Renal physiology - AbsoluteAstronomy.com

Renal physiology is the study of the physiology

Physiology

Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

of the kidney

Kidney

The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

. This encompasses all functions of the kidney, including reabsorption of glucose

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s, and other small molecules; regulation of sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

, potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

, and other electrolyte

Electrolyte

In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

s; regulation of fluid balance

Fluid balance

Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...

and blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

; maintenance of acid-base balance; the production of various hormone

Hormone

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

s including erythropoietin

Erythropoietin

Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...

, and the activation of vitamin D

Vitamin D

Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

Much of renal physiology is studied at the level of the nephron

Nephron

The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....

, the smallest functional unit of the kidney. Each nephron begins with a filtration component that filters blood entering the kidney. This filtrate then flows along the length of the nephron, which is a tubular structure lined by a single layer of specialized cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

and surrounded by capillaries

Capillary

Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...

. The major functions of these lining cells are the reabsorption of water and small molecules from the filtrate into the blood, and the secretion of wastes from the blood into the urine.

Proper function of the kidney requires that it receives and adequately filters blood. This is performed at the microscopic level by many hundreds of thousands of filtration units called renal corpuscle

Renal corpuscle

In the kidney, a renal corpuscle is the initial blood-filtering component of a nephron. It consists of two structures: a glomerulus and a Bowman's capsule. The glomerulus is a small tuft of capillaries containing two cell types. Endothelial cells, which have large fenestrae, are not covered by...

s, each of which is composed of a glomerulus

Glomerulus

A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...

and a Bowman's capsule

Bowman's capsule

The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...

. A global assessment of renal function

Renal function

Renal function, in nephrology, is an indication of the state of the kidney and its role in renal physiology. Glomerular filtration rate describes the flow rate of filtered fluid through the kidney...

is often ascertained by estimating the rate of filtration, called the glomerular filtration rate (GFR).

Functions of the kidney

The functions of the kidney can be divided into two groups: secretion of hormones, and extracellular homeostasis of pH and water. The nephron is the functional unit of the kidney.

Secretion of hormones

Secretion of erythropoietin
Erythropoietin
Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...

, which regulates red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...

production in the bone marrow
Bone marrow
Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

.
Secretion of renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

, which is a key part of the renin-angiotensin-aldosterone system.
Secretion of the active form of vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

(calcitriol
Calcitriol
Calcitriol , also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D3, is the hormonally active form of vitamin D with three hydroxyl groups...

) and prostaglandin
Prostaglandin
A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....

s.

Extracellular homeostasis

The kidney is responsible for maintaining a balance of the following substances:

