Renal physiology

Overview
Renal physiology is the study of the physiology of the kidneys.

Functions of the kidney
The functions of the kidney can be divided into two groups: secretion of hormones, and extracellular homeostasis.

Secretion of hormones

 * Secretion of erythropoietin, which regulates red blood cell production in the bone marrow.
 * Secretion of renin, which is a key part of the renin-angiotensin-aldosterone system.
 * Secretion of the active form of vitamin D, calcitriol, and prostaglandins.

Extracellular homeostasis
The kidney is responsible for maintaining a balance of several substances:

The body is very sensitive to its pH level. Outside the range of pH that is compatible with life, proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself. The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and losses of acid and base must be balanced. Acids are divided into "volatile acids" and "nonvolatile acids". See also titratable acid.

The major homeostatic control point for maintaining this stable balance is renal excretion. The kidney is directed to excrete or retain sodium via the action of aldosterone, antidiuretic hormone (ADH, or vasopressin), atrial natriuretic peptide (ANP), and other hormones. Abnormal ranges of the fractional excretion of sodium can imply acute tubular necrosis or glomerular dysfunction.

Mechanisms
The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion.

Filtration
The blood is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma (except that the ultrafiltrate has negligible plasma proteins) to enter Bowman's space. Filtration is driven by Starling forces.

The ultrafiltrate is passed through, in turn, the proximal tubule, the loop of Henle, the distal convoluted tubule, and a series of collecting ducts to form urine.

Renal plasma threshold
The renal plasma threshold is the minimum plasma concentration of a substance that results in the excretion of that substance in the urine.

For example, the renal plasma threshold for glucose is 180 to 200 mg per 100 ml. Glycosuria (sugar in urine) results when the plasma glucose concentration reaches and exceeds the renal plasma threshold of glucose. When the plasma glucose concentration is very high, the filtered glucose can saturate the carriers and reach the transport maximum of that molecule. Any amount past the transport maximum will continue through the renal tubules and be excreted in the urine.

Reabsorption
Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the intestines).

Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport.

Indirect reabsorption
In some cases, reabsorption is indirect. For example, bicarbonate (HCO3-) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. It begins with the active secretion of a hydrogen ion (H+) into the tubule fluid via a Na/H exchanger:


 * In the lumen
 * The H+ combines with HCO3- to form carbonic acid (H2CO3)
 * Luminal carbonic anhydrase enzymatically converts H2CO3 into H2O and CO2
 * CO2 freely diffuses into the cell
 * In the epithelial cell
 * Cytoplasmic carbonic anhydrase converts the CO2 and H2O (which is abundant in the cell) into H2CO3
 * H2CO3 readily dissociates into H+ and HCO3-
 * HCO3- is facilitated out of the cell's basolateral membrane

Hormones
Some key regulatory hormones for reabsorption include:
 * aldosterone, which stimulates active sodium reabsorption
 * antidiuretic hormone, which stimulates passive water reabsorption

Both hormones exert their effects principally on the collecting ducts.

Secretion
Tubular secretion is the transfer of materials from peritubular capillaries to renal tubular lumen. Tubular secretion is caused mainly by active transport.

Usually only a few substances are secreted. These substances are present in great excess, or are natural poisons.

Measurement of renal function
A simple means of estimating renal function is to measure pH, blood urea nitrogen, creatinine, and basic electrolytes (including sodium, potassium, chloride, and bicarbonate). As the kidney is the most important organ in controlling these values, any derangement in these values could suggest renal impairment.

There are several more formal tests and ratios involved in estimating renal function: