What Is the Principal Cation of the Ecf??

In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight;[citation needed] women and the obese typically have a lower percentage than lean men.[1] Extracellular fluid makes up about one-third of body fluid, the remaining two-thirds is intracellular fluid within cells.[2] The main component of the extracellular fluid is the interstitial fluid that surrounds cells.

The ECF composition is therefore crucial for their normal functions, and is maintained by a number of homeostatic mechanisms involving negative feedback . The volume of body fluid, blood glucose , oxygen , and carbon dioxide levels are also tightly homeostatically maintained.

The volume of extracellular fluid in a young adult male of 70 kg (154 lbs) is 20% of body weight about fourteen litres. The other major component of the ECF is the intravascular fluid of the circulatory system called blood plasma . The volume of extracellular fluid in a young adult male of 70 kg, is 20% of body weight about fourteen litres.

[6] Plasma and interstitial fluid are very similar because water, ions, and small solutes are continuously exchanged between them across the walls of capillaries, through pores and . [11] Substances in the ECF include dissolved gases, nutrients, and electrolytes , all needed to maintain life. Since carbon dioxide is about 20 times more soluble in water than oxygen, it can relatively easily diffuse in the aqueous fluid between cells and blood.

If hemoglobin in erythrocytes is the main transporter of oxygen in the blood , plasma lipoproteins may be its only carrier in the ECF. This causes the cell membrane to temporarily depolarize (lose its electrical charge) forming the basis of action potentials. The sodium ions in the ECF also play an important role in the movement of water from one body compartment to the other.

[21][26] The voltage gated sodium ion channels in the cell membranes of nerves and muscle have an even greater sensitivity to changes in the ECF ionized calcium concentration. Relatively small decreases in the plasma ionized calcium levels ( hypocalcemia ) cause these channels to leak sodium into the nerve cells or axons, making them hyper-excitable, thus causing spontaneous muscle spasms ( tetany ) and paraesthesia (the sensation of “pins and needles”) of the extremities and round the mouth. In addition, the pH of the ECF affects the proportion of the total amount of calcium in the plasma which occurs in the free, or ionized form, as opposed to the fraction that is bound to protein and phosphate ions.

This means that nutrients can be secreted into the ECF in one place (e.g. the gut, liver, or fat cells) and will, within about a minute, be evenly distributed throughout the body. Oxygen taken up by the lungs from the alveolar air is also evenly distributed at the correct partial pressure to all the cells of the body. Waste products are also uniformly spread to the whole of the ECF, and are removed from this general circulation at specific points (or organs), once again ensuring that there is generally no localized accumulation of unwanted compounds or excesses of otherwise essential substances (e.g. sodium ions, or any of the other constituents of the ECF).

However, this plasma is confined within the waterproof walls of the venous tubes, and therefore does not affect the interstitial fluid in which the body’s cells live. From the left atrium onward, to every organ in the body, the normal, homeostatically regulated values of all of the ECF‘s components are therefore restored. The tissue fluid is entering the blind ends of lymph capillaries (shown as deep green arrows)The arterial blood plasma, interstitial fluid and lymph interact at the level of the blood capillaries .

In addition the lymph which drains the small intestine contains fat droplets called chylomicrons after the ingestion of a fatty meal.

What is the principal extracellular cation quizlet?

Sodium (Na+) the principal extracellular cation needed to regulate the distribution of water throughout the body in the intravascular and interstitial fluid compartments. Potassium (K+) 98% of all the body’s potassium is found inside the cells of the body, making it the principal intracellular cation.

What is ECF ion?

Intracellular fluid (ICF) contains predominantly K+, while extracellular fluid (ECF) contains predominantly Na+ (Table 2.1), and it is this difference in ionic concentration between ICF and ECF that, first, allows the generation of the RMP and, second, provides the battery that drives the action potential.

– ADH is released in response to such stimuli as a rise in the concentration of electrolytes in the blood or a fall in blood volume or pressure. These stimuli occur when a person sweats excessively or is dehydrated.

