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Objective: A general overview on usual mechanisms that cattle use in maintaining homeostasis. What is homeostasis? It's simply the tendency of an organism or cell to maintain internal equilibrium by readjusting its physiological activities whenever it's facing a number of stress factors. Animal bodies continuously go through dynamic state of equilibrium , trying to reach a balance in which internal changes continuously attempt to compensate for external changes in a feedback control process to sustain relatively uniform physiological conditions. The significance of this concept can't be over-stressed, as it allows enzymes etc to be 'fine-tuned' to a particular set of conditions, and so to operate more efficiently. Much of the hormone system and autonomic nervous systems is dedicated to homeostasis, and their action is coordinated by the hypothalamus. Full body homeostasis is maintained by two mechanisms: thermoregulation and osmoregulation. Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. This process is one aspect of homeostasis Cattle (bovine) usually try to keep their rectal temperature between 37.5oC and 38.5oC. This is usually accomplished by varying the rates of heat production and heat loss through certain mechanisms. The other mechanism that ensures homeostasis is called Osmoregulation.Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; meaning it keeps the organism's fluids from becoming too diluted or too concentrated. Part of the osmoregulation process is to maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion of metabolic wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood via organs such as the skin and the kidneys. As a conclusion, keeping the amount of water and dissolved solutes in balance is referred to as osmoregulation. As a summary, maintaining full body homeostasis requires the careful coordinated activity of thermoregulation and osmoregulation.
All homeostatic mechanisms utilize negative feedback to retain a constant value (called the set point). This is the most important point in this topic! Negative feedback signifies that whenever a change arises in a system, this automatically causes a corrective mechanism to initiate, which reverses the original change and brings back system towards the set point (i.e. 'normal'). It also means that the larger the change the larger the corrective mechanism. Negative feedback equally pertains to central heating systems as well as to biological systems. When your oven overheats, the heating turns off; this allows the oven to cool down. Eventually it will get too cold, when the heating will switch back in, so raising the temperature once again.
So, in a system regulated by the negative feedback, the set level is never flawlessly maintained, but constantly fluctuates about the set point. A well-organized homeostatic system reduces the size of the oscillations as much as possible. Some variation must be permitted, however, or both corrective mechanisms would try to operate at once! This is particularly true in hormone-controlled homeostatic mechanisms (and most are), where there is a significant time-lag before the corrective mechanism can be activated.
One of the most significant examples of homeostasis is the "regulation of body temperature". Not all animals are physiologically capable of performing it. Animals that maintain a reasonably stable body temperature (birds and mammals) are called endotherms, whereas those that have an inconsistent body temperature (all others) are called ectotherms. Endotherms generally maintain their body temperatures at around 35 - 40Â°C, so are occasionally called warm-blooded animals, however ectothermic animals can as well have very warm blood during the day by relaxing under the sun, or by extended muscle activity 9e.g. bumble bees, tuna). The reason that these two groups are different is that endothermic animals use internal corrective mechanisms, yet ectotherms employ behavioral mechanisms (e.g. lying in the sun in cold weather moving into shade in hot weather). Such mechanisms can be incredibly effective, particularly when coupled with internal mechanisms to guarantee that the temperature of the blood going to vital organs (brain, heart) is kept constant. We use both!
In humans (being mammals themselves), body temperature is influenced by the thermoregulatory centre in the hypothalamus. It obtains input from two groups of thermoreceptors: receptors in the hypothalamus itself monitor the blood's temperature as it passes through the brain (the core temperature), and receptors in the skin watch out for the external environmental temperature. Both sets of information are required so that the body can make suitable adjustments. From the thermoregulatory center, several impulses are sent to numerous effectors to adjust body temperature:
The animal's preliminary response to encountering hotter or colder condition is voluntary - if too hot, it may decide to move into the shade; if too cold, it searches for the warmest place it can find! It is only when these responses are not sufficient that the thermoregulatory centre is stimulated. This is part of the autonomic nervous system, so the different responses are all involuntary.
When the animal gets too hot, the heat loss centre in the hypothalamus is stimulated; when it gets too cold, it is the heat conservation centre of the hypothalamus which is activated.
Notice that some of the responses to low temperature actually produce heat (thermogenesis), while others just conserve heat. Equally true some of the responses to cold actively cool the body down, while others just diminish heat production or transfer heat to the surface. The body thus has a range of responses available, depending on the internal and external temperatures.
First, smooth muscles in arterioles in the skin behave differently under different temperatures. In response to cold weather, muscles contract causing vasoconstriction. A reduced amount of heat is carried from the core to the surface of the body, preserving core temperature. Extremities can be damaged due to decreased blood flow (heat transfer).Yet in response to hot weather, muscles relax causing vasodilation. Additional heat is carried from the core to the surface, where it is lost by convection, radiation and to a lesser extent by conduction (when in water). Skin under the hair/fur coat turns red.
