Medical terms pertaining to renal function and structure are common in print and conversation. Knowledge of some of the more common terms is helpful in understanding kidney disorders and how they are treated. Such knowledge is basic for building health literacy in the area.
Creatinine clearance – Is a calculation of the amount of creatinine the kidneys excrete over a 24-hour period of time expressed in ml/min. It is useful for estimating glomerular filtration rate (GFR). The calculation is:
Creatinine clearance (L/minute) = urine creatinine concentration (ml/dL) x volume of urine (ml/24 hour) ÷ plasma concentration (mg/dl) x 1440 minutes/24 hour
In theory, the concentration of creatinine in the urine is the amount that freely filters across the glomerulus. The metabolism of creatine derived from muscle and meat in the diet determines the amount of creatinine filtered, since it is not subject to reabsorption or metabolism by the kidney. The red herring though is that secretion of creatinine by the proximal tubule makes up between 10% and 20% of the amount that shows up in the urine. Thus, based on the above formula creatinine clearance overestimates GFR to some degree. That disparity increases as kidney function declines.
Glomerular filtration rate (GFR) – Is the quantity of filtrate formed by the nephrons of both kidneys expressed in milliliters per minute. Direct measurement is most accurate but it requires a continuous infusion through a vein; multiple repeat blood and urine collections; and careful timing of blood sampling. This is cumbersome and invasive, so it is not practical for day-to-day use. Therefore, the methods in common use today are estimations based on calculations.
The more reliable type of estimation is based on the concentration of a substance in the blood and urine and the total volume of urine excreted in a 24-hour time period. A method based on creatinine is in most common usage since the body makes creatinine. A method using inulin is more accurate but requires an IV injection since it is a plant product which the body does not produce. Since both of these estimations require a 24-hour urine collection they are in less common use than in years past.
Although it is the least precise of the three methods of determining GFR, a calculation based on a single blood test is most popular today because it is convenient and cheap. Because it uses only the serum creatinine, it estimates creatinine clearance more so than GFR. Accordingly, it factors in variables that affect creatinine production. There are a number of equations for making the calculation, but two that are in common use. Both factor in age, gender and race. The other one also uses height and weight to calculate and adjust for body surface area.
Juxtaglomerular apparatus (JGA) is the part of a nephron that is involved in the control of the flow of blood into the nephron and the rate at which it filters blood. It sits between the glomerulus and distal convoluted tubule and makes contact with the latter. It is composed of the following:
- Juxtaglomerular cells – specialized smooth muscle cells containing granules which line the afferent arterial of each nephron. The granules secrete renin.
- Macula densa – thickened cells that line the distal convoluted tubule of each nephron and which act as sensors of sodium chloride (salt).
- Glomerular mesangial cells – Their function is not well known.
Mesangial cell – They are specialized cells around the small blood vessels of the kidney. Those located around capillaries of the glomerulus have three functions. They are structural support; regulation of filtration; and phagocytosis. Their effect on glomerular filtration is due to their ability to contract like smooth muscle. Those outside of the glomerulus are located between the afferent and efferent arterioles and the macula densa. They form a portion of the juxtaglomerular apparatus.
Renal artery – Is either of the two blood vessels (one on each side) which supplies blood flow to the kidney, the upper part of the ureter and part of the adrenal gland on its respective side. It is a branch of the abdominal aorta.
Renal calyx – Is the last part of the kidney from which urine exits before entering the urinary tract. It receives urine from one or more renal pyramids. There are two types; minor renal calyces join and drain into larger ones. Major calyces in turn join and drain into the renal pelvis. – Renal calyces Pl.
Renal pelvis – Is the funnel shaped cavity at the outlet of the kidney where urine flows before it enters the adjoining ureter. It arises from the convergence of renal calyces.
Renal pyramid – Is one of the several cone shaped masses of tissue that comprise the kidney medulla. The base of the pyramid is adjacent to the cortex of the kidney. The apex (peak) is adjacent to a renal calyx. Pyramids contain the loops of Henle and the collecting ducts of nephrons.
Renal tubular reabsorption – Is the transfer of a substance in renal tubular fluid back into blood flowing through peritubular capillaries. The purpose is to prevent its excretion in the urine. The substance is something the body needs. It might be water, ions nutrients, etc.
Renal tubular secretion – Is the transfer of a substance from blood in the peritubular capillaries to the lumen of a renal tubule near it. It is usually an active transport process which rids the body of only a few substances when they are in excess. The main ones are hydrogen ions (H+); potassium ions (K+); ammonium ions (NH4+); urea; creatinine; some hormones; and some drugs such as penicillin. The net result is their excretion in the urine.
Renal vein – Either of the two blood vessels (one on each side) which carries blood from the kidney and upper ureter on its respective side to the inferior vena cava. The one on the left also drains the left adrenal gland, the left testis of males and the left ovary of females.
Renin-angiotensin-aldosterone system (RAAS) – Is a cascade which regulates blood pressure and blood flow through the kidneys. It consists of hormones, enzymes, chemical reactions and a negative feedback loop. Activation of the cascade begins with secretion of renin by the kidney. The series of events that follow are:
- Renin acts in the liver to cause the conversion of angiotensinogen to angiotensin I.
- Angiotensin I then circulates to the lungs where the enzyme, angiotensin-converting enzyme (ACE), converts it to angiotensin II.
- Angiotensin II has multiple effects including constriction of arteries which raises blood pressure. It also causes constriction of the efferent arterioles of glomeruli preventing pressure within the structure from dropping too low, thus maintaining filtration.
- Angiotensin II also stimulates aldosterone secretion by the adrenal gland.
- Aldosterone causes increased reabsorption of salt and water by the distal tubules and collecting ducts of the kidney.
- As blood pressure rises it shuts off renin secretion via a negative feedback loop.
Serum osmolality – A measure of the number of dissolved particles per unit of water in serum. The higher it is the greater the number of particles and vice versa. The lower it is the less the amount of water and vice versa. It is a reflection of the hydration status of intracellular and extracellular compartments. Thus, it is an indicator of the hydration status of the total body. A specific lab test measures it. A calculation based on the concentration of sodium, BUN, and glucose in the serum estimates it.
Urea – It is a specific nitrogen containing compound that forms from protein metabolism. It is also the chief nitrogenous component of urine. The name of the blood test which measures it is blood urea nitrogen or BUN for short.
Urine osmolality – A measure of the number of dissolved particles per unit of water in urine. It is useful in diagnosing disorders of the kidney that affect its ability to concentrate urine and in assessing hydration status. It is more exact than the urine specific gravity.
Urine specific gravity – Is a measure of the density of all solutes in a sample of urine compared to water. It provides information on the kidney’s ability to concentrate urine. The latter reflects its ability to conserve water.