C. Hydrogen Ion Concentration

What is the normal pH of arterial plasma?

7.4

What is acidosis?

A condition where gain of H+ exceeds loss, arterial plasma H+ concentration increases, and pH falls below 7.4.

What is alkalosis?

A condition where loss of H+ exceeds gain, arterial plasma H+ concentration decreases, and pH exceeds 7.4

Why is H+ concentration in the body tightly regulated?

H+ concentration affects the tertiary structure of proteins, including enzymes, which can alter their function.

What are the four major sources of net H+ gain or loss in the body?

1) CO2 from metabolism,

2) Nonvolatile acids,

3) Gastrointestinal secretions,

4) Kidneys

How much H+ is typically produced from nonvolatile acids daily?

40 to 80 mmol per day

What happens when CO2 is retained in the body?

It causes a net gain of H+ (acidosis)

What happens during hyperventilation regarding H+ balance?

Net loss of CO2 occurs, causing net elimination of H+ (alkalosis)

What are nonvolatile acids?

Acids other than carbonic acid, including phosphoric acid, sulfuric acid (from protein catabolism), lactic acid, and other organic acids.

How does vomiting affect H+ balance?

Vomitus contains high H+ concentration, so vomiting constitutes a source of net H+ loss (causing alkalosis).

How does diarrhea affect H+ balance?

Gastrointestinal secretions lost in diarrhea are alkaline with higher HCO3- concentration than plasma, so diarrhea effectively constitutes a gain of H+ (causing acidosis).

What is a buffer?

Any substance that can reversibly bind H+.

What is the general form of buffering reactions?

Buffer + H+ ⇌ HBuffer

What is the major extracellular buffer system?

The CO2/HCO3- system

What are the major intracellular buffers?

Phosphates and proteins (including hemoglobin)

How do buffers stabilize H+ concentration?

When H+ concentration increases, buffers bind H+. When H+ concentration decreases, buffers release H+, minimizing changes in either direction.

Which organ is ultimately responsible for balancing hydrogen ion gains and losses?

Kidneys

How does the respiratory system help compensate for a hydrogen ion imbalance?

Increased arterial H+ stimulates ventilation, which lowers PCO2 and reduces H+ concentration. Decreased H+ inhibits ventilation, increasing PCO2 and H+ concentration.

What is the time frame difference between respiratory and renal compensation for acid-base imbalances?

Respiratory responses are rapid (minutes) while renal responses are slower (hours to days).

How do kidneys eliminate or replenish H+ from the body?

By altering plasma HCO3- concentration

What is the kidney's response to alkalosis?

The kidneys excrete large quantities of HCO3-, which increases plasma H+ concentration toward normal.

What is the kidney's response to acidosis?

The kidneys produce new HCO3- and add it to the plasma, which decreases H+ concentration toward normal.

How is HCO3- reabsorption in the kidneys accomplished?

Through tubular secretion of H+, which combines in the lumen with filtered HCO3-.

What happens to H+ secreted after all filtered HCO3- has been reabsorbed?

It combines with filtered nonbicarbonate buffers (primarily HPO42-) and is excreted in the urine.

What are the two mechanisms by which the tubules contribute new HCO3- to the plasma?

1) H+ secretion and excretion on nonbicarbonate buffers like phosphate

2) Renal production and secretion of ammonium ion (NH4+) from glutamine metabolism

What are the two major categories of acid-base imbalances?

1) Respiratory acidosis or alkalosis

2) Metabolic acidosis or alkalosis

What causes respiratory acidosis?

The respiratory system fails to eliminate carbon dioxide as fast as it is produced, resulting in increased arterial PCO2 and increased H+ concentration.

What causes respiratory alkalosis?

The respiratory system eliminates carbon dioxide faster than it is produced, resulting in decreased arterial PCO2 and decreased H+ concentration.

What are some common causes of metabolic acidosis?

Excessive production of lactic acid (during severe exercise or hypoxia), excessive production of ketone bodies (in uncontrolled diabetes mellitus or fasting), or excessive loss of HCO3- (as in diarrhea).

What is a common cause of metabolic alkalosis?

Persistent vomiting, with its associated loss of H+ as HCl from the stomach.

In metabolic acidosis, what happens to arterial PCO2?

It decreases due to reflexive stimulation of ventilation, helping restore H+ concentration toward normal.

In metabolic alkalosis, what happens to arterial PCO2?

It increases due to reflexive inhibition of ventilation, helping restore H+ concentration toward normal.

What is the key difference between PCO2 and H+ relationship in respiratory versus metabolic acid-base disorders?

In respiratory disorders, PCO2 and H+ concentration move in the same direction. In metabolic disorders, they move in opposite directions