Extracellular Fluid (ECF) – all fluids found outside the cells, comprises 40% of all body fluids Interstitial Fluid – 80% of ECF is found in localized areas: lymph, cerebrospinal fluid, synovial fluid, aqueous humor and vitreous body of eyes, between serous and visceral membranes, glomerular filtrate of kidneys. Blood Plasma – 20% of ECF found in circulatory system

Water is the main component of all body fluids making up 45-75% of the total body weight. Sources of water include Ingested foods and liquids (preformed water) Metabolic water produced during dehydration synthesis of anabolism. Solutes are broadly classified into Electrolytes are inorganic salts, all acids and bases, and some proteins Nonelectrolytes examples include glucose, lipids, creatinine, and urea Electrolytes have greater osmotic power than nonelectrolytes Water moves according to osmotic gradients

Electrolyte Composition of Body Fluids Each fluid compartment of the body has a distinctive pattern of electrolytes Extracellular Fluids ECFs are similar except for the high protein content of plasma Sodium (Na + ) is the major cation Chloride (Cl – )is the major anion

Intracellular Fluids Have low sodium and chloride Potassium (K + ) is the chief cation Phosphate (PO4 – ) is the chief anion Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites This reflects the activity of cellular ATP-dependent sodium-potassium pumps Electrolytes determine the chemical and physical reactions of fluids Ion fluxes are restricted and move selectively by active transport Nutrients, respiratory gases, and wastes move unidirectionally Plasma is the only fluid that circulates throughout the body and links external and internal environments Osmolalities of all body fluids are equal; changes in solute concentrations are quickly followed by osmotic changes Compartmental exchange is regulated by osmotic and hydrostatic pressures Net leakage of fluid from the blood is picked up by lymphatic vessels and returned to the bloodstream Exchanges between interstitial and intracellular fluids due to the selective permeability of the cellular membranes

If ECF becomes hypertonic relative to ICF, water moves from ICF to ECF If ECF becomes hypotonic relative to ICF, water moves from ECF into cells Homeostatic mechanisms respond to changes in ECF No receptors directly monitor fluid or electrolyte balance Response is to changes in plasma volume or osmotic concentrations All water moves passively in response to osmotic gradients Body content of water or electrolytes rises if intake exceeds outflow Water Balance and ECF Osmolality

To remain properly hydrated, water intake must equal water output Urine (60%) Feces (4%) Insensible losses through the skin and lungs (28%) Sweat (8%) Increases in plasma osmolality trigger thirst and release of antidiuretic hormone ( ADH ) The hypothalamic thirst center is stimulated by:

Decline in plasma volume of 10%15% Increases in plasma osmolality of 12% Baroreceptor input, angiotensin II, and other stimuli Thirst is quenched as soon as we begin to drink water Feedback signals that inhibit the thirst centers include: Moistening of the mucosa of the mouth and throat Activation of stomach and intestinal stretch receptors

Obligatory water losses include: Insensible water losses from lungs and skin Water that accompanies undigested food residues in feces Obligatory water loss reflects the fact that:

Kidneys excrete 900-1200 mOsm of solutes to maintain blood homeostasis Urine solutes must be flushed out of the body in water Antidiuretic hormone (ADH) (also called vasopressin ) Is a hormone made by the hypothalamus, and stored and released in the posterior pituitary gland Primary function of ADH is to decrease the amount of water lost at the kidneys (conserve water), which reduces the concentration of electrolytes ADH also causes the constriction of peripheral blood vessels, which helps to increase blood pressure ADH is released in response to such stimuli as a rise in the concentration of electrolytes in the blood or a fall in blood volume or pressure.

Sweating or dehydration increases the blood osmotic pressure. The increase in osmotic pressure is detected by osmoreceptors within the hypothalamus that constantly monitor the osmolarity (“saltiness”) of the blood Osmoreceptors stimulate groups of neurons within the hypothalamus to release ADH from the posterior pituitary gland.

