Acid-base homeostasis

Acid-base homeostasis

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Acid–base homeostasis is the part of human homeostasis
Human homeostasis
Human homeostasis is derived from the Greek, homeo or "same", and stasis or "stable" and means remaining stable or remaining the same.The human body manages a multitude of highly complex interactions to maintain balance or return systems to functioning within a normal range...

 concerning the proper balance between acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

s and bases, in other words, the pH
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...

. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it. Outside the acceptable range of pH, protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s are denatured and digested, enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s lose their ability to function, and death
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 may occur.

Mechanism


The body's acid–base balance is tightly regulated. Several buffering agent
Buffering agent
A buffering agent is a weak acid or base used to maintain the acidity of a solution at a chosen value. The function of a buffering agent is to prevent a rapid change in pH when acids or bases are added to the solution. Buffering agents have variable properties—some are more soluble than others;...

s that reversibly bind hydrogen ions and impede any change in pH exist. Extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...

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

 and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, whereas protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s and 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...

 act as intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...

 buffers. The bicarbonate buffering system
Bicarbonate buffering system
The bicarbonate buffering system is an important buffer system in the acid-base homeostasis of living things, including humans. As a buffer, it tends to maintain a relatively constant plasma pH and counteract any force that would alter it....

 is especially key, as carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 (CO2) can be shifted through carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 (H2CO3) to hydrogen ions and bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 (HCO3-) as shown below.


Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation
Ventilation (physiology)
In respiratory physiology, ventilation is the rate at which gas enters or leaves the lung. It is categorized under the following definitions:-Sample values:...

. This alters the concentration of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 in the blood, shifting the above reaction according to Le Chatelier's principle
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...

, which in turn alters the pH. For instance, if the blood pH drops too low (acidemia), the body will compensate by increasing breathing, expelling CO2, and shifting the above reaction to the left such that less hydrogen ions are free; thus the pH will rise back to normal. For alkalemia, the opposite occurs.

The kidneys are slower to compensate, but renal physiology
Renal physiology
Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including reabsorption of glucose, amino acids, and other small molecules; regulation of sodium, potassium, and other electrolytes; regulation of fluid balance and blood pressure;...

 has several powerful mechanisms to control pH by the excretion of excess acid or base. In responses to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH3 buffer. In responses to alkalosis, the kidney may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine
Glutamine
Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...

 metabolism and ammonia excretion.



Imbalance


Acid–base imbalance occurs when a significant insult causes the blood pH to shift out of the normal range (7.35 to 7.45). In the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

, the normal range differs based on which umbilical vessel is sampled (umbilical vein
Umbilical vein
The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta to the growing fetus.The blood pressure inside the umbilical vein is approximately 20 mmHg.-Development:...

 pH is normally 7.25 to 7.45; umbilical artery
Umbilical artery
The umbilical artery is a paired artery that is found in the abdominal and pelvic regions. In the fetus, it extends into the umbilical cord.-Umbilical arteries in the fetus:...

 pH is normally 7.18 to 7.38). An excess of acid in the blood is called acidemia and an excess of base is called alkalemia. The process that causes the imbalance is classified based on the etiology
Etiology
Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....

 of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis). There are four basic processes: 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...

, respiratory acidosis
Respiratory acidosis
Respiratory acidosis is a medical condition in which decreased ventilation causes increased blood carbon dioxide concentration and decreased pH ....

, metabolic alkalosis
Metabolic alkalosis
Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range . This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct result of increased bicarbonate concentrations.-Terminology:*Alkalosis...

, and respiratory alkalosis
Respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH...

. One or a combination may occur at any given time.

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