Homeostasis
Important Vocabulary
Homeostasis - the regulation of internal biochemistry to maintain a constant internal environment. Without this constant regulation, living organisms would be unable to function.
Capillaries - tiny blood vessels that exchange materials with body tissues.
Alveoli (singular: alveolus) - irregularly shaped air sacs that make up the lungs.
Glycogen - A polysaccharide, (C6H10O5)n, that is the main form of carbohydrate storage in animals and occurs primarily in the liver and muscle tissue. It is readily converted to glucose as needed by the body to satisfy its energy needs. Also called animal starch.
Urea - A water-soluble compound, CO(NH2)2, that is the major nitrogenous end product of protein metabolism and is the chief nitrogenous component of the urine in mammals and other organisms. Also called carbamide.
Nephron - kidney tubules, located within the cortex and medulla.
Filtration - the movement of solvents and dissolved substances across a selectively permeable membrane from regions of higher pressure to regions of lower pressure.
Glomerulus - A tuft of capillaries situated within a Bowman's capsule at the end of a renal tubule in the vertebrate kidney that filters waste products from the blood and thus initiates urine formation.
Reabsorbtion - molecules that are absorbed into cells a second time as they are needed.
Loop of Henle - The segment of the nephron of a vertebrate kidney that is situated between the proximal and distal convoluted tubules. It plays a role in the transport of ions and water and the concentrating of urine.
Collecting duct - A segment of the nephron of a vertebrate kidney that is situated after the Loop of Henle. It plays a role in the transport of ions and water and the concentrating of urine.
Tubular Excretion - the movement of molecules from the peritubular capillary network to the distal convoluted tubule.
Dialysis - the separation of molecules by a selectively permeable membrane that permits diffusion of some molecules but not others.
Urinalysis - a test used to determine the chemical composition of urine.
Homeostasis solves three basic internal environment problems: excretion of metabolic wastes, regulating the concentration of ions and other chemicals, and maintaining water balance. This lab focuses on how the lungs, the liver and the kidneys help to regulate homeostasis.
The Lungs
The lungs are the site of gas exchange. We inspire air that contains oxygen (O2) and expire air containing carbon dioxide (CO2). The air travels through the throat, the bronchial tubes, the bronchioles, to the alveoli (air sacs) that are covered by capillary beds. Blood that has depleted its oxygen supply enters the lung capillary beds, where the oxygen is replenished and carbon dioxide removed via simple diffusion. Oxygen is required in cellular respiration, and CO2 is a waste product of that process. CO2 is additionally an acid, lowering the pH of the blood.
Therefore the lungs have two homeostatic functions:
- Gas exchange (intake of O2 and release of waste CO2)
- Regulation of pH
The Liver
The liver aids in digestion and regulates blood sugar by storing excess sugar and releasing it when it is needed. Blood from the intestine (through the hepatic portal vein) may carry a lot of sugar if the organism has just eaten. That excess sugar is stored as glycogen. If an organism requires more sugar, perhaps because it is under stress (activity), then glycogen is broken back down and the simple sugars are released into the blood stream.
In addition, the liver helps in the breakdown of proteins. Proteins are long strings of amino acids, which are broken down to ammonia - very toxic! The liver converts the ammonia to urea, a relatively benign nitrogenous waste which is filtered out of the blood at the kidneys (more about that below).
Factoid: urea was the compound that disproved the idea of vitalism.
"Urea was first discovered in human urine by H.M. Rouelle in 1773. It was synthesized in 1828 by Friedrich Wohler and was the first organic compound to be synthesized from inorganic starting materials. It was found when Wohler attempted to synthesis ammonium cyanate, to continue a study of cyanates which he had been carrying out for several years. On treating silver cyanate with ammonium chloride solution he obtained a white crystalline material which proved identical to urea obtained from urine.
This discovery prompted Wohler to write triumphantly to Berzelius, "I must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea."
This organic synthesis dealt a severe blow to a widespread belief called "vitalism" which maintained that organic chemicals could be modified by chemistry but could only be produced through the agency of a vital force present in living plants and animals." - quoted from another webpage.
The liver has two homeostatic functions:
- blood sugar regulation
- nitrogenous waste conversion
[glucose regulation image]
The Kidneys
The kidneys filter the blood, removing nitrogenous wastes and regulating pH. They are composed of many nephrons which act as the filters.
Filtration is the first step in urine production. This step is simple diffusion across a hydrostatic pressure gradient, where smaller molecules can pass, but larger cannot. This happens in the part of the nephron called the glomerulus. We examined filtration with the simple filter apparatus showing only the small copper sulfate molecules could pass through the filter, not charcoal or starch.
Reabsorption is the next step in urine production. This step occurs in the loop of henle and the collecting duct. Here water and necessary salts and nutrients are reabsorbed by active transport into the blood. This is done to maintain blood volume and ionic balances.
Tubular excretion involves the movement of materials from the loop of henle to the distal convoluted tubule, again by active transport, for the purpose of excreting these materials from the body.
The Kidney's functions in homeostasis include:
- blood volume maintenance
- ion balance maintenance (includes pH)
- nitrogenous waste removal
Review Questions
- What does the term "homeostasis" mean?
- The lungs help regulate the gas content of your blood. In the lung, O2 passes from the ________________ into the _______________, and CO2 passes from the ____________________ into the ______________________.
- The liver is responsible for regulating ___________________ by storing excess ___________________ in the form of __________________.
- The kidneys help regulate the chemical balance of your blood by first filtering _____________________ from the blood and then reabsorbing the right amounts of ______________________.
- Name the three basic problems organisms face in the regulation of their internal environment.
- What is the name for the air sacs in the lungs?
- Name the process by which gas exchange occurs in the lungs.
- Consider the following substances: albumin, glucose, urea and water. Sate if you would expect to find each substance in the blood, in the kidney filtrate, in the urine, or a combination of the three.
- Fill out the following table:Blood vessel Carries blood from to Mesenteric artery Hepatic portal vein Hepatic vein - Which blood vessel carries the most glucose when one has not eaten in several hours? Why?
- Why does someone who is diabetic demonstrate excessive urination and thirst?
- T/F The pH of a solution becomes more basic by exhaling into the solution.
- T/F When you exercise, blood chemistry becomes more acidic. Elevated respiration associated with exercise helps to decrease blood pH.
- T/F The development of lactic acid in muscles after rigorous exercise results from the increasingly aerobic environment within your body.
- What is the role of insulin and where is it produced?