Second, sweat glands behave differently under different temperatures. Under cold conditions, hardly any sweat is produced. Under hot conditions however, the sweat glands secrete sweat onto surface of skin, where it evaporates. Given that water has a high latent heat of evaporation; it takes heat from the body. High humidity and dense/thick animal hair/fur coat reduces the ability of the sweat to evaporate and so makes the animal uncomfortable in hot weather.
Third, erector pili muscles in the skin (attached to skin hairs) act differently under various temperatures. Under cold climates, the muscles contract, raising skin hairs and conserving heat by trapping an insulating layer of still, warm air next to the skin. This is an effective mechanism in hairy/furry animals. But under warm conditions, the muscles relax, lowering the skin hairs and allowing air to circulate over the skin, encouraging heat loss convection and evaporation.
Fourth, Skeletal muscles behave differently under different temperatures. Under cold climate, it is observes that animals are seen sometimes shivering ; indicating that their Muscles are contracting and relaxing repeatedly, generating heat by friction and from metabolic reactions (respiration is merely 40% efficient: 60% of increased respiration hence generates heat).
Fifth, adrenal and thyroid glands activities vary according to different temperatures. Under cold conditions, the two glands secrete adrenaline and thyroxine hormones respectively, which boost the metabolic rate in different tissues, especially the liver, thus generating higher levels of body heat. Yet, under warm conditions, the two glands stop secreting adrenaline and thyroxine because the animal body already contains more heat that it can handle.
Finally, we observe different animal behavior under different climatic conditions. Under cold conditions, the animals usually coil up around themselves, cluster into groups, search shelter, grow denser body hair or fur, and increase the fat layer surrounding their bodies to insulate them from the cold .But in warm weather, the animals usually try to Stretch out their bodies, find a cool shaded area, cover themselves in water or mud to release some the built-in heat.
We have talked about the general physiological changes that animals go through due being exposed to temperatures outside their comfort zone, now we should talk about those that affect the animal's economic profitability, as in domesticated cattle. It has been observed over and over again that significant deviation from an animal's comfort zone has detrimental effects .In livestock, when large variations occur in the animal's rectum temperature( mostly observed in hot climates), it negatively affects its feed intake, growth rate, milk yield(in case of cows), reproductive functions and fertility( a decrease in semen production in bulls , and decreased rate of conception and embryonic development in cows).The level of these negative effects vary between breeds of the same species due different thermotolerance capabilities embedded in the genetic make up of each breed.
Now, with respect to the osmoregulation, there are three main types of osmoregulatory environments in which animals live: freshwater, marine, and terrestrial. Animals whose internal osmotic concentration is identical to the surrounding environment are regarded as osmoconformers, while those that maintain an osmotic difference between their body fluid and the surrounding environment are osmoregulators.Â Freshwater animals (all osmoregulators) include invertebrates, fishes, amphibians, reptiles, and mammals. The freshwater animals are generally hyperosmotic to their environment. The problems that they face because of this are that they are subject to swelling by movement of water into their bodies due to the osmotic gradient, and they are subject to the continual loss of body salts to the surrounding environment (which has a low salt content). The way these animals deal with these problems is to produce a large volume of dilute urine. The kidney absorbs the salts that are needed, and the rest of the water is excreted. Another way these animals deal with lack of salt is by obtaining it from the food they ingest. A key salt replacement mechanism for freshwater animals is active transport of salt from the external dilute medium across the epithelium into the interstitial fluid and blood. Amphibian's skin and fish gills are active in this process. Freshwater animals tend to take in water passively and to remove it actively through osmotic work of kidneys (in vertebrates) or kidney-like organs (in invertebrates).
Air breathing animals are subject to dehydration through their respiratory epithelia. Humans and most other air-breathing animals require a constant source of fresh drinking water to excrete accumulated salts and metabolic waste products.
Water regulation and temperature regulation are closely related. Animals living in harsh heat environments such as deserts have to compensate for the lack of water. The kangaroo rat avoids the daytime heat, and emerges late at night. These rats like other desert mammals have efficient kidneys, and excrete highly concentrated urine. 90% of the water that they use is called metabolic water and is the major source for all desert animals. Metabolic water is derived from cellular oxidation. Camels have a different way of dealing with the unforgiving heat and lack of water in the desert. They are too large to hide in a hole, so when deprived of drinking water they allow their body temperatures to rise. In doing this they limit the amount of water lost by evaporation/perspiration. At night the animals' body temperature can stay at 35 degrees Celsius and during the day rise to over 41 degrees Celsius. It too produces concentrated urine and dry feces. When water sources are too limited the camel will not produce urine but will store the urea in the tissues. This is particularly unusual because along with tolerating dehydration it can deal with high urea levels in its body. An interesting fact is that when water becomes available they will consume up to 80 liters in 10 minutes.
