Sonoma State University
Department of Biology - Hanes
Animal Physiology
Application
Discussion
Reflection
The earliest form of an excretory organ appears in the platyhelminthes. Some, especially fresh water ones have blind end tubes that end at "flame cells". Earthworms have nephridia as excretory organs. Vertebrates have nephrons that make up kidneys.
Problems of Osmoregulation - See tables 14-1 to 14-2 pp. 580-581
The following physical, behavioral or chemical actions may alleviate or add to an animals water balance problems:
Think of animals that illustrate the above.
Cell membranes of phospholipid are not especially permeable to water; however, a protein called aquaporin forms a tetrad in cell membranes that act as water channels. Cells can control water permeability by the numbers of these molecules in cell membranes. Some frogs have a belly patch of skin under the influence of arginine vasotocin a relative of ADH that allows them to take up water rapidly from the environment. Insects lose most water via tracheoles as their exoskeleton is pretty impermeable and they secrete a wax to cover it.
The camel and kangaroo rat have narrow, high surface area nasal passages that help to condense the water out of expired air. We use our nasal passages and turbinate bones to swirl air and conserve water. See Fig 14-6, p 587.
One source of water that we might ignore is metabolic water. See Table 14-4 for the amount of water we derive from digesting food. We also loose water through the skin. See Table 14-3 P 584 for some comparative numbers.
There Are a Number of Terms With Which You Should Become Familiar ( Fig 14-7 p 588)
Hyperosmotic, isosmotic, and hypo-osmotic are terms used by physicists, as they require perfect semipermeable membranes (allowing only water to pass through). These only exist in the minds of physicists, but they make mathematical formulas toe the line.
Hypertonic, isotonic, and hypotonic refer to osmotic water pressures comparing two compartments separated by a selectively permeable membrane (one that does not allow all dissolved substances and water through with equal ease). So, one can say that your body is hypertonic to SSU's ponds and if you sit in it, you will gain water. You can also say that your body is hyperosmotic to the pond meaning that it has a higher concentration of dissolved particles. Since there are many varieties of selectively permeable membranes and even many types of cell membranes, the only way that you can determine an isotonic solution to a cell is to put the cell in it and determine that it neither shrinks nor swells.
Osmotic regulators are animals that maintain a body osmotic pressure different than their environment (nearly all vertebrates, some crabs, all freshwater organisms). Osmotic conformers are animals that vary their osmotic pressure if the environment changes (marine bivalves, and most marine invertebrates).
Stenohaline animals can not survive in an environment that changes salt concentration very much. Euryhaline animals are those that can withstand large changes in environmental osmotic pressure (salmon).
Anadromous animals breed in fresh water, but live as adults in salt water (Salmon). Catadromous animals breed in salt water, but live as adults in fresh water (Eels).
Osmoregulation in Aqueous Environments -
Marine invertebrates are generally isotonic to their environment and are osmoconformers. They have no special mechanisms to move water, but they do regulate their salt contents so that they are different than seawater. They have cellular pumps and some impermeability to the salts. Estuarian and some coastal species regulate water balance. Some allow themselves to take up more water as seawater is diluted and even their cells increase in size to remain isotonic.
The barnacle has an interesting way of avoiding freezing when exposed to cold temperatures. It encourages ice crystal formation in intercellular spaces which concentrates the salts inside cells which in turn protects them from freezing. The cells may become quite hypertonic to seawater in a sense dehydrated.
Most fresh water invertebrates have osmotically low internal fluids to reduce the gradient with their environment. Mollusks can survive very dilute body fluids. Invertebrates all have Na+ pumps that remove Na+ from the water and add it to body fluids. Often these are on the gill or skin. All fresh water invertebrates are osmotic regulators. They often use excretory organs to filter body fluids and then retain salts as the fluid passes down a long tube, much like nephrons. Earthworms have two nephridia in each segment that work on this principle. The nephridia are a tube leading to the animals surface whose other end is an open ciliated funnel that collects body fluid.
Marine vertebrates have acquired a large variety of methods for dealing with their environments.
Hagfishes are isotonic to seawater and do not regulate water. They do regulate salts.
Fresh water vertebrates, like fish, do not drink water. The chloride cells in the gills pump Na+ into the body fluids. They also produce copious amounts of dilute urine with their kidney which is structured much like a mammal's without a Loop of Henle.
Terrestrial species may allow temperature changes in desert situations rather than cool with a water loss. Fig 14-10 p. 592. Some arthropods can absorb water from the air when humidity is as low as 50%. They do this with concentrated salt solutions.
Learn kidney anatomy: renal medulla, renal cortex, pelvis, ureter.
Learn the parts of the nephron: Bowman's capsule, proximal convoluted tubule, descending limb of Henle, ascending limb of Henle, (Henle's loop), distal convoluted tubule, collecting tubule, juxtaglomerular apparatus.
Learn the circulatory system of the kidney: afferent arteriole, glomerulus, efferent arteriole, paratubular capillaries.