Substance	Description	Proximal tubule Proximal tubule The proximal tubule is the portion of the duct system of the nephron of the kidney which leads from Bowman's capsule to the loop of Henle.-Structure and appearance:...	Loop of Henle Loop of Henle 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 F. G. J...	Distal tubule	Collecting duct >- \| Glucose Renal glucose reabsorption Renal glucose reabsorption is the part of renal physiology that deals with the retrieval of filtered glucose, preventing it from disappearing from the body through the urine....	If glucose is not reabsorbed by the kidney, it appears in the urine, in a condition known as glycosuria Glycosuria Glycosuria or glucosuria is the excretion of glucose into the urine. Ordinarily, urine contains no glucose because the kidneys are able to reclaim all of the filtered glucose back into the bloodstream. Glycosuria is nearly always caused by elevated blood glucose levels, most commonly due to... . This is associated with diabetes mellitus Diabetes mellitus Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced... .	reabsorption (almost 100%) via sodium-glucose transport proteins Sodium-glucose transport proteins Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa of the small intestine and the proximal tubule of the nephron . They contribute to renal glucose reabsorption... (apical) and GLUT Glucose transporter Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose over a plasma membrane. Because glucose is a vital source of energy for all life these transporters are present in all phyla... (basolateral).	-	-	>- \| Oligopeptides Renal oligopeptide reabsorption Renal oligopeptide reabsorption is the part of renal physiology that deals with the retrieval of filtered oligopeptides, preventing them from disappearing from the body through the urine.... , proteins Renal protein reabsorption Renal protein reabsorption is the part of renal physiology that deals with the retrieval of filtered proteins, preventing them from disappearing from the body through the urine.... , and amino acids	All are reabsorbed nearly completely.	reabsorption	-	-	>- \| Urea Renal urea handling Renal urea handling is the part of renal physiology that deals with the reabsorption and secretion of urea. Movement of large amounts of urea across cell membranes is made possible by urea transporter proteins....	Regulation of osmolality Osmole (unit) Osmolarity is the measure of solute concentration, defined as the number of osmoles of solute per litre 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"... . Varies with ADH	reabsorption (50%) via passive transport Passive transport Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...	secretion	-	>- \| Sodium Renal sodium reabsorption Renal reabsorption of sodium is a part of renal physiology. It uses Na-H antiport, Na-glucose symport, sodium ion channels . It is stimulated by angiotensin II and aldosterone, and inhibited by atrial natriuretic peptide....	Uses Na-H antiport Sodium-hydrogen antiporter The sodium–hydrogen antiporter or sodium–hydrogen exchanger is a membrane protein found in many cells, and especially in those of the nephron of the kidney... , Na-glucose symport, sodium ion channel Sodium ion channel Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions through a cell's plasma membrane. They are classified according to the trigger that opens the channel for such ions, i.e... s (minor)	reabsorption (65%, isosmotic)	reabsorption (25%, thick ascending, Na-K-2Cl symporter Na-K-2Cl symporter The Na-K-Cl cotransporter is a protein that aids in the active transport of sodium, potassium, and chloride into and out of cells. There are two varieties, or isoforms, of this membrane transport protein, called NKCC1 and NKCC2. NKCC1 is widely distributed throughout the body; it has important... )	reabsorption (5%, sodium-chloride symporter Sodium-chloride symporter The sodium-chloride symporter is a cotransporter in the kidney which has the function of reabsorbing sodium and chloride ions from the tubular fluid into the cells of the distal convoluted tubule of the nephron.It is a member of the SLC12... )	aldosterone Aldosterone Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that... via ENaC ENAC ENAC may refer to: - a French civil aeronautics academy* Epithelial sodium channel * Italian Civil Aviation Authority... >- \| Chloride Renal chloride reabsorption Renal reabsorption of chloride is a part of renal physiology, in order not to lose too much chloride in the urine.-Overview table:...	Usually follows sodium Sodium Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride... . Active (transcellular) and passive (paracellular)	reabsorption	reabsorption (thin ascending, thick ascending, Na-K-2Cl symporter Na-K-2Cl symporter The Na-K-Cl cotransporter is a protein that aids in the active transport of sodium, potassium, and chloride into and out of cells. There are two varieties, or isoforms, of this membrane transport protein, called NKCC1 and NKCC2. NKCC1 is widely distributed throughout the body; it has important... )	reabsorption (sodium-chloride symporter Sodium-chloride symporter The sodium-chloride symporter is a cotransporter in the kidney which has the function of reabsorbing sodium and chloride ions from the tubular fluid into the cells of the distal convoluted tubule of the nephron.It is a member of the SLC12... )	>- \| Water Water Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...	Uses aquaporin Aquaporin Aquaporins are proteins embedded in the cell membrane that regulate the flow of water.Aquaporins are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells.... water channels. See also diuretic Diuretic A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :... .	absorbed osmotically along with solutes	reabsorption (descending)	-	arginine vasopressin receptor 2 Arginine vasopressin receptor 2 Arginine vasopressin receptor 2 is a protein that acts as receptor for arginine vasopressin. AVPR2 belongs to the subfamily of G-protein-coupled receptors... ) >- \| Bicarbonate Bicarbonate In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...	Helps maintain acid-base balance.	reabsorption (80-90%)	reabsorption (thick ascending)	-	band 3 Band 3 Anion Exchanger 1 or Band 3 is a phylogenetically preserved transport protein responsible for mediating the exchange of chloride for bicarbonate across a plasma membrane. Functionally similar members of the AE clade are AE2 and AE3.It is ubiquitous throughout the vertebrates... and pendrin Pendrin Pendrin also known as sodium-independent chloride/iodide transporter is a protein that in humans is encoded by the SLC26A4 gene .- Function :Pendrin is an ion exchanger found in the cortical collecting duct.... ) >- \| Protons	Uses vacuolar H+ATPase	-	-	-	>- \| Potassium Potassium Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...	Varies upon dietary needs.	reabsorption (65%)	reabsorption (20%, thick ascending, Na-K-2Cl symporter Na-K-2Cl symporter The Na-K-Cl cotransporter is a protein that aids in the active transport of sodium, potassium, and chloride into and out of cells. There are two varieties, or isoforms, of this membrane transport protein, called NKCC1 and NKCC2. NKCC1 is widely distributed throughout the body; it has important... )	-	Na+/K+-ATPase Na+/K+-ATPase Na+/K+-ATPase is an enzyme located in the plasma membrane in all animals.- Sodium-potassium pumps :Active transport is responsible for cells containing relatively high... , increased by aldosterone Aldosterone Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that... ), or reabsorption (rare, hydrogen potassium ATPase Hydrogen potassium ATPase Gastric hydrogen potassium ATPase is also known as H+/K+ ATPase- Function and location :The gastric hydrogen potassium ATPase or H+/K+ ATPase is the proton pump of the stomach and, as such, is the enzyme primarily responsible for the acidification of the stomach contents... ) >- \| Calcium Calcium Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...	Uses calcium ATPase Calcium ATPase Calcium ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The calcium ATPase are:Plasma membrane Ca2+ ATPase Sarcoplasmic reticulum Ca2+ ATPase - Plasma membrane Ca2+ ATPase :... , sodium-calcium exchanger Sodium-calcium exchanger The sodium-calcium exchanger is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions...	reabsorption	reabsorption (thick ascending) via passive transport Passive transport Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...	-	>- \| Magnesium Magnesium Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...	Calcium and magnesium compete, and an excess of one can lead to excretion of the other.	reabsorption	reabsorption (thick ascending)	reabsorption	>- \| Phosphate Phosphate A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...	Excreted as titratable acid Titratable acid Titratable acid is a term to describe acids such as phosphoric acid, sulfuric acid which are involved in renal physiology.It is used to explicitly exclude ammonium as a source of acid, and is part of the calculation for net acid excretion.... .	reabsorption (85%) via sodium/phosphate cotransporter Sodium/phosphate cotransporter The sodium/phosphate cotransporter is a protein found in the proximal tubule of the nephron. It is responsible for reabsorbing phosphate that has been filtered out at the glomerulus.-See also:* Renal physiology* Cotransporter... . Inhibited by parathyroid hormone Parathyroid hormone Parathyroid hormone , parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids... .	-	-	>- \| Carboxylate		reabsorption (100%) via carboxylate transporter Carboxylate transporter A carboxylate transporter is a membrane transport protein that transports carboxylate.They are responsible for the reabsorption of filtered carboxylate in renal physiology, resulting in a 100% reabsorption in the proximal tubule.-In proximal tubule:... s.	-	-	-