ADH travels through the bloodstream to its target organs : a. ADH tavels to the collecting tubules in the kidneys and makes the membrane more permeable to water (that is it increases water reabsorption ) which leads to a decrease in urine output. b. ADH also travels to the sweat glands where it stimulates them to decrease perspiration to conserve water.

c. ADH travels to the arterioles , where it causes the smooth muscle in the wall of the arterioles to constrict. This narrows the diameter of the arterioles which increases blood pressure. Alcohol inhibits the production of ADH which is one of the reasons a person has increased fluid excretion after drinking alcohol!

Click here for an animation on the release of ADH in response to decreased blood volume. Is a hormone made by cells in the adrenal cortex (zona glomerulosa) Controls the levels of Na + and K + ions in extracellular fluids such as the blood Net result of its action is to reabsorb Na + ions into the blood and simultaneously excrete K + ions into the urine; because “water follows the ions,” as Na + is reabsorbed, water is also reabsorbed. Atrial natriuretic peptide (ANP) is a hormone made by cells in the right atrium whenever blood volume increases (atria are stretched) Brain natriuretic peptide (BNP) is a hormone made by cells in the ventricles in response to excessive stretching of the ventricles In general, the effects of ANP and BNP are opposite to those of angiotensin II Both ANP and BNP promote the loss of sodium ions and water at the kidneys in the urine, inhibit rennin release, and inhibit the secretion of ADH and aldosterone By inducing blood vessels to dilate and water to be excreted in the urine, ANP and BNP reduce both blood volume and blood pressure

Water loss exceeds water intake and the body is in negative fluid balance Causes include: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, and diuretic abuse Signs and symptoms: cottonmouth, thirst, dry flushed skin, and oliguria (decreased production of urine) Renal insufficiency or an extraordinary amount of water ingested quickly can lead to cellular overhydration, or water intoxication ECF is diluted sodium content is normal but excess water is present resulting hyponatremia promotes net osmosis into tissue cells These events must be quickly reversed to prevent severe metabolic disturbances, particularly in neurons Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance Salts are important for: Essential minerals Controls osmosis between fluid compartments Help maintain acid-base balance Carry electrical (ionic) current for action potentials

Sodium in Fluid and Electrolyte Balance Sodium holds a central position in fluid and electrolyte balance Sodium is the single most abundant cation in the ECF Accounts for 90-95% of all solutes in the ECF Contribute 280 mOsm of the total 300 mOsm ECF solute concentration The role of sodium in controlling ECF volume and water distribution in the body is a result of: Sodium being the only cation to exert significant osmotic pressure Sodium ions leaking into cells and being pumped out against their electrochemical gradient

Sodium concentration in the ECF normally remains stable Rate of sodium uptake across digestive tract directly proportional to dietary intake Sodium losses occur through urine and perspiration Changes in plasma sodium levels affect: Plasma volume, blood pressure ICF and interstitial fluid volume Large variations in sodium are corrected by homeostatic mechanisms If sodium levels are too low, antidiuretic hormone (ADH) and aldosterone are secreted If sodium levels are too high, atrial natriuretic peptide (ANP) is secreted

Regulation of Sodium Balance: Aldosterone A decrease in Na + levels in the plasma stimulates aldosterone release The kidneys detect the decrease in Na + levels and cause a series of reactions referred to as the renin-angiotensin-aldosterone mechanisms. This is mediated by the juxtaglomerular apparatus, which releases renin in response to: Sympathetic nervous system stimulation Decreased filtrate osmolality Decreased stretch (due to decreased blood pressure)

Sodium reabsorption 65% of sodium in filtrate is reabsorbed in the proximal tubules 25% is reclaimed in the loops of Henle When aldosterone levels are high, all remaining Na + is actively reabsorbed Water follows sodium if tubule permeability has been increased with ADH Atrial Natriuretic Hormone (ANH)

