E10 Kidney

Topic

The kidney continuously filters plasma through approximately one million functional units called nephrons. Each nephron begins at the glomerulus, a cluster of capillaries where a portion of the plasma passes through a barrier that retains blood cells and large proteins, forming the filtrate that will eventually become urine.

The amount of plasma filtered depends on the balance of forces within the glomerular capillaries. Hydrostatic pressure promotes the movement of fluid into the filtration space, while the oncotic pressure exerted by plasma proteins opposes this movement. The combined effect of these forces across all nephrons determines the glomerular filtration rate, which is approximately 125 mL per minute in a healthy person. This means that the kidneys process a daily volume of filtrate that far exceeds the body's total plasma volume.

However, approximately 99% of this filtrate is subsequently reabsorbed by the renal tubules. The handling of any given substance can be described by its renal clearance—that is, the volume of plasma completely cleared of that substance per unit of time. If the clearance rate matches the glomerular filtration rate, the substance is filtered without significant tubular reabsorption or secretion. If the clearance is lower, reabsorption into the blood occurs; if it is higher, the substance is not only filtered but also secreted from the blood into the tubule. Thus, the kidney can selectively regulate the elimination or retention of various plasma components.

Glucose serves as a key example of tubular reabsorption. Normally, filtered glucose is recovered via specific transporters that have a maximum capacity. As long as the filtered amount remains below this capacity, virtually all of the glucose returns to the bloodstream and does not appear in the urine. When blood glucose levels exceed approximately 180 mg/dL, the filtered load increases sufficiently to saturate these transporters, resulting in glucosuria. Some medications used for diabetes deliberately inhibit these transporters, increasing urinary glucose excretion and thereby helping to lower blood glucose levels.

Creatinine is used to estimate kidney filtration function because it is freely filtered and undergoes limited tubular processing. Its blood concentration, combined with patient characteristics, allows for the estimation of the glomerular filtration rate without the need for urine collection. This estimated rate is used to classify chronic kidney disease into different stages and, when monitored over time, helps determine the rate at which kidney function declines.

In addition to filtering and selectively handling substances, the kidney controls the final concentration of urine. To do this, it establishes a concentration gradient in the renal medulla via the loop of Henle, whose limbs transport fluid in opposite directions, creating a multiplier mechanism. This gradient allows water to be recovered from the filtrate when the body needs to conserve it.

Effective use of this gradient depends on antidiuretic hormone. When present, this hormone promotes the insertion of water channels into the collecting duct, increasing its permeability. Water then leaves the filtrate, moving into the concentrated medulla and back into the circulation, resulting in a lower volume of more concentrated urine. When levels of antidiuretic hormone drop, the collecting duct remains less permeable to water, and a larger volume of dilute urine is excreted.

If the body fails to produce sufficient antidiuretic hormone, or if the kidney stops responding to it, diabetes insipidus develops. The kidney loses its ability to adequately concentrate urine, leading to a significant increase in urine volume; if the lost water is not replenished, severe dehydration can occur.

Overall, kidney function relies on glomerular filtration, tubular reabsorption and secretion, and the regulation of urine concentration. Clearance allows for the description of the renal fate of each substance; Glucosuria reveals the limits of tubular reabsorption; creatinine allows for the estimation of filtration capacity; and the medullary gradient, together with antidiuretic hormone, regulates water conservation. Through these mechanisms, nephrons maintain the stability of the internal environment's composition while eliminating products that must be removed from the body.

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