The body is very sensitive to its pH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. 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

Acid-base homeostasis

Acid–base homeostasis is the part of human homeostasis concerning the proper balance between acids and bases, in other words, the pH. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it...

by regulating the pH of the blood plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

. Gains and losses of acid and base must be balanced. Acids are divided into "volatile acids" and "nonvolatile acids". See also titratable acid

Titratable acid

Titratable acid is a term to describe acids such as phosphoric acid, sulfuric acid which are involved in renal physiology.It is used to explicitly exclude ammonium as a source of acid, and is part of the calculation for net acid excretion....

.

The major homeostatic

Homeostasis

Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

control point for maintaining this stable balance is renal excretion. The kidney is directed to excrete or retain sodium via the action of aldosterone

Aldosterone

Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

, antidiuretic hormone (ADH, or vasopressin), atrial natriuretic peptide

Atrial natriuretic peptide

Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...

(ANP), and other hormones. Abnormal ranges of the fractional excretion of sodium can imply acute tubular necrosis

Acute tubular necrosis

Acute tubular necrosis or is a medical condition involving the death of tubular cells that form the tubule that transports urine to the ureters while reabsorbing 99% of the water . Tubular cells continually replace themselves and if the cause of ATN is removed then recovery is likely...

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, whose sum is renal excretion. That is:

Urinary excretion rate = Filtration rate - Reabsorption rate + Secretion rate

Filtration

The blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

is filtered by nephron

Nephron

s, the functional units of the kidney. Each nephron begins in a renal corpuscle

Renal corpuscle

, which is composed of a glomerulus

Glomerulus

enclosed in a Bowman's capsule

Bowman's capsule

. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration

Ultrafiltration (renal)

In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent...

, 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 convoluted tubule, the loop of Henle

Loop of Henle

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 F. G. J...

, the distal convoluted tubule

Distal convoluted tubule

The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.- Physiology :It is partly responsible for the regulation of potassium, sodium, calcium, and pH...

, and a series of collecting ducts to form urine

Urine

Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

Reabsorption

Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid

Tubular fluid

Tubular fluid is the fluid in the tubules of the kidney. It starts as a renal ultrafiltrate in the glomerulus, changes composition through the nephron, and ends up as urine leaving through the ureters.-Composition table:...

and transported into the blood. It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the intestine

Intestine

In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

s).

Reabsorption is a two-step process beginning with the active

Active transport

Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...

or passive

Passive transport

Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...

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

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

, and active transport

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
  Carbonic anhydrase
  The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
  
  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
Aldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

, which stimulates active sodium reabsorption (and water as a result)
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

Peritubular capillaries

In the renal system, peritubular capillaries are tiny blood vessels that travel alongside nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron....

to renal tubular lumen. Tubular secretion is caused mainly by active transport

Active transport

.

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

Many drugs are eliminated

Clearance (medicine)

In medicine, the clearance is a measurement of the renal excretion ability. Although clearance may also involve other organs than the kidney, it is almost synonymous with renal clearance or renal plasma clearance. Each substance has a specific clearance that depends on its filtration characteristics...

by tubular secretion. Further reading: Table of medication secreted in kidney

Measurement of renal function

A simple means of estimating renal function is to measure pH

, blood urea nitrogen

Blood urea nitrogen

The blood urea nitrogen test is a measure of the amount of nitrogen in the blood in the form of urea, and a measurement of renal function. Urea is a by- product from metabolism of proteins by the liver and is removed from the blood by the kidneys.-Physiology:The liver produces urea in the urea...

, creatinine

Creatinine

Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...

, and basic electrolyte

Electrolyte

s (including sodium

Sodium

, potassium

Potassium

, chloride

Chloride

The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

, and bicarbonate

Bicarbonate

In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

). 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:

Measurement	Calculation	Details >- \| renal plasma flow		blood plasma Blood plasma Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid... delivered to the kidney per unit time. PAH clearance PAH clearance Para aminohippurate clearance or PAH clearance is a method used in renal physiology to measure renal plasma flow, which, in turn, is a measure of renal function.... is a renal analysis method used to provide an estimate. Approximately 625 ml/min. >- \| renal blood flow Renal blood flow In the physiology of the kidney, renal blood flow is the volume of blood delivered to the kidneys per unit time. In humans, the kidneys together receive roughly 22% of cardiac output, amounting to 1.1 L/min in a 70-kg adult male...	(HCT is hematocrit Hematocrit The hematocrit or packed cell volume or erythrocyte volume fraction is the percentage of the concentration of red blood cells in blood. It is normally about 45% for men and 40% for women... )	blood Blood Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.... delivered to the kidney per unit time. In humans, the kidneys together receive roughly 20% of cardiac output, amounting to 1 L/min in a 70-kg adult male. >- \| glomerular filtration rate	(estimation using creatinine clearance)	renal Kidney The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and... glomerular capillaries into the Bowman's capsule Bowman's capsule The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac... per unit time. Estimated using inulin Inulin Inulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes... . Usually a creatinine clearance test is performed but other markers, such as the plant polysaccharide inulin or radiolabelled EDTA, may be used as well. >- \| filtration fraction Filtration fraction In renal physiology, the filtration fraction is the ratio of the glomerular filtration rate to the renal plasma flow .Filtration Fraction , FF = GFR/RPF...		>- \| anion gap Anion gap The anion gap is the difference in the measured cations and the measured anions in serum, plasma, or urine. The magnitude of this difference in the serum is often calculated in medicine when attempting to identify the cause of metabolic acidosis...	AG = [Na⁺] − ([Cl⁻] + [HCO₃⁻])	+ (usually), Ca²⁺, H₂PO₄⁻. Aids in the differential diagnosis Differential diagnosis A differential diagnosis is a systematic diagnostic method used to identify the presence of an entity where multiple alternatives are possible , and may also refer to any of the included candidate alternatives A differential diagnosis (sometimes abbreviated DDx, ddx, DD, D/Dx, or ΔΔ) is a... of metabolic acidosis Metabolic acidosis In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, i.e., blood pH is low due to increased production of hydrogen by the body or the... >- \| Clearance Clearance (medicine) In medicine, the clearance is a measurement of the renal excretion ability. Although clearance may also involve other organs than the kidney, it is almost synonymous with renal clearance or renal plasma clearance. Each substance has a specific clearance that depends on its filtration characteristics... (other than water)	where U = concentration, V =urine volume / time, U*V = urinary excretion, and P = plasma concentration	>- \| free water clearance Free water clearance In the physiology of the kidney, free water clearance is the volume of blood plasma that is cleared of solute-free water per unit time. An example of its use is in the determination of an individual's state of hydration.-Overview:...	or	blood plasma Blood plasma Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid... that is cleared of solute Solution In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :... -free water Water Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a... per unit time. >- \| Net acid excretion Net acid excretion In renal physiology, net acid excretion is the net amount of acid excreted in the urine per unit time. Its value depends on urine flow rate, urine acid concentration, and the concentration of bicarbonate in the urine...		Net amount of acid excreted in the urine Urine Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream... per unit time

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