AP II Chapter 26(Fluid, Electrolyte, and Acid-base Balance )


  1. Body Fluids and Fluid Compartments: (26.1). Water moves through semi-permeable membranes from one compartment of the body to another by a process called osmosis. 

Body Water Content: (In adults).

  1. Water is the main component of all body fluids 

  • 60% of body mass in males.

  • 50% of body mass in females.

  • As low as 45 % in old age.

  1. About one-third (27-33%) of the body’s mass is located in cells (intracellular fluid - ICF).

  2. The remaining fluid (23-28%) is called extracellular fluid (ECF).

  • Some of the ECF fluid is localized in specific places.

  • ICF and ECF are called fluid compartments.

  1. Fluid Compartments: A fluid compartment is a location that is largely separate from another compartment by some form of a physical barrier. 

  1. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes 

  2. Extracellular fluid (ECF) surrounds all cells in the body. ECF compartments: 

  • The fluid component of the blood (called plasma).

  • The interstitial fluid (IF) that surrounds all cells not in the blood. 

  1. Composition of Body Fluids:

  1. The compositions of the two components of the ECF (plasma and IF) are more similar to each other than either is to the ICF.

  2. Blood plasma has high concentrations of sodium, chloride, bicarbonate, and protein. 

  • The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein. 

  1. In contrast, the ICF has elevated amounts of potassium and protein. 

  1. Fluid Movement between Compartments: Hydrostatic pressure, the force exerted by a fluid against a wall, causes movement of fluid between compartments.

  1. The hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart. 

  2. In capillaries, hydrostatic pressure (capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood (primarily the protein, albumin) at the arteriolar end of the capillary, forcing plasma and nutrients out of the capillaries and into surrounding tissues. 

  3. Fluid enters the capillaries at the venule end, where the hydrostatic pressure is less than the osmotic pressure in the vessel. 

  4. Net filtration pressure forces fluid from the plasma in the blood to the IF around the tissue cells. 

  5. The surplus fluid in the interstitial space that is not returned to the capillaries is drained from tissues by the lymphatic system, and then re-enters the vascular system at the subclavian veins.




  1. Solute Movement between Compartments:

  1. Active transport allows cells to move a substance against its concentration gradient through a membrane protein, requiring energy in the form of ATP. 

  • The sodium-potassium pump employs active transport to pump sodium out of cells and potassium into cells, with both substances moving against their concentration gradients.

  1. Passive transport of a molecule or ion depends on its ability to pass through the membrane, as well as a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration

  • This process does not require energy (passive). For example, glucose is transferred passively into cells by glucose transporters (facilitated transport).

  1. Disorders of the Fluids:

Fluid Balance: Edema: Edema is the accumulation of excess water in the tissues.

  1. It is most common in the soft tissues of the extremities. 

  • The causes of edema include water leakage from blood capillaries. 

  • Edema is almost always caused by an underlying medical condition, by the use of some therapeutic drugs, by pregnancy, by localized injury, or by an allergic reaction. 

  1. Pulmonary edema is excess fluid in the lungs, a common symptom of heart and/or kidney failure.

  • Pulmonary edema resulting from heart failure, leakage of water occurs because fluids get “backed up” in the pulmonary capillaries of the lungs, when the left ventricle of the heart is unable to pump sufficient blood into the systemic circulation.

  1. Other causes of edema include damage to blood vessels and/or lymphatic vessels, or a 🡻 in osmotic pressure in chronic and severe liver disease, where the liver is unable to manufacture plasma proteins 



  1. Therapy for edema usually focuses on elimination of the underlying cause.



  1. Water Balance: (26.2). Water is the largest single constituent in the body, varying from 45% to 75% of body weight, depending on age and the amount of fat present.   

      Fluid intake (gain) normally = fluid output (loss)       

  1.  Regulation of Water Intake:

  1. Metabolic water (generated by metabolism) volume depends mostly on the level of aerobic cellular respiration, which reflects the demand for ATP in body cells.

