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. The blood then collects in larger and larger vessels and leaves the kidney in the renal vein. The renal vein joins the inferior vena cava, the main vein that returns blood to the heart from the lower body. The amounts of water and ions reabsorbed into the circulatory system are carefully regulated and this is an important way the body regulates its water content and ion levels.
The waste is collected in larger tubules and then leaves the kidney in the ureter, which leads to the bladder where urine, the combination of waste materials and water, is stored.
The bladder contains sensory nerves, stretch receptors that signal when it needs to be emptied. These signals create the urge to urinate, which can be voluntarily suppressed up to a limit. The conscious decision to urinate sets in play signals that open the sphincters, rings of smooth muscle that close off the opening, to the urethra that allows urine to flow out of the bladder and the body. Dialysis is a medical process of removing wastes and excess water from the blood by diffusion and ultrafiltration.
When kidney function fails, dialysis must be done to artificially rid the body of wastes and fluids. This is a vital process to keep patients alive. In some cases, the patients undergo artificial dialysis until they are eligible for a kidney transplant. In others who are not candidates for kidney transplants, dialysis is a lifelong necessity.
Dialysis technicians typically work in hospitals and clinics. While some roles in this field include equipment development and maintenance, most dialysis technicians work in direct patient care. Their on-the-job duties, which typically occur under the direct supervision of a registered nurse, focus on providing dialysis treatments. This can include reviewing patient history and current condition, assessing and responding to patient needs before and during treatment, and monitoring the dialysis process.
Homeostasis is a dynamic equilibrium that is maintained in body tissues and organs. It is dynamic because it is constantly adjusting to the changes that the systems encounter. Water levels and mineral ions in the blood are controlled to keep the concentrations the same inside the cells as around them. This protects cells by stopping too much water from entering or leaving them by osmosis. If body cells lose or gain too much water, they do not function efficiently.
If the concentration of water is the same inside and out the cells, they remain in their normal state. If the water concentration is too high outside, water enters the cell by osmosis and they may burst. On the other hand, if the water concentration is too low outside compared to the inside of the cells, water will leave by osmosis and the cells may shrivel.
If body cells lose or gain too much water by osmosis they do not function efficiently. The process of glomerular filtration filters out most of the solutes due to the high blood pressure and specialized membranes in the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. All solutes in the glomerular capillaries, including sodium ions and negatively and positively charged ions, pass through by passive diffusion; the only exception is macromolecules such as proteins.
There is no energy requirement at this stage of the filtration process. Glomerular filtration rate GFR is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.
Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed; this occurs either by passive or active transport. Reabsorption of water and key electrolytes are regulated and influenced by hormones.
Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries.
Every solute, however, has a transport maximum; the excess solute is not reabsorbed. In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb.
Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration. It tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, it acts as a countercurrent multiplier. The vasa recta around the loop of Henle acts as the countercurrent exchanger. Loop of Henle : The loop of Henle acts as a countercurrent multiplier that uses energy to create concentration gradients.
The descending limb is water permeable. Water flows from the filtrate to the interstitial fluid, so osmolality inside the limb increases as it descends into the renal medulla. At the bottom, the osmolality is higher inside the loop than in the interstitial fluid. Additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is the opposite process to tubular reabsorption.
The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. 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.
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