Topics in Depth
The Theme of Homeostasis in Animal Movement
At every moment an animal's body is a beautifully choreographed performance of cells working together. Think of it like it like a theater production with trillions of cast members all on the stage at the same time. In order to maintain organization, all of the cells have standard biological conditions under which they can function. There are baseline levels for salt concentration, pH, nutrients, fluids, and body temperature.
The reason that solutes, pH, and temperature need to be at such specific levels is because of enzymes. Enzymes are the keys that catalyze chemical reactions in the cells and allow them to take place. As shown in the graphs below, enzyme activity levels can vary greatly depending on temperature and pH levels. If the proper levels of everything are not maintained in the cell, the consequences can be deadly. For example, an imbalance of potassium and sodium can cause the heart muscle to fail.
The graph on the left shows the % of activity (vertical y-axis) that an enzyme has over a range of temperatures (horizontal x-axis). Notice that the enzyme only reaches near 100% activity at one temperature. The graph on the right shows the % activity (y-axis) for three different enzymes (blue, yellow, and green) over a range oh pHs (x-axis). Notice that each enzyme works best at only a single pH.
The process of regulating and maintaining the internal environment is called homeostasis. All forms of homeostasis involve three stages: sensing, processing, and responding. The sensing stage is completed by thousands of internal receptors that detect the levels of a variety of things in the body. These sensors then report the information to the brain, which processes the information. If necessary, the brain will start orchestrating an appropriate response to fix the disruption of homeostasis.
Think of it like a thermometer, a thermostat, and a furnace. The thermometer senses the temperature in the house is low, the thermostat processes the measurement and sends the signal to the furnace, the furnace carries out the response and increases the temperature of the house.
The circle of homeostasis.
The communication steps that divide the three stages of homeostasis are performed by the nervous system and the endocrine system. The nervous system communicates via electrical signals, and the endocrine system communicates via biological signals. The endocrine system is a system of glands that secrete chemical messengers throughout the body. It is also an important communication tool, especially in the final response stage.
Just like the circulatory system, which has different names for the blood vessels to and from the heart, there are different names for the neural pathways leading to and from the brain. The neuron signaling pathway leading towards the brain is the afferent pathway, and the pathway traveling away from the brain is the efferent pathway. Electrical signals of the nervous system are often translated into the biological signals of the endocrine system.
Diagram of the afferent and efferent nerves which lead towards and away from the brain.
Most processes are regulated by negative feedback loops. Negative feedback means that a sensor somewhere in the body has reported a level occurring outside the tolerated limits. The body then must correct the level until it returns to suitable levels.
We discussed one type of homeostasis in the respiration section: gas exchange. In this situation the sensors are located within the walls of major arteries. They are constantly sending the brain reports of the carbon dioxide in the blood by measuring carbon dioxide and blood pH. If the brain receives a report that carbon dioxide levels are too high and pH levels are too low, it sends out the proper signals to increase respiration and circulation to bring in more oxygen.
Solutes are substances which are dissolved in water. If you put water in a tube and let it evaporate, the residue leftover would be the solutes that had been dissolved in the water. If you have ever taken a swim in the ocean then you have likely seen the white salt residue that is left on your skin afterwards. This was the salt that had been dissolved in the salt water.
Sodium chloride, or salt, is a common solute found in the blood, and the levels of sodium ions and chloride ions are among many solutes that are regulated. Solutes are important for chemical reactions to occur, but only in the proper concentrations. Too much or too little of something can cause severe consequences. Iron deficiency, copper deficiency, and iodine deficiency are just a few examples that can cause serious problems in your body. Apart from specifics, overall ion levels control the pH inside the body, which is important for all biological processes.
Luckily, the blood is filtered by the kidneys. It is the kidneys that remove excess solutes from the blood so that they may be excreted as urine.
Another form of regulation is called osmoregulation. This is the control of the levels of water in the body, and the consequent control of the solutes dissolved within it.
Diagram of water movement during osmosis from an area of low salt concentration to an area of high salt concentration.