  2. The main way to regulate body water balance is by adjusting the volume of oral water intake.

  3. When water loss is greater than water gain, dehydration occurs.

  4. The stimulus for fluid intake (gain) is dehydration resulting in thirst sensations

  • By renin-angiotensin II stimulation of osmorecetors in the thirst center in the hypothalamus.   Renin-angiotensin II increases electrolyte and water resorption in the kidneys

  • Receptors in the mouth that detect dryness or lack of saliva.

  • Baroreceptors in blood vessels detect falling BP

  1. Regulation of Water Output: 

  1.  Water loss from the body occurs predominantly through the renal system. 

  2. The kidneys also must make adjustments in the event of ingestion of too much fluid. 

  • Diuresis, is the production of urine in excess of normal levels, begins soon after drinking a large quantity of fluid. 

  1. ADH (also known as Vasopressin), controls the amount of water reabsorbed from the collecting ducts and tubules in the kidney. 

  • This hormone is produced in the hypothalamus and is delivered to the posterior pituitary for storage and release. 

  • When the osmoreceptors in the hypothalamus detect an 🡹 in the concentration of blood plasma, the hypothalamus signals the release of ADH from the posterior pituitary into the blood.

  • ADH has two major effects. 

1). It constricts the arterioles in the peripheral circulation, which reduces the flow of blood to the extremities and thereby 🡹 the blood supply to the core of the body. 

2). ADH also causes the collecting tubules to insert aquaporins  (hollow proteins) into the cell membrane which leads to more reabsorption of water into the bloodstream.

  • A diuretic is a compound that 🡹 urine output and therefore  🡻 water conservation by the body. Diuretics are used to treat hypertension, congestive heart failure, and fluid retention associated with menstruation. 



  1. Electrolyte Balance: (26.3). The body contains a large variety of ions, or electrolytes, which perform a variety of functions.

  1.  Roles of Electrolytes:

     Electrolytes serve four general functions in the body.

  1. Because they are more numerous than nonelectrolytes, electrolytes control the osmosis of water between body compartments.

  2. Acid-base balance required for normal cellular activities.

  3. Electrical current, which allows production of action potentials in nerves and muscles. 

  4. Cofactors needed for optimal activity of enzymes.

Concentrations of Electrolytes in Body Fluids

  • The chief difference between plasma and interstitial fluid is that plasma contains many proteins, interstitial fluid (IF) has few since plasma proteins generally cannot move out of blood vessel walls. 

  • Intracellular fluid (ICF) differs considerably from extracellular fluid (ECF).

Sodium (Na+) 

  • Most abundant extracellular cation.

  • Functions: impulse transmission, muscle contraction, and participates in fluid and electrolyte balance by creating most of the osmotic pressure of extracellular fluid.

  • The average daily intake of sodium far exceeds the body’s normal daily requirements. The kidneys excrete excess sodium and conserve it during periods of sodium restriction.

  • Hyponatremia is a lower-than-normal concentration of sodium, usually associated with excess water accumulation in the body, which dilutes the sodium.

  • Hypernatremia is an abnormal 🡹 of blood sodium. It can result from water loss from the blood, resulting in the hemoconcentration of all blood constituents.

  • Excess Na+ in the body can result in edema. Excess loss of Na+ causes excessive loss of water, which results in hypovolemia, and low blood volume. 

 Chloride (Cl-) 

  • Most abundant extracellular anion.

  • The secretion and reabsorption of chloride ions follows the paths of sodium ions.

  • Functions: Move easily between fluid compartments and regulate osmotic pressure between compartments and forming HCl in the stomach.

Potassium (K+) 

  • Most abundant intracellular cation.

  • Functions: It is involved in maintaining fluid volume, impulse conduction, muscle contraction, and regulating pH.

  • Hypokalemia is an abnormally low potassium blood level.

  • Hyperkalemia, an elevated potassium blood level, also can impair the function of skeletal muscles, the nervous system, and the heart.

Bicarbonate (HCO3-)

  • Second most abundant extracellular anion.

  • Functions: plasma acid-base buffer system.

  • The kidney reabsorbs or secretes bicarbonate to make the final balance for acid-base conditions.

 Calcium (Ca+2)

  • Most abundant mineral in the body (98% is stored in bone and teeth)

  • Functions: structural component of bones and teeth. Functions in blood coagulation, neurotransmitter release, maintenance of muscle tone, and excitability of nervous and muscle tissue.

  1. Regulation of Sodium and Potassium: Sodium is reabsorbed from the renal filtrate, and potassium is excreted into the filtrate in the renal collecting tubule. The control of this exchange is governed principally by two hormones—aldosterone and angiotensin II.

  1. Aldosterone:

  • Aldosterone 🡹 the excretion of potassium and the reabsorption of sodium in the distal tubule. 

  • Its effect is to conserve and 🡹 water levels in the plasma by reducing the excretion of sodium, and thus water, from the kidneys. 

  1. Angiotensin II causes vasoconstriction and an 🡹 in systemic blood pressure. 

  • This action 🡹 the glomerular filtration rate (GFR), resulting in more material filtered out of the glomerular capillaries and into Bowman’s capsule. 

  • Angiotensin II also signals an increase in the release of aldosterone from the adrenal cortex.







            



  1. Regulation of Calcium and Phosphate.

  1. Calcium and phosphate are both regulated through the actions of three hormones: 

  • Parathyroid hormone (PTH)- Activates osteoclasts and 🡻 kidney loss.

  • Vitamin D (calcitriol)- Necessary for GI absorption of calcium.

  • Calcitonin- Activates osteoblasts.



  1.  Acid-Base Balance: (26.4). The acid-base balance of the body is maintained by controlling the H+ concentration of body fluids, especially extracellular fluid. The normal pH of extracellular fluid is 7. 4 (7.35-7.45).

  1. Homeostasis of pH is maintained by:

  • Buffer systems

  • Exhalation of carbon dioxide.

  • Kidney excretion.

  1. Buffer Systems

  • Most buffer systems of the body consist of a weak acid and the salt of that acid (which functions as a weak base); together they prevent rapid, drastic changes in the pH of a body fluid by changing strong acids and bases into weak acids and bases. Buffers work immediately.

  • Important buffer systems:

  1. Protein buffer system 

  • Is the most abundant buffer in body cells and plasma

  • Inside RBCs, the protein hemoglobin is an especially good buffer for carbonic acid.

  • Terminal groups of every protein act as buffers:

  1. Carbonic acid-bicarbonate buffer system 

  • Is an important regulator of blood pH 

  • (sodium bicarbonate) + (strong acid) → (weak acid) + (salt) 

  • Is based on the bicarbonate ion.

  1. Phosphate buffer system 

  • Important regulator of pH, both in red blood cells and in the kidney tubular fluids.

2.   Exhalation of CO2

  • The pH of body fluids may be adjusted by a change in the rate and depth of respirations, which usually takes from 1 to 3 minutes.

  1. An 🡹 in the rate and depth of breathing causes more carbon dioxide to be exhaled, thereby 🡹 pH  (less acidic).

  2. A 🡻 in respiration rate and depth means that less carbon dioxide is exhaled, causing the blood pH to fall. (more acidic).

  1. The pH of body fluids, in turn, affects the rate of breathing.

3.   Kidneys help maintaining pH; they excrete H+ and reabsorb HCO3- 

B. Respiratory and renal regulation of acid-base balance:

  1. The respiratory system contributes to acid/base balance by regulating the blood levels of carbonic acid. Carbon dioxide in the blood readily reacts with water to form carbonic acid (CO2 and carbonic acid are in equilibrium). When the CO2 rises in the blood (by 🡻 the respiratory rate) more carbonic acid forms lowering the pH and when the level of CO2 falls (by 🡹 the respiratory rate), carbonic acid 🡻 raising the pH.

  2. The kidneys regulate of the bloods pH by controlling the blood level of bicarbonate (HCO3-), if there is excess acid (low pH) the renal tubular cells convert carbonic acid into acid and bicarbonate (excreting the acid into the urine and conserving the bicarbonate into the blood). If there is too little acid (High pH) the renal tubular cells convert carbonic acid into acid and bicarbonate (excreting the bicarbonate into the urine and conserving the acid into the blood).

