16.1 Homeostasis and Osmoregulation

• Homeostasis refers to the relatively stable state inside the body.
• The animal organs and organ systems adapt to internal and external changes in order to maintain a steady state.
• There are internal conditions that are maintained homeostatically.
• Negative feedback relationships result in stable conditions.
• If the bloodglucose or calcium goes up, this signals to the organs that are supposed to lower it.
• Negative feedback can be seen in the signals that restore the normal levels.
• The results can be bad for the animal when homeostatic mechanisms fail.

• Homeostatic mechanisms keep the body in a constant state of equilibrium.
• This homeostatic equilibrium is maintained by an animal that is inactive.
• The temperature and water content are two examples of factors that are regulated homeostatically.
• The processes that regulate these two factors are called thermoregulation and osmoregulation.

• The body's systems try to go back to this point when there are normal fluctuations from the set point.
• The response of the system is to adjust the activities of the system so the value moves back toward the set point after a change in the internal or external environment.
• Changes are made to cool the animal if it becomes too warm.
• If the blood sugar levels go up after a meal, it's important to lower them to get the nutrition into tissues that need it or to store it for later use.

• An adjustment must be made to keep the internal environment of the body and cells stable when there is a change in an animal's environment.
• There is a feedback mechanism that senses the change in the environment.
• The stimuli are detected by the receptor.
• The brain relays appropriate signals to an effector organ that is able to cause an appropriate change, either up or down, depending on the information the sensor was sending.

• Animals can be divided into two groups, those that maintain a constant body temperature in the face of differing environmental temperatures, and those that have a body temperature that is the same as their environment and thus varies with the environmental temperature.
• The body temperature of these organisms is similar to the environment, although the individual organisms may do things that keep their bodies slightly below or above the environment temperature.
• burrowing underground on a hot day or resting in the sunlight on a cold day are examples.
• The term cold-blooded may not apply to an animal in the desert with a very warm body temperature.

• The internal heat generated by these animals allows them to maintain a level of activity that other animals can't.

• You can watch a Discovery Channel video on thermoregulation to see illustrations of the process in a variety of animals.

• There are a variety of ways animals conserve heat.
• Endothermic animals have insulation.
• They have fur, fat, or feathers.
• Animals with thick fur or feathers have an insulation layer between their skin and internal organs.

• The polar bears and seals live and swim in a warm environment.
• The fluffy tail of the arctic fox helps it sleep in cold weather.
• It is possible for mammals to increase body heat production by shivering.
• Individual hairs stand up when the individual is cold because of arrector pili muscles.
• This increases the strength of the hair.
• The reaction causes "goose bumps" instead of having the intended effect on our hairless bodies.
• Animals use layers of fat as insulation.
• Losing body fat will affect an individual's ability to conserve heat.

• The circulatory systems of ectotherms and endotherms help maintain body temperature.
• Vasodilation, the opening up of arteries to the skin by relaxation of their smooth muscles, brings more blood and heat to the body surface, facilitating radiation and cooling the body.
• Vasoconstriction, the narrowing of blood vessels to the skin by contraction of their smooth muscles, reduces blood flow in peripheral blood vessels and conserves heat.
• Some animals have adaptions to their circulatory system that allow them to transfer heat from arteries to veins that are flowing next to each other.
• The countercurrent heat exchange prevents the cold blood from cooling the heart and other internal organs.
• The countercurrent adaptation is found in many animals.

• Some animals use changes in their behavior to regulate their body temperature.
• During the hottest part of the day in the desert, they seek cooler areas.
• The same animals may climb onto rocks in the evening to get some heat in the desert.

• The processes of temperature control are coordinated by the nervous system in the animal brain.
• The set point for body temperature is maintained by the hypothalamus through reflexes.
• The sympathetic nervous system under control of the hypothalamus is in charge of the body's temperature loss and gain.
• Some instances the set point may be adjusted.
• During an infection, compounds called pyrogens are produced and circulate to the hypothalamus to reset the thermostat.
• The body's temperature can increase to a new homeostatic equilibrium point when this occurs.
• The increase in body heat makes the body less efficient in fighting infections and increases the activities of cells so they are better able to fight them.

• The body is able to regulate temperature.

• The blood contains pyrogens, which are released whenbacteria are destroyed.
• The body's thermostat is set to a higher temperature.

• The fluids inside and around the cells are made of water, electrolytes, and non-electrolytes.
• An ion is a compound that is dissolved in water.
• A nonelectrolyte does not form ion in water.
• The "membranes" made of cells in the body are semipermeable.
• Cells are impermeable to water and certain types of solutes.

• The body is not isolated.
• The system has a constant input of water and electrolytes.
• osmotic balance is maintained by excess water, electrolytes, and waste being transported to the kidneys.
• Insufficient fluid intake causes the kidneys to excrete more fluid.
• There is a tendency to accumulate toxic waste and water if there is no mechanism to regulate osmotic pressure.

• Specific concentrations of important electrolytes in the three major fluid compartments are regulated by mammalian systems.
• The volume of the fluid compartments can change temporarily because osmotic pressure is regulated by the movement of water.
• osmotic pressures have a direct bearing on blood pressure since blood is one of the fluid components.

• The human excretory system removes waste from the body through the skin, lungs, and urinary system.
• The three systems are involved in waste removal.
• The OpenStax book is available for free at http://cnx.org/content/col11487/1.9 blood containing the metabolic wastes from cells.
• The human body has a kidneys that filters blood about 60 times a day.
• The urine is collected from the bladder.

• An outer cortex, a medulla in the middle, and the expanded end of the ureter are the internal regions of the kidneys.

• The human excretory system is made up of several organs.
• The urine is stored in the bladder until it is eliminated through the urethra.
• The internal structure of the kidneys is shown on the right.
• It is distributed in smaller vessels.
• The blood comes in contact with the waste-collecting tubules in a structure called the glomerulus.
• Water and many solutes present in the blood, as well as wastes and valuable substances, leave the blood and enter the tubule system of the nephron.
• When materials pass through the tubule, they are reabsorbed back into the capillaries that surround the tubules.
• Active transport is required for some of this reabsorption.
• Drugs and ion are taken up by tubule cells when they diffuse out of the capillaries into the interstitial fluid.
• The tubules are active in the production of these wastes.
• The blood collects in larger vessels and leaves the kidneys in the vein.
• The amount of water and ion reabsorption into the circulatory system is regulated by the body.
• The urine is stored in the bladder and the waste is collected in larger tubules.

• When the bladder needs to be emptied, it has sensory nerves.
• The urge to urinate can be suppressed up to a limit by these signals.