In order to understand why osmoregulation is important, it is first necessary to understand the movement of water. Water movement between two places is determined by the concentration of solutes in each location, and the process by which water moves is called osmosis. The endgame in osmosis is equalizing the concentrations of solutes by changing the amount of water in each location. Water flows from areas with low concentrations of solute into area of high concentrations of solute. The movement of water consequently equalizes the concentration of solute.
Think about it. Adding more water in the highly concentrated area will decrease the concentration of solute. Conversely, removing water from the lowly concentrated area will increase the concentration of solute. When both areas have equal concentration of solute, the water stops moving. Since water movement is dependent on solute concentration, regulating solutes is one form of osmoregulation.
Osmosis is an important player in all animals because water in the cell and water in the blood need to have similar concentrations of solute or water will move one direction or the other. Cells and blood vessels are finite spaces. If too much water moves in, it can cause the cell or the blood vessel to rupture.
Diagram of what happens to red blood cells in three different environments due to osmosis causing water to move either into or out of the cells.
Osmoregulation in Water Animals
Osmoregulation is even more complicated for animals that live in the water. They have to worry about osmosis occurring between their environment and the insides of their bodies. There are two ways that water dwelling animals can handle this: osmoconformation and osmoregulation.
Jellyfish and sea anemones are osmoconformers. They are unable to regulate the water levels inside of their bodies and are at the mercy of osmosis at all times. To prepare for water movement, they have elastic bodies that can handle slight changes in size, like a balloon. Osmoconformers also often live in environments that are relatively stable.
However, most animals are osmoregulators. They need to keep a constant solute concentration. Think about it. What would happen to a fish if the inside of its body had a higher salt concentration than the surrounding water, like a freshwater fish? How would the water move? What would happen to their salt concentration?
Diagram showing the movement of water and solutes (ions) in freshwater fish.
Freshwater fish are naturally hypertonic, which means that they have a higher solute concentration than their surrounding water. This causes two things:
1) Water is always flowing into the fish towards the high concentration of solute.
2) Salt is diffusing out of the fish toward the lower concentration in the water.
Both processes will increase the water level in the fish and reduce the salt level. Yet, the fish needs to keep its high salt concentration and its low water level. Therefore, fish osmoregulate by expelling highly diluted urine to remove the excess water. They also require special ion pumps in their gills to actively extract ions from the water.
Marine fish have the opposite situation and are naturally hypotonic. They osmoregulate by removing concentrated urine. This allows them to reduce their internal salt concentration without losing too much water. They also actively transport ions out of their gills.
As mentioned above, internal temperature is very important for the body's chemical reactions to take place. Temperature regulation is a crucial part of homeostasis.
Failure of Homeostasis
Homeostasis can fail at any stage of the process. A problem with the sensor, the brain, or the communication pathway can wreak havoc on the entire body's homeostasis. Sometimes, the sensors and the brain work fine, but the body is not capable of completing the response. Diabetes mellitus is an example of this type of homeostatic failure.
Blood glucose levels are controlled by sensors in the blood vessels. When the levels rise it signals the pancreas to produce a middle-man called insulin. Insulin is a hormone that is released from the pancreas and it travels around telling specific organs to remove glucose from the blood.
However, if a person has Type I diabetes they are unable to produce insulin. If they have Type II diabetes, then the cells of the body do not respond when insulin tells them to suck up glucose. In both types, the body is not capable of completing the task that the brain is telling it to do, and it cannot remove the excess glucose from the blood.
An excess of glucose in the blood is an example of a breakdown in maintaining homeostasis. It shows how if one part of the body's natural status is disrupted, multiple other systems can be affected. This is why diabetes causes symptoms throughout the body as diagrammed in the picture below.
Diagram of the main symptoms of diabetes showing the wide-spread effects that failure of homeostasis can have.
Did you know that salmon are capable of swimming in salt-water and fresh-water? As they as they travel to spawn each year they have to navigate many types of waterways, and it is beneficial for them to be able to osmoregulate in either situation since they might have to swim in salt-water or freshwater.
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