V. Disorders of Acid-Base Balance: (Module 26.5).

The normal pH range of systemic arterial blood is between 7.35-7.45.

  1. Acidosis: a blood pH below 7.35. 

Its principal effect is depression of the central nervous system and synaptic transmission. 

  1. Alkalosis: a blood pH above 7.45. 

Its principal effect is over excitability of the central nervous system. 

Compensation: the physiological response to an acid-base imbalance.

  1. A change in blood pH that leads to acidosis or alkalosis can be compensated to return pH to normal. 

  1. Respiratory acidosis and respiratory alkalosis: disorders of blood PCO2. 

  1. Respiratory acidosis: is characterized by a 🡹 PCO2 and 🡻 pH and is caused by hypoventilation or other causes of reduced gas exchange in the lungs.

  2. Respiratory alkalosis: characterized by a 🡻 arterial blood PCO2 and 🡹 pH and is caused by hyperventilation.

  1. Metabolic acidosis and metabolic alkalosis: disorders of bicarbonate concentration.

  • Metabolic acidosis: characterized by a 🡻 bicarbonate level and 🡻 pH, and results from an abnormal 🡹 in acid metabolic products (other than CO2), loss of bicarbonate, or failure of the kidneys (renal failure) to excrete H+ ions derived from metabolism of dietary proteins

  • Metabolic alkalosis: characterized by 🡹 bicarbonate concentration and 🡹 pH resulting from non-respiratory loss of acid (e.g., excessive vomiting) or excess intake of alkaline drugs.

C.  Metabolic acidosis: characterized by a 🡻 bicarbonate level and 🡻 pH, and results from an abnormal 🡹 in acid metabolic products (other than CO2), loss of bicarbonate, or failure of the kidneys (renal failure) to excrete H+ ions derived from metabolism of dietary proteins.

1.  Respiratory response to acidosis (compensation) is 🡹 the respiratory rate (removing CO2)

  • If CO2 falls then: CO2  + H20 🡸 H2CO3  🡸  H+ + HCO3-  (decrease in acid)

 2.  Renal response to acidosis (compensation):

a. Secrete H+ ions at kidney.

b. Reabsorb bicarbinate ion.

D.  Metabolic alkalosis: characterized by 🡹 bicarbonate concentration and 🡹 pH, and results from non-respiratory loss of acid (e.g., excessive vomiting-🡹pH or excess intake of alkaline drugs).

1.  Respiratory response to alkalosis (compensation) is 🡻 the respiratory rate (adding CO2)

  • If CO2 is  increased: CO2 + H20 🡺H2CO3  🡺 H+ + HCO3- (increase in acid)

2.   Renal response to alkalosis (compensation)

a. Conserving H+ ions at kidney.

b. Secreting bicarbinate ion.

E.  Respiratory acidosis: characterized by an 🡹 PCO2 and 🡻 pH and is caused by hypoventilation or other causes of reduced gas exchange in the lungs (i.e., lung disease). 

If CO2 is  🡹: CO2 + H20 🡺 H2CO3 🡺 H+ + HCO3- (increase in acid)

1.  Renal response (compensation) to respiratory acidosis is conservation of bicarbonate (excrete H+).

F.  Respiratory alkalosis: characterized by a 🡻 arterial blood PCO2 and 🡹 pH and is caused by hyperventilation.

If CO2 falls then: CO2  + H20 🡸 H2CO3  🡸  H+  + HCO3- (decrease in acid)

  1. Renal response (compensation) to respiratory alkalosis is to secrete bicarbonte ions (conserve H+).

G.  Summary (Not in Text):

   Diagnosis of acid-base imbalances employs a general four-step process, taken in order:

  1. Note whether the pH is high or low relative to the normal range.

  2. Decide which value of PCO2 or HCO3- could cause the abnormality.

  3. Specify the problem source as respiratory or metabolic.

  4. Look at the noncausative value and determine if it is compensating for the problem.