Is released in the heart atria as a response to stretch (elevated blood pressure), It has potent diuretic and natriuretic effects It promotes excretion of sodium and water, inhibits angiotensin II production Potassium ion concentrations in ECF are low Not as closely regulated as sodium Potassium ion excretion increases as ECF concentrations rise, aldosterone secreted, pH rises Potassium retention occurs when pH falls Relative ICF-ECF potassium ion concentration affects a cells resting membrane potential Potassium controls its own ECF concentration via feedback regulation of aldosterone release An increase in K + levels stimulates the release of aldosterone through the renin-angiotensin-aldosterone mechanism or through the direct release of aldosterone from the adrenal cortex cells Aldosterone stimulates potassium ion excretion from the kidneys In cortical collecting ducts, for each Na + reabsorbed, a K + is excreted When K + levels are low, the amount of secretion and excretion is kept to a minimum Excessive ECF potassium (hyperkalemia) decreases membrane potential Too little potassium (hypokalemia) causes hyperpolarization and nonresponsiveness Hyperkalemia and hypokalemia can Disrupt electrical conduction in the heart Lead to sudden death

Hydrogen ions shift in and out of cells lead to corresponding shifts in potassium in the opposite direction and interferes with activity of excitable cells Ionic calcium in ECF is important for blood clotting, cell membrane permeability, and secretory behavior Hypocalcemia increases excitability and causes muscle tetany Hypercalcemia inhibits neurons and muscle cells and may cause heart arrhythmias Two hormones regulate blood calcium levels: Parathyroid Hormone (PTH) (made by the parathyroid glands) Calcitonin (CT) (made by the thyroid glands) As calcium-rich foods are ingested, blood calcium levels rise.

The thyroid gland releases calcitonin (CT) . CT binds to receptors on osteoblasts (bone-forming cells). This triggers the osteoblasts to deposit calcium salts into bone throughout the skeletal system.

This causes the blood calcium levels to fall. CT stops being produced when blood calcium levels return to normal. When blood calcium levels fall, the parathyroid glands (located on posterior surface of the thyroid gland) release PTH.

PTH binds to receptors on osteoclasts (bone-degrading cells) within the skeletal system The osteoclasts decompose bone and release calcium into the blood. The blood calcium level rises PTH stops being produced when blood calcium levels return to normal. – Calcium reabsorption and phosphate excretion go hand in hand Filtered phosphate is actively reabsorbed in the proximal tubules In the absence of PTH, phosphate reabsorption is regulated by its transport maximum and excesses are excreted in urine High or normal ECF calcium levels inhibit PTH secretion Release of calcium from bone is inhibited Larger amounts of calcium are lost in feces and urine More phosphate is retained

Chloride (Cl-) is the major anion accompanying sodium in the ECF 99% of chloride is reabsorbed under normal pH conditions When acidosis occurs, fewer chloride ions are reabsorbed Other anions have transport maximums and excesses are excreted in urine Molecules that are dissolved in water may dissociate into charged ions. An acid is a substance that increases the number of H + ions in a solution.

A base is a substance that decreases the number of H + ions in a solution. Normal pH of body fluids: Arterial blood is 7.4 Venous blood and interstitial fluid is 7.35 Intracellular fluid is 7.0 Important part of homeostasis because cellular metabolism depends on enzymes, and enzymes are sensitive to pH.

