Transcript Unit One: Introduction to Physiology: The Cell and General

Chapter 30: Acid-Base Regulation
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Acid-Base Regulation
• Hydrogen Ion Concentration is Precisely Regulated
• Acid- molecules that release H+ in solution
• Base- ion or molecule that can accept an H+
• Alkali- formed by the combination of one or more
of the alkali metals (i.e. Na) with a highly basic
ion (i.e. OH); the base portion reacts quickly
with hydrogen ions and remove them from
solution---therefore they act as bases
Acid-Base Regulation
• Strong Acid- one that rapidly dissociates and
releases large amounts of H+ in solution
• Strong Base- one that reacts rapidly and strongly
with H+ and quickly removes them from
solution
Acid-Base Regulation
The normal H+ concentration is 40nEq/L
(0.00000004); therefore, the normal pH is
Acid-Base Regulation
Table 30.1 pH and Hydrogen Ion Concentration of Body Fluids
H+ Concentration
pH
ECF
Arterial Blood
4.0 x 10-5
7.40
Venous blood
4.5 x 10-5
7.35
Interstitial Fluid
4.5 x 10-5
7.35
Intracellular Fluid
1 x 10-3 to 4 x 10-5
6.0-7.4
Urine
3 x 10-2 to 1 x 10-5
4.5-8.0
Gastric HCl
160
0.8
Defending Against Changes in H+
• Three primary systems regulate H+ concentration
to prevent acidosis or alkalosis
a. Chemical acid-base buffer systems of body fluids
(1st line of defense)
a. The respiratory center which regulates the
removal of CO2 and therefore H2CO3 (2nd line
of defense)
c. The kidneys which can excrete either acid or
alkaline urine
Bicarbonate Buffer System
• Consists of (1) a weak acid and (2) a bicarbonate salt
Bicarbonate Buffer System
Fig. 30.1 Titration curve for bicarbonate buffer system
Phosphate Buffer System
• Addition of a Strong Acid
•Addition of a Strong Base
Phosphate Buffer System
• Role of Phosphate Buffer
a. Relatively insignificant as an extracellular buffer
b. Important in the tubular fluids of the kidney
1. Phosphate becomes greatly concentrated in
the tubules
2. Tubular fluid usually has a considerably
lower pH than extracellular fluid
c. Important in intracellular fluid because of the
phosphate concentration
Proteins As Important Intracellular Buffers
• Proteins are the most plentiful buffer due to high
concentrations inside cells
• In the rbc, hemoglobin is an important buffer
• Approximately 60-70% of the total chemical
buffering of body fluids is inside the cells,
and most of this comes from intracellular
proteins
Respiratory Regulation of Acid-Base Balance
• Pulmonary Expiration of CO2 Balances Metabolic
Formation of CO2
• Increasing Alveolar Ventilation Decreases
Extracellular Fluid H+ Concentration and
Raises pH
Fig. 30.2 Change in ECF pH caused by increased or decreased rate of alveolar
ventilation, expressed as times normal
Respiratory Regulation (cont.)
• Increased H+ Concentration Stimulates
Alveolar Ventilation
Fig. 30.3 Effect of blood pH on the rate of alveolar ventilation
Respiratory Regulation (cont.)
• Feedback Control of H+ Concentration By the
Respiratory System (Negative Feedback)
a. Increased H+ concentration stimulates respiration
b. Increased alveolar ventilation decreases H+
concentration
•
Efficiency of Respiratory Control of H+
Concentration- cannot return the concentration
back to normal when a disturbance outside
the respiratory system has altered the pH
Respiratory Regulation (cont.)
• Buffering Power of the Respiratory System
a. Acts as a physiologic type of buffering system
•
Impairment of Lung Function Can Cause
Respiratory Acidosis
Renal Control of Acid-Base Balance
• Secretion of H+ and Reabsorption of HCO3- By
the Renal Tubules
30.4 Reabsorption of bicarbonate in different
segments of the renal tubule
Renal Control of Acid-Base Balance
• H+ is Secreted by Secondary Active Transport in
the Early Tubular Segments
30.5 Cellular mechanisms for (1)active secretion
of hydrogen ions into the renal tubule, (2)
tubular reabsorption of bicarbonate by
formation of carbonic acid, and (3) sodium
ion reabsorption in exchange for hydrogen
ion secretion
Renal Control of Acid-Base Balance
• Filtered HCO3 is Reabsorbed by Interaction with
H+ in the Tubules
a. Each time an hydrogen ion is formed in the
tubular epithelium, an HCO3 is also formed
and released back into the blood
b. HCO3 is “titrated” against H+ in the tubules
Renal Control of Acid-Base Balance
• Primary Active Secretion of H+ in the Intercalated
Cells of Late Distal and Collecting Tubules
Fig. 30.6 Primary active secretion of H ion
through the membrane of the
intercalated cells
Renal Control of Acid-Base Balance
• Phosphate Buffer System Carries Excess H+ into
the Urine and Generates New HCO3
Fig. 30.7
Renal Control of Acid-Base Balance
• Excretion of Excess H+ and Generation of New
HCO3 by the Ammonia Buffer System
Fig. 30.8 Production and secretion of
ammonium ion by the proximal
tubular cells
Fig. 30.9 Buffering of the hydrogen ion secretion
by ammonia in the collecting tubules
Quantifying Renal Acid-Base Excretion
• Bicarbonate excretion is calculate as the urine
flow rate multiplied by urinary HCO3
concentration
• The amount of new HCO3 contributed to the blood
at any given time is equal to the amount of H+
secreted that ends up in the tubular lumen
• The rest of the non-bicarbonate, non-ammmonia
buffer excreted is measured by determining a
value known as titratable acid
Quantifying Renal Acid-Base Excretion
• Regulation of Renal Tubular H+ Secretion
Increase H+ Secretion and
HCO3 Reabsorption
Increase PCO2
Increase H+
Decrease H+ Secretion and
HCO3 Reabsorption
Decrease PCO2
Decrease HCO3
Decrease H+
Increase HCO3
Decrease ECF volume
Increase ECF volume
Increase Angiotensin II
Decrease Angiotensin II
Increase Aldosterone
Decrease Aldosterone
Hypokalemia
Hyperkalemia
Renal Correction of Acidosis
• Acidosis Decreases the ration of HCO3/H+
in Renal Tubular Fluid
a. In metabolic acidosis, an excess of H+ over
HCO3 occurs in the tubular fluid primarily
because of decreased filtration of HCO3
b. There is also a decrease in pH and a rise in
ECF H+ concentration
Renal Correction of Alkalosis
• Alkalosis Increases the Ratio of HCO3/H+ in
Renal Tubular Fluid
Table. 30.3 Characteristics of Primary Acid-Base Disturbance
Normal
pH
H+
PCO2
HCO3
7.4
40 mEq/L
40 mm Hg
24 mEq/L
Respiratory
Acidosis
Respiratory
Alkalosis
Metabolic
Acidosis
Metabolic
Alkalosis
The primary event is indicated by the double arrows. Respiratory acid-base disorders are initiated
By an increase or decrease in PCO2; metabolic disorders are initiated by an increase or decrease in HCO3