Functions of The Glomerulus and Bowman's Capsule
The glomerulus is an especially leaky capillary knot surrounded by cells of Bowman's capsule. The capsular cells that surround the glomerulus are Podocytes with fingers of Pedicels that leave filtration slits between them. Note the relative pressures that occur as in Starling's law of the capillary. Table 14-7 p.601. The human kidney, for example, receives 625 ml/min of blood plasma. Of this amount, 125 ml/min is expressed from the glomerulus and is picked up by Bowman's capsule and delivered to the proximal tubule. The glomerulus acts like a pressure filter and passes molecules smaller than about 68,000 daltons. Blood albumin at ~ 69,000 daltons usually is retained. Hemoglobin free of red cells will pass somewhat and myoglobin from crushed muscle will readily pass. The retained plasma (500 ml/min) and RBC's exit the efferent arteriole.
Functions of the Proximal Convoluted Tubule
The filtered plasma is slightly hypotonic to blood, as it contains no protein. Cells lining the proximal tubule have very active Na+ pumps on their basement membrane surfaces. These cells also pump glucose and amino acids from the filtrate into the intertubular space. Water and Cl- follow by diffusion so that there is very little osmotic difference between intratubular and intertubular fluid. pH is also partly regulated in the proximal tubule. Acid (H+) can be removed from the blood by adding H+ to HCO3-, converting this to CO2 and H2O with carbonic anhydrase and removing the CO2 and H2O via the blood. Other conversions would be:
Acid can be retained in the blood by excreting H+ deficient compounds and allowing the metabolism of the body to build up H+.
The proximal tubule also acts to save Na+ which would be in low supply in a normal mammalian diet. It does this by substituting either K+ or H+ for Na+ with its Na+ pumps. So the final conversions might look like this:
The results of all this pumping action and diffusion is the loss of most of the solutes and fluid filtered at the glomerulus to the intertubular space and the concentration of all substances not pumped out of the tubule by the lining epithelium. The cellular pumps of the proximal tubule have maximum levels of transfer. If the tubular fluid is overloaded with say glucose or vitamin C, the pumps can not transfer it all and it can be carried into the urine as in diabetes or huge Vitamin C dosages. By the end of the proximal tubule, 3/4 of the solutes and water have been removed and returned to the blood.
Other chemicals are secreted into the proximal tubule. See the table 14-9 p 607.
Functions of Henle's Loop
So perhaps 30 ml/min of fluid enter Henle's loop at an osmotic concentration of about 300 mOsm. Henle's loop is a counter current multiplier system that does an amazing job of developing a concentration gradient that increases from the top of the medulla to the pelvis of the kidney. Cells lining the descending limb of Henle are thin and not very metabolically active. They are permeable to water, but not very permeable to Na+ and urea. The intertubular fluid becomes more concentrated toward the tip of the loop so that water is drawn out of the tubule concentrating the Na+ and Cl-. After the loop makes its turn at the bottom, the cells change character and become impermeable to water (not letting it back in) and the cells actively pump out both Na+ and Cl- concentrating the intertubular fluid and diluting intratubular fluid on the way back up. The ascending loop has a low permeability to water. By the time the fluid has reached the renal cortex, intratubular fluid is again nearly isotonic to the blood or hypotonic.
The ascending thin limb of Henle's Loop is permeable to urea, the lower part of the collecting tubule is permeable to urea. When the kidney is conserving water and there is little water loss, the concentration of urea will build up and recirculate from collecting tubule to ascending thin limb and around again. This adds to the osmotic pressure of the lower pelvic kidney. When the kidney is getting rid of excess water in volume, the concentration of urea will be lessened as the concentration of urea that is lost is the same as that in the pelvic kidney and urine volume changes will alter this concentration. Fig 14-28 p 605.
Functions of the Distal Convoluted Tubule and the Collecting Ducts
Water permeability of the distal and collecting duct epithelia depends upon the presence of Antidiuretic Hormone (from the posterior pituitary). If ADH is present, the epithelium is water permeable. The distal cells continue to pump out Na+ and water follows so that volume is reduced and the isotonicity of the intratubular fluid is due more and more to the concentration of waste molecules. Leaving the distal tubule, the intratubular fluid is still near isotonicity, but as it continues down the collecting ducts through the salt and urea gradient, water is lost to the intertubular space, but nothing else can penetrate the collecting duct epithelium. So, the urine becomes concentrated with waste molecules and relatively little water is lost. Of the 625 ml/min of blood plasma received by the kidneys, only about 1 ml/min of urine may be formed.
If ADH is not present, the epithelia of the distal tubule and collecting ducts are impermeable to water. The distal tubule continues to pump out Na+ very efficiently so that the remaining fluid in the tubule becomes more dilute. Even the early collecting ducts can still pump out the small amount of Na+ remaining in the tubules. Water can not follow the Na+ nor can it be drawn out by the Na+ gradient so that a dilute urine is excreted. Urea is permeable to the lower collecting ducts. During water retention the urea concentration in the kidney medulla helps to establish the intertubular concentration gradient. When water is being lost, much of the urea is also lost thus reducing the efficiency of the gradient.