Challenges to acid-base balance due to cellular metabolism: produces acids hydrogen ion donors Acidosis (physiological acidosis) is a blood pH below 7.35. Its principal effect is depression of the central nervous system by depressing synaptic transmissions Alkalosis is a blood pH above 7.45. Its principal effect is overexcitability of the central nervous system through facilitation of synaptic transmission

Most hydrogen ions originate from cellular metabolism Breakdown of phospho rus-containing proteins releases phosphoric acid into the ECF Anaerobic respiration of glucose produces lactic acid Fat metabolism yields organic acids and ketone bodies Transporting carbon dioxide as bicarbonate releases hydrogen ions Concentration of hydrogen ions is regulated sequentially by:

Chemical buffer systems act within seconds The respiratory center in the brain stem acts within 1-3 minutes Renal mechanisms require hours to days to affect pH changes A buffer is a solution whose function is to minimize the change in pH when a base or an acid is added to the solution Most buffers consist of a weak acid (which releases H + ions) and a weak base (which binds H + ions) If an acidic solution is added to a buffer solution, the buffer will combine with the extra H + ions and help to maintain the pH If a basic solution is added to a buffer solution, the buffer will release H + ions to help maintain the pH There are many different buffers and each one stabilizes the pH of the solution within a specific pH range One buffer may be effective within a range of pH 2 to pH 6, while another buffer may be effective within a range of pH 10 to pH 12 Strong Acids all their H + is dissociated completely in water Weak Acids dissociate partially in water and are efficient at preventing pH changes Strong Bases dissociate easily in water and quickly tie up H + Weak Bases accept H + more slowly (e.g., HCO3 and NH3) The three main buffer systems in our bodies are the:

Is a mixture of carbonic acid (H 2 CO 3 ) and its salt, sodium bicarbonate (NaHCO 3 ) (potassium or magnesium bicarbonates If strong acid is added: Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid) The pH of the solution decreases only slightly If strong base is added: It reacts with the carbonic acid to form sodium bicarbonate (a weak base) The pH of the solution rises only slightly This system is the only important ECF buffer

Nearly identical to the bicarbonate system Its components are: Sodium salts of dihydrogen phosphate (NaH 2 PO 4 ), a weak acid Monohydrogen phosphate (Na 2 HPO 4 2 ), a weak base This system is an effective buffer in urine and intracellular fluid Plasma and intracellular proteins are the bodys most plentiful and powerful buffers Some amino acids of proteins have: Free organic acid groups (weak acids) Groups that act as weak bases (e.g., amino groups)

Amphoteric molecules are protein molecules that can function as both a weak acid and a weak base The respiratory system regulation of acid-base balance is a physiological buffering system There is a reversible equilibrium between: Dissolved carbon dioxide and water Carbonic acid and the hydrogen and bicarbonate ions CO 2 + H 2 O H 2 CO 3 H + + HCO 3 When hypercapnia or rising plasma H + occurs: Deeper and more rapid breathing expels more carbon dioxide Hydrogen ion concentration is reduced

Alkalosis causes slower, more shallow breathing, causing H + to increase Respiratory system impairment causes acid-base imbalance (respiratory acidosis or respiratory alkalosis) Chemical buffers can tie up excess acids or bases, but they cannot eliminate them from the body The lungs can eliminate carbonic acid by eliminating carbon dioxide Only the kidneys can rid the body of metabolic acids (phosphoric, uric, and lactic acids and ketones) and prevent metabolic acidosis The ultimate acid-base regulatory organs are the kidneys The most important renal mechanisms for regulating acid-base balance are conserving (reabsorbing) or generating new bicarbonate ions and excreting bicarbonate ions Losing a bicarbonate ion is the same as gaining a hydrogen ion; reabsorbing a bicarbonate ion is the same as losing a hydrogen ion Carbonic acid formed in filtrate dissociates to release carbon dioxide and water Carbon dioxide then diffuses into tubule cells, where it acts to trigger further hydrogen ion secretion For each hydrogen ion secreted, a sodium ion and a bicarbonate ion are reabsorbed by the PCT cells Secreted hydrogen ions form carbonic acid Thus, bicarbonate disappears from filtrate at the same rate that it enters the peritubular capillary blood

