A Summary of Kidney Function

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Welcome to another spotlight video where we walk you through a spotlight figure from your textbook. This video walks you through the kidney function spotlight found in chapter 18. After watching this video, you should be able to summarize how the kidneys work. Have you ever heard of someone having blood in their urine and wondered why? This graphic summarizes how a nephron, the functional unit of the kidney, plays a critical role in regulating blood volume, composition and pressure. Notice the key at bottom left indicating the meaning of the different symbols in the diagram. Blood arrives at the nephron from an afferent arteriole and enters the renal corpuscle. Blood pressure forces solutes and a small amount of plasma through the glomerulus and into the capsular by a process called filtration, resulting in a fluid called filtrate. Most plasma proteins, as well as blood cells, are too large to pass through the filtration membrane and exit the nephron at an efferent arteriole with the remaining plasma. The filtrate then moves into the renal corpuscle and into the renal tubule. This fluid, now called tubular fluid, contains many essential solutes that must be reclaimed or they will be lost in the urine. Re-absorption, the process of solids moving from the renal tubule and into the vasa recta reclaims these essential solutes. Secretion adds additional waste products into the renal tubule from the vasa recta. Along the proximal convoluted tubule, or PCT, 60% to 70% of the tubular fluid is reabsorbed, along with glucose, amino acids, and other organic nutrients. Sodium, potassium, calcium, and other ions are actively reabsorbed by ion pumps. At the descending limb, additional water is reabsorbed and the tubular fluid becomes increasingly concentrated. The ascending limb, however, is impermeable to water and most solutes, so sodium and chloride ions are actively pumped out of the tubule. At the distal convoluted tubule, or DCT, active secretion and re-absorption occur. Two hormones, aldosterone and antidiuretic hormone, or ADH, function here as well as in the collecting ducts. As aldosterone levels rise, more sodium ions are reabsorbed and more potassium ions are secreted. As ADH levels rise, more water is reabsorbed resulting in a more concentrated tubular fluid. In summary, kidney function begins with the formation of filtrate in the renal corpuscles. As this fluid progresses through the renal tubule, it is modified by tubular re-absorption where water and ions move out of the tubular fluid. And then tubular secretion, where substances are transported into the tubular fluid. These processes can be active or passive, and some are hormonally regulated by hormones, aldosterone and ADH. So what? Why is it important to understand kidney function? Well, this knowledge is important for understanding kidney injuries. For example, although blood pressure is required to form filtrate, blood pressure that's too high can force red blood cells into the renal tubule and then into urine, a condition called hematuria. This can damage a kidney. And as it fails, ions that would ordinarily be removed can rise to toxic levels in the blood. Also, blunt trauma, such as from boxing or other contact sports, can also damage a kidney and cause hematuria. Kidney function is critical to our health, so it is important to control your blood pressure and wear protective sports equipment to lessen your risk of kidney damage.