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41.4C: Kidney Function and Physiology

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  • Urine is a byproduct of the osmoregulatory function of kidneys, which filter blood, reabsorb water and nutrients, and secrete wastes.


    Outline the process by which kidneys filter blood, reabsorb nutrients and water, and produce urine


    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

    Blood Filtration and Nutrient and Water Reabsorption

    Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus. Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called glomerular filtration. Second, the renal tubules collect the filtrate. Most of the solutes are reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network.


    Nephron structure: Each part of the nephron performs a different function in filtering waste and maintaining homeostatic balance.

    Finally, some substances, such as electrolytes and drugs, are removed from blood through the peritubular capillary network into the distal convoluted tubule or collecting duct. Urine is a collection of substances that have not been reabsorbed during glomerular filtration or tubular reabsorbtion.

    Glomerular Filtration

    The formation of urine occurs through three steps: glomerular filtration, tubular reabsorption, and tubular secretion. 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. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. 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 and Secretion

    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. Sodium (Na+) is the most abundant ion; most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na+ is actively transported out of the tubule, water follows to even out the osmotic pressure. 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. Kidneys’ osmolarity of body fluids is maintained at 300 milliosmole (mOsm).

    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. Thus, as filtrate enters the ascending limb, Na+ and Cl- ions exit through ion channels present in the cell membrane. Further up, Na+ is actively transported out of the filtrate and Cl- follows. Osmolarity is given in units of milliosmoles per liter (mOsm/L).

    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.



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