Acid base balance

The Quantities of water and electrolytes entering the body must equal the quantities leaving it. maintaining this balance requires mechanisms to ensure that lost water and electrolytes will be replaced and that any excess will be excreted .

water balance and electrolyte balance are interdependent because electrolytes are dissolved in the water of body fluids.

water intake must equal water output.

Intake : 2500ml/day- via ingested fluid 60%, food 30%, and cellular metabolism 10%

Output: water vapor from lungs  and skin, perspiration 8%, feces 4%, and kidneys 60%

overview: water balance is primarily determined by thirst centers  in the brain to vary intake an the kidneys ability to vary water output.

Primary regulator= Thirst mechanism

As the body looses water , osmoreceptors stimulate the hypothalamic thirst center, which causes the person to feel thirsty and seek water. Thirst mechanism is triggered whenever the total water decreases by a little as 1%. 

The act of drinking and the resultant distention of the stomach wall trigger impulses that inhibit the thirst mechanism. thus, a person is prevented from drinking too much.

water output must balance water input. If water intake is insufficient, water out put must be reduced.

Primary regulator= anti-diuretic hormone

when the concentration of blood solutes rises, the hypothalamus causes. ADH to be secreted from the posterior pituitary gland into the blood . ADH circulates to the kidney , where it increases the permeability of the collecting duct to water. this allows more water to be reabsorbed, reducing the amout of water excreted in the urine.

When blood solutes are to dilute, ADH secretion is inhibited and water is eliminated in the urine. 

↑  Solutes, → Hypothalamus→ Posterior pituitary→ ADH is in blood. 


EX: alcohol inhibits ADH secretion 

In the absence of ADH the collecting duct is impermeable to water

Acid are in electrolytes that release H+ in solution, and thus lower the PH of a solution

Bases are electrolytes that accept H+ in solution, and thus raise the PH of a solution . bases can also be thought of as electrolytes that release OH+ ion (hydroxyl ions) in solution

Normal PH range : 7.35-7.45

Acid base balance entails regulation of the H+ of body fluids . This is important because slight changes in H+  can alter the rates of enzyme- controlled metabolic reactions , shift the distribution of other ions, or modify hormones actions . 

Breathing helps control PH, tubular secretions rids body of H+  

Blood PH is important to change the structure  of proteins.

Most H+ in body fluids originates as byproducts of metabolic process. The digestive tract may directly absorb some H+ 

• The acids resulting from metabolism vary in strength. Thus, their effects  on the 
• H+  of the body fluids vary

Note: The H+ at equilibrium determines PH

1. Aerobic respiration of glucose produces CO2 and H2O ,CO2 combines with H20 to form carbonic acid H2CO2. Carbonic acid ionizes, which forms hydrogen ions H+ and bicarbonate Hco3

co2 + H2O↔ H2co3↔H+ +HCO3-


2. Anaerobic respiration of glucose produces lactic acid, which adds H+ to bodily fluids.

3. Incomplete oxidation of fatty acids produces acidic ketone bodies, which increase H+.

Hydrolysis of phosphoproteins  and nucleic acids, followed by the oxidation of phosphorus , produces phosphoric acid (H3po4) , which ionizes to release H+.

Strong indicates that the acid/base had a large influence on PH

Weak indicates that the acid/base has a small influence on PH

Acids - strong acids ionize more completely and release more H+

Weak acids ionize less completely and release fewer H+ 

Bases- bases release ions, such as hydroxide ions (OH-), which can combine with H+ , thereby lowering (H+)

Strong bases ionize more completely and release more OH-

Weak bases ionize less completely and release fewer OH-

• To many acids/bases= bad
• To few acids/bases= bad

Normal metabolic reactions generally produce more acid than base.

Implication: The maintenance of acid-base balance usually involves the elimination of acids, via one of three ways:

Buffer= Protection that prevents large changes in PH

• 1. Acid-base buffer systems
• 2. Respiratory excretion of CO2 ,(breathing out co2)
• 3. Renal excretions of H+( Tubular secretion)

Acids - base buffer systems are in all body fluids and are based on chemicals that combine with excess acids of bases 

Buffers are substances that stabilize the PH of a solution. :

Buffers may combine with strong acids to convert them into weak acids. 

Buffers may combine with strong bases to convert them to weak bases

1. Bicarbonate buffer system- present in ECF and ICF 

Bicarbonate converts a strong acid to a weak acids. 

H+ + HCO3=H2CO3

Carbonic acid converts a strong base to a weak base.

H2CO3= H+ +hco3

FYI : although this reaction releases HCO3-  , it is the increase of free H+ at equalibrium that is important in minimizing the shift toward a more  alkaline ph.

2. Phosphate Buffer System- present in EFC and ICF

The monohydrogen phosphaste ion converts a strong acid to a weak acid 

H+ + HPO4= H2PO4

The dihydrogen phosphate ion converts a strong base to a weak base.

H2po4= H+ = HPO4

3. Protein Buffer System- consists of the plasma proteins  (albumin and hemoglobin) 

A) Some amino acids (comprising proteins) have freely exposed carboxyl groups (-COOH)  and /or amino groups (-NH2), which act as buffers.

If H+ drops, the carboxyl group (-COOH) can become ionized, releasing H+, thus resisting the ph change: 

-COOH - -coo + H+

This is a reversible reaction :- COO- accepts  H+  in the presence of excess acid. 

• If H+ rises, amino groups (-NH2) can accept H+
• -NH2 + H+⇄ -NH3

This is a reversible reaction NH3 group releases H+ in the presence of excess base. 

b) Hemoglobin is an especially important protein that buffers  H+. RBCs contain carbonic anhydrase that speeds the reaction between CO2 and H20, Producing carbonic acid, which quickly dissociates to release H+ and HCO3

The respiratory center in the medulla helps regulate H+ in the body fluids by controlling the rate and depth of breathing.

↑ CO2 levels - ↑ H+ levels -↓ PH - Hyperventilation

↓ CO2 levels - ↓ H+ levels -↑ PH - Hypoventilation

Nephrons help regulate  H+ of body fluids by excreting H+ in the urine.

(Recall: H+ is secreted into the tubular fluid. Tubular secretion of H+  is linked to tubular  reabsorption of HCO3

Phosphates buffer H+ urine.

Ammonia (NH3),a weak base produced by renal cells , helps transport H+ to the out of the body.

Normal blood PH  7.35-7.45, important for normal functioning  of enzymes, hemoglobins, receptors,protein channels , structural proteins, etc.

Many of the bodys metabolic reactions generates H+.if not eliminated , these acids will accumulate

Any change in blood PH  by more than a few tenths of a unit beyond this range could be fatal A person usually cannot survive if the PH drops drops to 6.8 or rises to 8.0 for more than a few hrs . Thus the body has several mechanisms to deal with excess H+

Acidosis occurs when the PH falls below the normal  range . It results from an accumulation of acids or a loss of bases , both of which cause a an abnormal increase in the H+ of body. 

Alkalosis occurs when the PH of the blood above the normal range. It results from a loss of acids or an accumulation of bases accompanied by a decrease in H+

Acid-base disturbances can either be metabolic or respiratory in their origin.

Respiratory acid-base disturbances occur due to primary changes in the PCO2 

Metabolic (non respiratory) acid-base disturbances occurs due to primary changes in HCO3

Acid base disturbances can be partially or almost completely compensated, compensation for acid base disorders is either respiratory or renal.

when the primary disorder is metabolic compensation is respiratory.

When the primary disorder is respiratory in origin , compensation is Renal (metabolic)

• Metabolic alkalosis 
• Caused by an increase in HCO3 and lead to an increase in PH

Metabolic alkalosis is caused by a loss of fixed H+ from the body or gain of HCO3 .Although metabolic alkalosis can be caused by administration of HCO3 most often it is caused by loss of fixed acid from the body. Metabolic alkalosis can result from the following

• Loss of fixed H+ from the GI tract
• Loss of fixed H+ from the kidney
• Administration of solutions containing HCO3
• EFC volume contraction (e.x, diuretics)

Arterial blood profile: PH ↑-HCO3↑-PCO2↑

Metabolic acidosis- is a decrease in HCO3 that leads to a decrease in PH

Metabolic acidosis is caused by a gain of fixed H+ in the body or loss of HCO3  it can result from the following 

Increased production of ketoacids of lactic acid

Ingestion of fixed acids, such as salicylic acid

An inability of the kidneys to excrete the fixed acids produced from normal metabolism

Loss of HCO3 via the kidneys of the GI tract

Arterial blood profile: PH↓ HCO3↓ PCO2↓

Respiratory alkalosis is caused by hyperventilation , which results in excessive loss of CO2 decreased PCO2 and increased PH 

Hyperventilation can be caused by

Direct stimulation of the medullary respiratory center (anxiety, fever) 

Hypoxemia- stimulates peripheral chemoreceptors

Mechanical ventilation

In each case , rapid, deep breathing depletes co2, and the PH of the bodily fluids increases. 

Arterial blood profile: PH ↑ PCO2↓ HCO3↓

Respiratory Acidosis-  is caused by hypoventilation , which results in co2 retention, increased PCO2, and decreased in PH

Retention of CO2 can be caused by: 

Inhibition of the medullary respiratory center

• paralysis of respiratory muscle 
•  
• airway obstruction

 failure to exchange CO2 between pulmonary capillary blood and alveolar gas.

any of these conditions can increase the level of H+ in body fluids thus lowering PH 

Arterial blood profile: PH ↓ CO2↑ HCO3↑