Urinary ExcretionThe primary route in which the body eliminates substances is through the kidneys. The main function of the kidney is the excretion of body wastes and harmful chemicals into the urine. The functional unit of the kidney responsible for excretion is the nephron. Each kidney contains about one million nephrons. The nephron has three primary regions that function in the renal excretion process: the glomerulus,
proximal tubule, and the distal tubule (Figure 2). Show
Figure 1. Kidney Structure Three processes are involved in urinary excretion:
Figure 2. Nephron of the kidney Filtration Molecules with molecular weights greater than 60,000 (which include large protein molecules and blood cells) cannot pass through the capillary pores and remain in the blood. If urine contains albumin or blood cells, it indicates that the glomeruli have been damaged. Binding to plasma proteins will influence urinary excretion. Polar substances usually do not bind with the plasma proteins and thus can be filtered out of the blood into the tubule filtrate. In contrast, substances extensively bound to plasma proteins remain in the blood. Secretion Reabsorption A factor that greatly affects reabsorption and urinary excretion is the pH of the urine. This is especially the case with weak electrolytes. If the urine is alkaline, weak acids are more ionized and excretion is increased. Weak acids (such as glucuronide and sulfate conjugates) are less ionized if the urine is acidic and undergo reabsorption and renal excretion is reduced. Since the urinary pH varies in humans, the urinary excretion rates of weak electrolytes also vary.
The physical properties (primarily molecular size) and polarity of a substance in the urinary filtrate greatly affect its ultimate elimination by the kidney. Small toxicants (both polar and lipid-soluble) are filtered with ease by the glomerulus. In some cases, large molecules (including some that are protein-bound) may be secreted (by passive transfer) from the blood across capillary endothelial cells and nephron tubule membranes to enter the urine. The major difference in ultimate fate is governed by a substance's polarity. Those substances that are ionized remain in the urine and leave the body. Lipid-soluble toxicants can be reabsorbed and re-enter the blood circulation, which lengthens their half-life in the body and potential for toxicity. Kidneys, which have been damaged by toxins, infectious diseases, or because of age, have diminished ability to eliminate toxicants thus making those individuals more susceptible to toxins that enter the body. The presence of albumin in the urine indicates that the glomerulus filtering system is damaged, letting large molecules pass through. The presence of glucose in the urine is an indication that tubular reabsorption has been impaired.
1) The reason that much of the blood plasma filters into the renal tubule is due to: 2) In which area of the nephron does active secretion take
place? 3) Most of the material filtered through the glomerulus is reabsorbed in the proximal convoluted tubule of the nephron. The primary property of a xenobiotic that determines whether it will be reabsorbed is: What is renal secretion from the nephron?Renal secretion occurs as: substances pass from the peritubular capillaries to the nephron tubules. All substances filtered from the blood by the nephron are excreted in the urine. false Describe the composition of glomerular filtrate.
Why does more filtration occur at the glomeruli?A greater amount of filtration occurs at glomeruli than at other capillary beds of the body because: glomeruli have a higher hydrostatic pressure. Water reabsorption from the proximal convoluted tubule is closely linked to the active reabsorption of:
What is the main function of the kidneys Quizlet?The kidneys function to produce hormones, excrete wastes, regulate blood pressure, and maintain ion balance. Which of the following is another function of the kidneys? Blood leaves the glomerulus via which vessel? Nice work! You just studied 12 terms!
What is the composition of glomerular filtrate?Describe the composition of glomerular filtrate. Like plasma, but lacking larger protein molecules. A greater amount of filtration occurs at glomeruli than at other capillary beds of the body because: glomeruli have a higher hydrostatic pressure.
How much blood is filtered through the glomerulus?According to the National Kidney Foundation, normal results range from 90 to 120 mL/min/1.73 m2.
What percentage of blood is filtered through the kidneys?About 20% of the plasma volume passing through the glomerulus at any given time is filtered. This means that about 180 liters of fluid are filtered by the kidneys every day. Thus, the entire plasma volume (about 3 liters) is filtered 60 times a day!
What percentage of glomerular filtration is reabsorbed?Nearly 70–80 percent of electrolytes and water from the glomerular filtrate is reabsorbed in.
What percentage of plasma entering the glomerulus is filtered and enters Bowman's capsule?Glomerular Histology
Approximately 20% of the plasma that enters the glomerulus is filtered through the capillary filtration barrier and out into the urinary space (US) inside Bowman's capsule. The remaining 80% of the plasma and all the blood cells leave the glomerulus via the efferent arteriole.
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