Generating New Bicarbonate Ions Two mechanisms carried out by tubule cells generate new bicarbonate ions Both involve renal excretion of acid via secretion and excretion of hydrogen ions or ammonium ions (NH 4 + ) In response to acidosis hydrogen ions must be counteracted by generating new bicarbonate Kidneys generate bicarbonate ions and add them to the blood An equal amount of hydrogen ions are added to the urine Dietary The excreted hydrogen ions must bind to buffers (phosphate buffer system) in the urine and excreted Bicarbonate generated is: Moved into the interstitial space via a cotransport system Passively moved into the peritubular capillary blood

This method uses ammonium ions produced by the metabolism of glutamine in PCT cells Each glutamine metabolized produces two ammonium ions and two bicarbonate ions Bicarbonate moves to the blood and ammonium ions are excreted in urine When the body is in alkalosis, tubular cells: Secrete bicarbonate ions and reclaim hydrogen ions and acidify the blood The mechanism is the opposite of bicarbonate ion reabsorption process Result from failure of the respiratory system to balance pH PCO 2 is the single most important indicator of respiratory inadequacy PCO 2 levels Normal PCO 2 fluctuates between 35 and 45 mm Hg Values above 45 mm Hg signal respiratory acidosis Values below 35 mm Hg indicate respiratory alkalosis

Respiratory acidosis is the most common cause of acid-base imbalance Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema Respiratory alkalosis is a common result of hyperventilation Metabolic acidosis is the second most common cause of acid-base imbalance.

Typical causes are: Ingestion of too much alcohol and excessive loss of bicarbonate ions Other causes include accumulation of lactic acid, shock, ketosis in diabetic crisis, starvation, and kidney failure -Metabolic alkalosis due to a rise in blood pH and bicarbonate levels. Vomiting of the acid contents of the stomach Intake of excess base (e.g., from antacids) Constipation, in which excessive bicarbonate is reabsorbed

Rate and depth of breathing are elevated As carbon dioxide is eliminated by the respiratory system, PCO 2 falls below normal Kidneys secrete H + and retain/generate bicarbonate to offset the acidosis Pulmonary ventilation is slow and shallow allowing carbon dioxide to accumulate in the blood Kidneys generate H + and eliminate bicarbonate from the body by secretion Acid-base imbalance due to inadequacy of a physiological buffer system is compensated for by the other system.

The respiratory system will attempt to correct metabolic acid-base imbalances The kidneys will work to correct imbalances caused by respiratory disease

Extracellular fluid

In cell biology,Extracellular fluid is the internal environment of all multicellular animals, and in those animals with a blood circulatory system, a proportion of this fluid is blood plasma.The extracellular fluid, in particular the interstitial fluid, constitutes the body’s internal environment that bathes all of the cells in the body. The ECF composition is therefore crucial for their normal functions, and is maintained by a number of homeostatic mechanisms involving negative feedback. Homeostasis regulates, among others, the pH, sodium, potassium, and calcium concentrations in the ECF. The volume of body fluid, blood glucose, oxygen, and carbon dioxide levels are also tightly homeostatically maintained.The volume of extracellular fluid in a young adult male of 70 kg (154 lbs) is 20% of body weight – about fourteen litres. Eleven litres is interstitial fluid and the remaining three litres is plasma.

Components[edit]

The main component of the extracellular fluid (ECF) is the

Interstitial fluid[edit]

TheInterstitial fluid is the body fluid between blood vessels and cells,Interstitial fluid consists of a water solvent containing sugars, salts, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, white blood cells and cell waste-products. This solution accounts for 26% of the water in the human body. The composition of interstitial fluid depends upon the exchanges between the cells in the biological tissue and the blood.The plasma that filters through the blood capillaries into the interstitial fluid does not contain red blood cells or platelets as they are too large to pass through but can contain some white blood cells to help the immune system.Once the extracellular fluid collects into small vessels (lymph capillaries) it is considered to beThe ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs–Donnan effect. This causes a slight difference in the concentration of cations and anions between the two fluid compartments.

Function[edit]

The extracellular fluid provides the medium for the exchange of substances between the ECF and the cells, and this can take place through dissolving, mixing and transporting in the fluid medium.

