Cells placed in a hypertonic environment tend to shrink due to loss of water. In a hypotonic environment, cells tend to swell due to the intake of water. The blood maintains an isotonic environment so that cells neither shrink nor swell. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.
Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, extracellular fluid, and intracellular fluid.
Because blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure. Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to the number of solute atoms or molecules and not dependent on the size of the solute molecules.
Because electrolytes dissociate into their component ions, they, in essence, add more solute particles into the solution and have a greater effect on osmotic pressure, per mass than compounds that do not dissociate in water, such as glucose. Water can pass through membranes by passive diffusion. If electrolyte ions could passively diffuse across membranes, it would be impossible to maintain specific concentrations of ions in each fluid compartment, therefore, they require special mechanisms to cross the semi-permeable membranes in the body.
This movement can be accomplished by facilitated diffusion and active transport. Facilitated diffusion requires protein-based channels for moving the solute. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.
Persons lost at sea without any fresh water to drink, are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90 percent of all bony fish are restricted to either freshwater or seawater.
They are incapable of osmotic regulation in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life in freshwater and part in sea water. Organisms like the salmon and molly that can tolerate a relatively wide range of salinity are referred to as euryhaline organisms.
This is possible because some fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in fresh water, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure 4. In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very dilute urine, and they achieve electrolyte balance by active transport of salts through the gills.
When they move to a hypertonic marine environment, these fish start drinking sea water; they excrete the excess salts through their gills and their urine, as illustrated in Figure 4. Most marine invertebrates, on the other hand, maybe isotonic with sea water osmoconformers. Semipermeable membranes are permeable to certain types of solutes and to water, but typically cell membranes are impermeable to solutes. The body does not exist in isolation.
There is a constant input of water and electrolytes into the system. Excess water, electrolytes, and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance. Insufficient fluid intake results in fluid conservation by the kidneys. Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces.
Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences. Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, interstitial fluid, and intracellular fluid.
Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Since blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure. The human excretory system functions to remove waste from the body through the skin as sweat, the lungs in the form of exhaled carbon dioxide, and through the urinary system in the form of urine. All three of these systems participate in osmoregulation and waste removal.
Here we focus on the urinary system, which is comprised of the paired kidneys, the ureter, urinary bladder and urethra Figure The kidneys are a pair of bean-shaped structures that are located just below the liver in the body cavity. Each of the kidneys contains more than a million tiny units called nephrons that filter blood containing the metabolic wastes from cells.
All the blood in the human body is filtered about 60 times a day by the kidneys. The nephrons remove wastes, concentrate them, and form urine that is collected in the bladder.
Internally, the kidney has three regions—an outer cortex, a medulla in the middle, and the renal pelvis, which is the expanded end of the ureter. The renal cortex contains the nephrons—the functional unit of the kidney. The renal pelvis collects the urine and leads to the ureter on the outside of the kidney. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder. Blood enters each kidney from the aorta, the main artery supplying the body below the heart, through a renal artery.
It is distributed in smaller vessels until it reaches each nephron in capillaries. Within the nephron the blood comes in intimate contact with the waste-collecting tubules in a structure called the glomerulus. Water and many solutes present in the blood, including ions of sodium, calcium, magnesium, and others; as well as wastes and valuable substances such as amino acids, glucose and vitamins, leave the blood and enter the tubule system of the nephron.
As materials pass through the tubule much of the water, required ions, and useful compounds are reabsorbed back into the capillaries that surround the tubules leaving the wastes behind. Some of this reabsorption requires active transport and consumes ATP. Some wastes, including ions and some drugs remaining in the blood, diffuse out of the capillaries into the interstitial fluid and are taken up by the tubule cells.
These wastes are then actively secreted into the tubules. Chapter Osmoregulation and Excretion. Chapter 1: Scientific Inquiry. Chapter 2: Chemistry of Life. Chapter 3: Macromolecules. Chapter 4: Cell Structure and Function. Chapter 5: Membranes and Cellular Transport.
Chapter 6: Cell Signaling. Chapter 7: Metabolism. Chapter 8: Cellular Respiration. Chapter 9: Photosynthesis. Chapter Cell Cycle and Division. Chapter Meiosis. Chapter Classical and Modern Genetics. Chapter Gene Expression. Chapter Biotechnology. Chapter Viruses. Chapter Nutrition and Digestion. Chapter Nervous System. Chapter Sensory Systems. Chapter Musculoskeletal System.
Chapter Endocrine System. Chapter Circulatory and Pulmonary Systems. Chapter Immune System. Chapter Reproduction and Development. Chapter Behavior. Chapter Ecosystems. Chapter Population and Community Ecology. Chapter Biodiversity and Conservation. Chapter Speciation and Diversity. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis.
Privacy Policy. Skip to main content. Osmotic Regulation and the Excretory System. Search for:. Human Osmoregulatory and Excretory Systems. Learning Objectives Explain how the kidneys serve as the main osmoregulatory organs in mammalian systems, using the functional properties of nephrons. Kidneys filter the blood; urine is the filtrate that eliminates waste from the body via the ureter into the bladder. The kidneys are surrounded by three layers: renal fascia, perirenal fat capsule, and the renal capsule.
Key Terms renal : pertaining to the kidneys. Nephron: The Functional Unit of the Kidney The functional unit of the kidney, the nephron, removes waste from the body. Learning Objectives Explain the role of the nephron as the functional unit of the kidney.
Key Takeaways Key Points Kidneys contain two types of nephrons, each located in different parts of the renal cortex: cortical nephrons and juxtamedullary nephrons. A nephron comprises a renal corpuscle, a renal tubule, and the associated capillary network.
Internally, kidneys are mainly composed of over one million nephrons and an extensive network of blood vessels and capillaries. Diagram of a nephron The nephron is the functional unit of the kidney. Kidney Function and Physiology Urine is a byproduct of the osmoregulatory function of kidneys, which filter blood, reabsorb water and nutrients, and secrete wastes.
Learning Objectives Outline the process by which kidneys filter blood, reabsorb nutrients and water, and produce urine. Key Takeaways Key Points Glomerular filtration, tubular reabsorption, and tubular secretion are the three primary steps in which kidneys filter blood and maintain proper electrolyte balance.
Glomerular filtration removes solutes from the blood; it is the first step of urine formation. In tubular reabsoption, the second step of urine formation, almost all nutrients are reabsorbed in the renal tubule by active or passive transport. Tubular secretion is the last step of urine formation, where solutes and waste are secreted into the collecting ducts, ultimately flowing to the bladder in the form of urine.
Key Terms arteriole : one of the small branches of an artery, especially one that connects with capillaries countercurrent : a current that flows against the prevailing one electrolyte : any of the various ions such as sodium or chloride that regulate the electric charge on cells and the flow of water across their membranes. Licenses and Attributions.
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