited in a small opening called a papilla near the center of the kidney. From there, it flows into the central chamber or renal pelvis, which dumps its contents out of the kidney via the ureter tubes to the urinary bladder. This is the organ that feels "full" and makes you want to hunt up the nearest WC. The opening of the urinary bladder is under learned voluntary control; when a human releases the sphincter control, the urine flows out the urethra and out of the body.

The actual tube structure in the renal cortex is quite complex. Urine flows through various tubes before reaching the papilla, during which small molecules are filtered out, filtered nutrients are reabsorbed, and the certain specific wastes are actively secreted. First, the urine solution may pass by a glomerulus, or collection of blood vessels, where some of the plasma in the blood is filtered into the kidney tubules through a collecting structure called a Bowman's capsule. Only certain small molecules, such as glucose, salts, urea, and various ions, make it through the membranes into the urine solution. The solution continues through loops of renal tubule, which has an interior surface that is highly efficient in reabsorbing useful nutrients and passing them back into another set of blood capillaries. Some molecules, such as potassium ions, are actively "pushed" from the blood supply through the renal walls into the urine solution. By the time the filtrate reaches the main collection area of the renal pelvis, it is highly concentrated with wastes.

Various hormones control the actual amounts of water and ions retained or eliminated as wastes. How much water is secreted depends on the antidiuretic hormone ADH which is released when the body is dehydrated, so that more water is absorbed through the renal tubules. ADH also controls the sensation of thirst. A set of hormones control the secretion and absorption of sodium.

Temperature Control

Animals regulate their internal temperatures to maximize metabolic processes. Ectotherms (cold-blooded animals) get their heat from their environment as well as from the exothermic metabolic processes within their bodies; their body temperatures fluctuate with their surroundings. Reptiles will bask in the sun, and many insects must utilize sun and shade areas to achieve body temperatures in acceptable ranges. Endotherms (warm-blooded animals, which includes mammals), retain a constant body temperature despite their surroundings. They need to use various mechanisms (increased movement, shivering, sweating) to control body temperature in response to heat or cold in the environment.