There are different sources of water gain or loss in animals. Animals acquire most of their water in food, drink and a smaller amount by oxidative metabolism. Animals lose water by urinating, defecating, and by evaporative loss due to sweating and breathing. For aquatic animals, evaporation is unimportant, but these animals experience the uptake and loss of water across the body surface by osmosis. Animals that are protected by a covering that stops water loss and gain have specialized epithelia which are not waterproof, that must be exposed to the environment in order to exchange gases. Examples of these epithelia occur in gills, lungs, and tracheae. The nasal passages of mammals play an important role in reducing water loss through this pathway. Respiratory surfaces are the major source of water loss in air-breathing animals. The internalization of the respiratory surfaces in a body cavity such as the lungs reduces evaporative loss in terrestrial vertebrates. Because the body temperature of birds and mammals is generally higher than external temperatures, evaporative loss of water is greater. Warm expired air contains more water than the cooler inspired air, as the water holding capacity of air increases with temperature. A mechanism, termed a temporal countercurrent system retains most of the respiratory water vapor by condensing it on cooled nasal passages during expiration. The nasal passages of mammals play an important role in reducing the loss of water and heat from the body. The importance of the nasal passages in cooling expired air can be detected easily by placing your hand in front of your nose when breathing, and comparing this to putting your hand in front of your mouth when breathing.
With regard to osmoregulation, animals face several major problems. In most animals, the majority of cells are not in direct contact with the external environment but are bathed by an internal body fluid. Homeostatic mechanisms hamper changes in an animal's body fluid, which both gives protection from harmful external environments and impedes quick exchange between intracellular compartments. The cells of the animal cannot survive much additional water gain or loss. Water continuously enters and leaves an animal cell across the plasma membrane; however, uptake and loss must balance. Animal cells swell and burst if there is a net uptake of water or shrivel and die it there is a net loss of water. Additional troubles related to osmoregulation are body and environment temperatures. The enzyme activity in the body functions between temperatures of 0o - 40o Celsius. The way animals deal with temperature and regulating it is by way of water loss. So animals in hot environments need to limit the amount of water loss due to evaporation and respiration. The importance of water in temperature regulating leads to conflicts and compromises between physiological adaptations to environmental temperatures and osmotic stresses in terrestrial animals.Â
Waste products generated in metabolic processes are often toxic, and therefore must be eliminated before they can harm the organism. The major metabolic wastes produced by animals include carbon dioxide, metabolic water, and nitrogenous wastes. Small aquatic organisms are able to get rid of wastes by simple diffusion across membranes. More complex animals with circulatory systems rely on kidneys to filter wastes out of the blood and eliminate them from the body.
Carbon dioxide and metabolic water produced in respiration simply diffuse into the environment from respiratory surfaces. Nitrogenous waste excretion is more difficult, yet necessary. Elevated ammonia levels in the body can lead to convulsions, coma, and even death. This is because ammonium ions can substitute for potassium ions in ion-exchange mechanisms. Ammonia can also adversely affect metabolism and amino acid transport. Excessive amounts of ammonia in the system elevate bodily pH, which causes changes in the tertiary structure of proteins, and thus cellular functions can be altered.
There are three main types of nitrogenous wastes: ammonia, urea, and uric acid. The type of waste an animal excretes depends on its living environment, because nitrogenous waste excretion is accompanied by a certain amount of water loss. Ammonotelic (ammonia-excreting) animals generally live only in aquatic habitats, because ammonia is extremely toxic, and a large volume of water is required to maintain the excreted ammonia level lower than the body level. This is needed because ammonia excretion relies on passive diffusion, so a gradient is required between the organism and the environment in order for the ammonia to flow from high concentration to low concentration.
Whereas most excretion of ammonia occurs across the gills of aquatic animals, mammals do excrete some ammonia in the urine. Amino groups are enzymatically transformed into glutamate, and then changed to glutamine in the liver. Glutamine can cross the kidney membranes (whereas amino acids can not). In the kidney tubules, the glutamine is deaminated to ammonia and then excreted in the urine.
Although ammonia excretion is present in some forms in mammals, the major nitrogenous waste excreted is urea. Urea is less toxic than ammonia, and requires less water for elimination. Therefore, ureotelic (urea-excreting) animals are most often (but not exclusively) terrestrial. A downside to urea excretion is that urea synthesis requires energy, in the form of ATP. Vertebrates synthesize urea in the liver using the ornithine-urea cycle. Teleosts and invertebrates produce urea from uric acid via the uricolytic pathway.
Birds, reptiles, and most terrestrial arthropods often are subject to very limited water availability, so even urea excretion is not possible. Therefore, these uricotelic (uric acid-excreting) animals synthesize uric acid, which requires even less water than urea for elimination.Â The ability to produce uric acid, which is relatively insoluble, is quite important to birds and reptiles prior to hatching.Â Nitrogenous wastes can be safely stored within the egg in the form of uric acid, whereas a build-up of either ammonia or urea would be deadly.Â
In conclusion, for any animal body to attain homeostasis, different physiobiochemical processes relating to thermoregulation and osmoregulation must work hand in hand, because even slight synchronization between the two can pose risk to animal health.