Clinical Chemistry: Nitrogen Metabolism and Acid-Base Balance

This flashcard set covers the metabolic pathways of ammonia and uric acid, sample handling requirements, and the step-by-step interpretation of acid-base disorders and compensation.

Ammonia ($NH_3$)

A toxic nitrogen-containing waste product formed during amino acid metabolism that is especially toxic to the brain.

Urea Cycle (Ornithine Cycle)

The metabolic process in the liver that detoxifies ammonia by converting it into urea.

BUN

Blood Urea Nitrogen; the laboratory measurement of the nitrogen portion of urea circulating in the blood.

Hyperammonemia

An elevated level of ammonia in the blood, often caused by liver conditions such as cirrhosis, acute liver failure, or urea cycle disorders.

Asterixis

A flapping tremor developed by patients when ammonia crosses the blood-brain barrier, often seen in hepatic encephalopathy.

Ammonia Specimen Handling

Requirements including collection on ice, rapid separation of plasma, avoidance of hemolysis, and immediate transport to prevent falsely elevated results from ongoing cell metabolism.

Uric Acid

The final waste product of purine metabolism, derived from the breakdown of DNA, RNA, ATP, and GTP.

Purines

The nitrogenous bases (DNA, RNA) which, when metabolized, follow a pathway through hypoxanthine and xanthine to become uric acid.

Xanthine Oxidase

The key enzyme that converts hypoxanthine to xanthine and subsequently to uric acid.

Hyperuricemia

High levels of uric acid in the blood, classically associated with gout, kidney stones, and tumor lysis syndrome.

Gout

A condition caused by excessive uric acid forming needle-like crystals in the joints, commonly affecting the big toe.

Tumor Lysis Syndrome

A condition where chemotherapy causes rapid cell death in leukemia patients, leading to a massive release of purines and a subsequent spike in uric acid.

Uricase Method

The modern, most specific laboratory method for measuring uric acid levels.

Phosphotungstic Acid (PTA) Method

An older, less specific historical method for uric acid measurement susceptible to interference from glucose and vitamin C.

Carbonic Acid ($CO_2$)

The acidic component of the acid-base balance regulated primarily by the lungs.

Bicarbonate ($HCO_3$)

The basic (alkaline) component of the acid-base balance regulated primarily by the kidneys.

Normal ABG Values

Standard reference ranges: pH $7.35-7.45$, $pCO_2$ $35-45\,mmHg$, and $HCO_3$ $22-26\,mEq/L$.

Respiratory Acidosis

An acid-base disorder characterized by a low pH and high $CO_2$, often due to hypoventilation or COPD.

Respiratory Alkalosis

An acid-base disorder characterized by a high pH and low $CO_2$, often caused by hyperventilation, anxiety, or high altitude.

Metabolic Acidosis

An acid-base disorder characterized by a low pH and low $HCO_3$, often caused by diarrhea, kidney failure, or diabetic ketoacidosis.

Metabolic Alkalosis

An acid-base disorder characterized by a high pH and high $HCO_3$, often caused by vomiting or excessive use of antacids.

Full Compensation

An acid-base state where the pH has returned to the normal range ($7.35-7.45$) through the opposing actions of the lungs or kidneys, despite the original abnormal values.

Hepatic Encephalopathy

A condition where impaired liver function allows ammonia to accumulate, leading to confusion, tremors, and potentially coma.