Kidney and Na+ K+ Balance.
As we have already seen, the mammalian kidney is not only designed to save water, but also to conserve Na+. K+ is found in plant and animal cells. Na+ may not be present in many plants, but is common in animal blood. The kidney also acts to balance these two salts. A low Na+/K+ ratio will stimulate the release of the hormone Aldosterone (a steroid hormone from the adrenal cortex). Aldosterone brings the Na+ pumps of the kidney epithelium to high activity removing Na+ from the tubular fluid and substituting either H+ or K+. In this way Na+ is preserved.
One of the causes of high blood pressure is a restricted blood flow to one of the kidneys. A low blood flow is detected by the juxtaglomerular apparatus. This is found where a loop of the distal convoluted tubule comes into contact with Bowman's capsule where the arterioles enter and exit. There is a small mass of specialized cells growing outside the tubule at this point called the juxtaglomerular apparatus. It somehow influences the afferent arteriole which has specialized endothelial cells that contain an enzyme called renin. Renin is released into the blood where it can come into contact with and cleave a liver synthesized protein called Angiotensinogen forming Angiotensin I. Angiotensin I is in turn cleaved by an enzyme in the lungs to form Angiotensin II. Angiotensin II is a powerful vasoconstrictor. Arterioles all over the body except for the brain are constricted increasing blood pressure and blood flow to the kidney.
The Concept of Kidney Clearance
This term comes from the idea that the kidney can clear substances from blood plasma. It is described by the formula:
where: V
= Urine accumulation rate; Ux = Urine concentration of substance x;
and Px = Plasma concentration of substance x.
As an example if urine is produced at 1.2 ml/min, substance "x" is in the urine at 0.28 mg/ml, and the plasma concentration is .003 mg/ml, then the plasma clearance would be 112 ml/min.
Note that clearance values are in ml of plasma/min. These are the ml of plasma that would be cleared of substance x to account for its accumulation in the urine. This doesn't necessarily mean that the plasma is completely cleared in fact. It is only a way to describe the effectiveness of kidney excretion of any substance.
Let us take an easy example. Inulin a polysaccharide composed of galactose units is small enough to be well filtered, but there are no enzymes that pump it nor can it cross tubular epithelium. Therefore however much of the inulin is filtered into Bowman's capsule will appear in the urine. Inulin clearance given in ml plasma/min is used to determine the glomerular filtration rate (GFR). It is used commonly to determine filtration efficiency in many types of animals.
Another common use of clearance values is that for para-aminohippuric acid (PAH). This is also used in kidney function tests. It is a chemical that is very actively pumped into tubules. It is so efficiently pumped that all of the blood that goes to the kidney is really cleared in one pass. This includes the blood plasma that is not filtered. If all of the blood that supplies the kidney is cleared of PAH and it all ends up in urine, then the clearance value for PAH is the renal plasma flow rate (RPF). If we know the percentage of blood that is plasma, we can then calculate renal blood flow.
Vascular Reabsorption
After leaving the glomerulus, blood has lost some pressure as it traverses the efferent arteriole and from there through the paratubular capillaries. These capillaries follow the tubules and one limb even follows the Loop of Henle. Because the blood still contains proteins and intertubular fluid does not, the blood is always slightly hypertonic to intertubular fluid even as it follows the Loop into the lower medulla. This means that the paratubular capillaries will always attract fluid into themselves. It is by this means that the large amounts of fluid accumulated by the intertubular space is removed. Capillary endothelium is always permeable to water and salts.
The variations that terrestrial vertebrates, or desert vertebrates exhibit are most apparent when examining their mode of excretion. Amphibians, reptiles, and almost all birds can not produce a hypertonic urine to conserve water. The best that they can do is produce a urine isotonic with their blood. Water needed to get rid of nitrogenous wastes is the biggest water waster for these animals. Frogs and toads can switch to urea (Fig 14-51) from ammonia when they become dehydrated. Urea is much less toxic so that it can be concentrated in a little water and one has the benefit of two nitrogen atoms per molecule instead of one. Birds and reptiles use uric acid (Fig 14-52)as a nitrogenous waste. It is nearly insoluble in water (comes out as a white crystal), and so does not add to an osmotic burden. It also has four nitrogen atoms per molecule.
There are also several interesting and weird animals who have developed systems that are completely different from the others.
1. Be able to define the new terms used.
2. What are the possible inputs of water into animals and what are
the ways water is lost?
3. What are the advantages and disadvantages of ammonia, urea, &
uric acid excretion?
4. For each of the animals mentioned be able to enumerate their
osmotic problems and their unique behavioral, anatomical and
physiological solutions to these problems.
5. Describe the kidneys control mechanisms for Na+ control, osmotic
control, K+ control, blood pH control, control of blood flow to the
kidney.
6. Match a part of the nephron with its functions.
Which is more osmotically stressful for a marine fish - eating other fish or eating squid.
What animals would benefit from using ammonia as a nitrogenous waste product? urea? uric acid?
Why does alcohol cause dehydration? How can desert animals cool themselves without wasting water?