Oxygenation[edit]

One of the main roles of extracellular fluid is to facilitate the exchange of molecular oxygen from blood to tissue cells and for carbon dioxide, COHowever, hydrophobic molecular oxygen has very poor water solubility and prefers hydrophobic lipid crystalline structures.If hemoglobin in erythrocytes is the main transporter of oxygen in the blood, plasma lipoproteins may be its only carrier in the ECF.The oxygen-carrying capacity of lipoproteins, OCCL, reduces in ageing or in inflammation. This results in changes of ECF functions, reduction of tissue O

Regulation[edit]

The internal environment is stabilised in the process of homeostasis. Complex homeostatic mechanisms operate to regulate and keep the composition of the ECF stable. Individual cells can also regulate their internal composition by various mechanisms.There is a significant difference between the concentrations of sodium and potassium ions inside and outside the cell. The concentration of sodium ions is considerably higher in the extracellular fluid than in the intracellular fluid.This potential is created by sodium-potassium pumps in the cell membrane, which pump sodium ions out of the cell, into the ECF, in return for potassium ions which enter the cell from the ECF. The maintenance of this difference in the concentration of ions between the inside of the cell and the outside, is critical to keep normal cell volumes stable, and also to enable some cells to generate action potentials.In several cell types voltage-gated ion channels in the cell membrane can be temporarily opened under specific circumstances for a few microseconds at a time. This allows a brief inflow of sodium ions into the cell (driven in by the sodium ion concentration gradient that exists between the outside and inside of the cell). This causes the cell membrane to temporarily depolarize (lose its electrical charge) forming the basis of action potentials.The sodium ions in the ECF also play an important role in the movement of water from one body compartment to the other. When tears are secreted, or saliva is formed, sodium ions are pumped from the ECF into the ducts in which these fluids are formed and collected. The water content of these solutions results from the fact water follows the sodium ions (and accompanying anions) osmotically.Calcium ions have a great propensity to bind to proteins.The tertiary structure of proteins is also affected by the pH of the bathing solution. In addition, the pH of the ECF affects the proportion of the total amount of calcium in the plasma which occurs in the free, or ionized form, as opposed to the fraction that is bound to protein and phosphate ions. A change in the pH of the ECF therefore alters the ionized calcium concentration of the ECF. Since the pH of the ECF is directly dependent on the partial pressure of carbon dioxide in the ECF, hyperventilation, which lowers the partial pressure of carbon dioxide in the ECF, produces symptoms that are almost indistinguishable from low plasma ionized calcium concentrations.The extracellular fluid is constantly “stirred” by the circulatory system, which ensures that the watery environment which bathes the body’s cells is virtually identical throughout the body. This means that nutrients can be secreted into the ECF in one place (e.g. the gut, liver, or fat cells) and will, within about a minute, be evenly distributed throughout the body. Hormones are similarly rapidly and evenly spread to every cell in the body, regardless of where they are secreted into the blood. Oxygen taken up by the lungs from the alveolar air is also evenly distributed at the correct partial pressure to all the cells of the body. Waste products are also uniformly spread to the whole of the ECF, and are removed from this general circulation at specific points (or organs), once again ensuring that there is generally no localized accumulation of unwanted compounds or excesses of otherwise essential substances (e.g. sodium ions, or any of the other constituents of the ECF). The only significant exception to this general principle is the plasma in the veins, where the concentrations of dissolved substances in individual veins differ, to varying degrees, from those in the rest of the ECF. However, this plasma is confined within the waterproof walls of the venous tubes, and therefore does not affect the interstitial fluid in which the body’s cells live. When the blood from all the veins in the body mixes in the heart and lungs, the differing compositions cancel out (e.g. acidic blood from active muscles is neutralized by the alkaline blood homeostatically produced by the kidneys). From the left atrium onward, to every organ in the body, the normal, homeostatically regulated values of all of the ECF‘s components are therefore restored.