Calculation
Since urine flow is determined by the rate at which plasma is cleared of solutes and water (as discussed above), urine flow (V) is given as the sum of osmolar (Cosm) and free water clearance (CH2O):
Rearranging yields CH2O
Since osmolar clearance is given as the product of urine flow rate and the ratio of urine to plasma osmolality, this is commonly represented as
For example, for an individual with a urine osmolality of 140 mOsm/L, plasma osmolality of 280 mOsm/L, and a urine production of 4 ml/min, the free water clearance is 2 ml/min, obtained from
Renal functions include maintaining an acid-base balance; regulating fluid balance; regulating sodium, potassium, and other electrolytes; clearing toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer, the German anatomist Friedrich Gustav Jakob Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.
Assessment of kidney function occurs in different ways, using the presence of symptoms and signs, as well as measurements using urine tests, blood tests, and medical imaging.
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by excessive unsuppressible release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source. Unsuppressed ADH causes an unrelenting increase in solute-free water being returned by the tubules of the kidney to the venous circulation.
In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy.
In pharmacology, clearance is a pharmacokinetic measurement of the volume of plasma from which a substance is completely removed per unit time. Usually, clearance is measured in L/h or mL/min. The quantity reflects the rate of drug elimination divided by plasma concentration. Excretion, on the other hand, is a measurement of the amount of a substance removed from the body per unit time. While clearance and excretion of a substance are related, they are not the same thing. The concept of clearance was described by Thomas Addis, a graduate of the University of Edinburgh Medical School.
The urea reduction ratio (URR), is a dimensionless number used to quantify dialysis treatment adequacy.
Standardized Kt/V, also std Kt/V, is a way of measuring (renal) dialysis adequacy. It was developed by Frank Gotch and is used in the United States to measure dialysis. Despite the name, it is quite different from Kt/V. In theory, both peritoneal dialysis and hemodialysis can be quantified with std Kt/V.
The trans-tubular potassium gradient (TTKG) is an index reflecting the conservation of potassium in the cortical collecting ducts (CCD) of the kidneys. It is useful in diagnosing the causes of hyperkalemia or hypokalemia. The TTKG estimates the ratio of potassium in the lumen of the CCD to that in the peritubular capillaries.
Osmol gap in medical science is the difference between measured serum osmolality and calculated serum osmolality.
Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution. The osmolarity of a solution is usually expressed as Osm/L, in the same way that the molarity of a solution is expressed as "M". Whereas molarity measures the number of moles of solute per unit volume of solution, osmolarity measures the number of osmoles of solute particles per unit volume of solution. This value allows the measurement of the osmotic pressure of a solution and the determination of how the solvent will diffuse across a semipermeable membrane (osmosis) separating two solutions of different osmotic concentration.
Plasma osmolality measures the body's electrolyte–water balance. There are several methods for arriving at this quantity through measurement or calculation.
In the physiology of the kidney, renal blood flow (RBF) is the volume of blood delivered to the kidneys per unit time. In humans, the kidneys together receive roughly 25% of cardiac output, amounting to 1.2 - 1.3 L/min in a 70-kg adult male. It passes about 94% to the cortex. RBF is closely related to renal plasma flow (RPF), which is the volume of blood plasma delivered to the kidneys per unit time.
A countercurrent mechanism system is a mechanism that expends energy to create a concentration gradient.
The fractional excretion of sodium (FENa) is the percentage of the sodium filtered by the kidney which is excreted in the urine. It is measured in terms of plasma and urine sodium, rather than by the interpretation of urinary sodium concentration alone, as urinary sodium concentrations can vary with water reabsorption. Therefore, the urinary and plasma concentrations of sodium must be compared to get an accurate picture of kidney clearance. In clinical use, the fractional excretion of sodium can be calculated as part of the evaluation of acute kidney failure in order to determine if hypovolemia or decreased effective circulating plasma volume is a contributor to the kidney failure.
Volume contraction is a decrease in the volume of body fluid, including the dissolved substances that maintain osmotic balance (osmolytes). The loss of the water component of body fluid is specifically termed dehydration.
Urine osmolality is a measure of urine concentration, in which large values indicate concentrated urine and small values indicate diluted urine. Consumption of water affects the osmolality of urine. Osmolality is measured by osmometer, which evaluates the freezing point depression of a solution and supplies results as milliosmoles per kilogram of water while specific gravity is measured by colorimetric strips, refractometer, hydrometer and pyknometer. In healthy humans with restricted fluid intake, urine osmolality should be greater than 800 mOsm/kg, while a 24-hour urine osmolality should average between 500 and 800 mOsm/kg.
Potomania is a specific hypo-osmolality syndrome related to massive consumption of beer, which is poor in solutes and electrolytes. With little food or other sources of electrolytes, consumption of large amounts of beer or other dilute alcoholic drinks leads to electrolyte disturbances, where the body does not have enough nutrients known as electrolytes, namely sodium, potassium, and magnesium. The symptoms of potomania are similar to other causes of hyponatremia and include dizziness, muscular weakness, neurological impairment and seizures, all related to hyponatremia and hypokalaemia. While the symptoms of potomania are similar to other causes of hyponatremia and acute water intoxication, it should be considered an independent clinical entity because of its often chronic nature of onset, pathophysiology, and presentation of symptoms.
In pharmacology the elimination or excretion of a drug is understood to be any one of a number of processes by which a drug is eliminated from an organism either in an unaltered form or modified as a metabolite. The kidney is the main excretory organ although others exist such as the liver, the skin, the lungs or glandular structures, such as the salivary glands and the lacrimal glands. These organs or structures use specific routes to expel a drug from the body, these are termed elimination pathways:
