Acid-Base Balance and Fluid Compartments

Hydrogen Ion Concentration and the pH Scale

• Hydrogen ions can be toxic to the body if not properly controlled. Consequently, the body must strictly regulate the concentration of hydrogen ions moving within body fluids.

• In biochemistry and medical contexts, the concentration of a substance is symbolized by placing brackets around it. For instance, the concentration of hydrogen ions is written as $[H^+]$.

• In body fluids and blood, the concentration of hydrogen ions is typically very small.

The Mathematicians' Invention: Scientific Notation and pH

• Scientific notation was developed as a way to manage very small or very large numbers without the risk of dropping zeros. Scientists and mathematicians often look for ways to simplify complex numerical data.

• Examples of small hydrogen ion concentrations:

  • A small concentration such as $0.001$ can be written as $1 \times 10^{-3}$.

  • A much smaller concentration such as $0.000000001$ can be written as $1 \times 10^{-9}$.

• The pH scale (the "power of hydrogen" scale) was invented as an even simpler method to represent these concentrations. Instead of using the full scientific notation, scientists focus on the exponent.

• The formal book definition of pH is the negative log of the hydrogen ion concentration:

  • $pH = -\log[H^+]$

• Examples of pH conversion:

  • If the concentration is $1 \times 10^{-3}$, the pH is $3$.

  • If the concentration is $1 \times 10^{-9}$, the pH is $9$.

• Crucially, a lower pH number indicates a higher concentration of hydrogen ions. For example, a pH of $3$ has significantly more hydrogen ions than a pH of $9$.

Properties of the pH Scale

• The scale typically ranges from $0$ to $14$.

• The Acidic End: The range from $0$ to just under $7$ is considered acidic. This end of the scale represents the highest hydrogen ion concentrations.

• The Basic (Alkaline) End: The range from above $7$ to $14$ is considered basic or alkaline. This end represents lower concentrations of hydrogen ions.

• Bases: These molecules attract and bind with hydrogen ions to remove them from body fluids where they might cause problems. On the periodic table, the "alkaline" and "alkaline earths" columns are known for producing strong bases.

• Neutral: A pH of $7$ is considered neutral.

Specialized pH Indicators in Nature and Lab

• Certain substances in nature change color based on pH levels.

• Hydranges: These flowers serve as natural indicators. They turn blue when grown in alkaline soil and turn pink or reddish when grown in acidic soil.

• pH Paper: Laboratory pH paper contains substances that change color depending on exposure to acids or bases.

  • Colors closer to red indicate more acidic substances.

  • Colors closer to purple indicate more alkaline substances.

Blood pH and Clinical Significance

• The normal pH range for human blood is narrow and must be strictly maintained between $7.35$ and $7.45$.

• In scientific notation, this range corresponds to $1 \times 10^{-7.35}$ to $1 \times 10^{-7.45}$.

• Even minor deviations in blood pH can be fatal. For example, if a patient's blood pH drops to $7.2$, it is common to "lose them" (death), as this level of acidosis significantly complicates medical treatment.

pH of Common Substances

• Highly Acidic (pH $0$ to $2$):

  • Battery acid and strong hydrofluoric acid: pH near $0$. These can dissolve tissues and bodies.

  • Hydrochloric acid ($HCl$): pH of approximately $1$. This is secreted by the stomach and can dissolve tissues and tooth enamel.

  • Gastric acid/Lemon juice: pH around $2$. These can also dissolve tooth enamel.

• Moderately Acidic (pH $3$ to $5$):

  • Soda pop and vinegar: pH around $3$. Coca-Cola is acidic enough to be used as a toilet cleaner.

  • Citrus juices (grapefruit, orange) and less intense sodas: pH around $3$.

  • Tomato juice and acid rain: pH around $4$.

  • Black coffee and soft drinking water: pH around $5$.

• Weakly Acidic to Neutral (pH $6$ to $7$):

  • Urine and saliva: pH around $6$.

  • Pure water: pH of $7$ (requires removing all electrolytes and impurities).

• Alkaline/Basic (pH $8$ to $14$):

  • Seawater: Slightly alkaline.

  • Baking soda and Great Salt Lake water: Alkaline.

  • Milk of Magnesia: Alkaline; used as a laxative.

  • Ammonia: Household cleaner.

  • Soapy water: Alkaline; effective at killing bacteria and viruses because soap disrupts biological membranes.

  • Bleach: pH around $13$. It kills everything except spores (which require an autoclave).

  • Oven cleaner and liquid drain cleaner: pH of $14$.

The Bicarbonate Buffer System

• A buffer system is a chemical reaction that resists changes in pH by either removing acids/bases from circulation or adding them back to maintain balance.

• The most important buffer system in the human body involves carbon dioxide ($CO_2$) and water ($H_2O$).

• The Chemical Equation:

  • $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

  • $H_2CO_3$ is carbonic acid.

  • $HCO_3^-$ is the bicarbonate ion.

• This equation is reversible (indicated by the double arrows), meaning it can shift left or right to balance the "teeter-totter" of pH.

• Respiratory End: Controlled by the respiratory system, which regulates the amount of $CO_2$ in the blood.

• Metabolic/Renal End: Controlled by the urinary (renal) system, which regulates hydrogen ions and bicarbonate. It is also called the metabolic end because metabolism produces acids.

Respiratory Control of pH

• Accumulation of $CO_2$: If breathing stops or slows, $CO_2$ increases. This drives the equation to the right, producing more $H^+$ and lowering the pH (Acidosis).

• Blowing off $CO_2$: Hyperventilation decreases $CO_2$. The equation shifts to the left, consuming $H^+$ to replace the lost $CO_2$. This increases the pH (Alkalosis).

Blood pH Disorders: Acidosis and Alkalosis

• Acidosis: Blood pH below $7.35$.

• Alkalosis: Blood pH above $7.45$.

• Diagnosis is typically made using Arterial Blood Gas (ABG) tests. There are four primary categories of acid-base disorders:

  1. Respiratory Acidosis: Caused by high $CO_2$ levels due to hypoventilation, COPD, airway obstruction, drug overdose (narcotics/alcohol), chest trauma, pulmonary edema, or neuromuscular diseases (ALS, botulism).

  2. Respiratory Alkalosis: Caused by low $CO_2$ levels due to hyperventilation, hypoxia (high altitude), anxiety, fear, pain, or fever.

  3. Metabolic Acidosis: Caused by adding acid or losing base. Examples include diabetic ketoacidosis (burning fats creates ketones), diarrhea (loss of potassium/bicarbonate), renal failure, shock/sepsis (poor circulation and acid production), and aspirin ($\text{acetylsalicylic acid}$) overdose.

  4. Metabolic Alkalosis: Caused by losing too much acid. Examples include prolonged vomiting (loss of gastric secretions), overuse of antacids, or potassium-wasting diuretics.

Compensation Mechanisms

• The body does not just accept a pH imbalance; one system will try to compensate for the failure of the other.

• Respiratory Compensation: Very fast. The lungs can quickly change the breathing rate to blow off or retain $CO_2$.

• Renal/Metabolic Compensation: Slower than the respiratory system but much more precise, as kidneys can excrete individual hydrogen ions.

• Example: A patient with COPD (respiratory acidosis) will have kidneys that try to compensate by secreting more hydrogen ions to bring the pH back to normal. This would be described as "respiratory acidosis with metabolic compensation."

Clinical Signs and Symptoms

• Signs (measurable) and symptoms (reported by patient) for acidosis and alkalosis are often non-specific and overlapping, making blood tests like ABGs essential for diagnosis.

• Acidosis Symptoms: Headache, sleepiness, confusion, loss of consciousness, coma, shortness of breath, coughing, arrhythmias, increased heart rate, seizures, weakness, nausea, vomiting, and diarrhea.

• Alkalosis Symptoms: Confusion, lightheadedness, stupor, coma, twitching, prolonged muscle spasms, nausea, vomiting, hand tremors, and numbness/tingling.

Fluid Compartments and Water Balance

• Humans are water-based creatures, and body tissues cannot function efficiently without adequate hydration.

• Symptoms of dehydration include fatigue, mental cloudiness, and (in children) extreme irritability.

• Fluid Compartments (The "Rooms"):

  1. Intracellular Fluid (ICF): Fluid inside the cells. This is where the majority of body water is located.

  2. Extracellular Fluid (ECF): Everything outside the cells. It is subdivided into:

     • Interstitial Fluid: Fluid surrounding the cells in the tissue spaces.

     • Plasma: Fluid inside the circulatory system (blood volume is only about $5$ liters).

     • Transcellular Fluid: Minor amounts of specialized fluids like synovial fluid in joints and aqueous humor in eyes.

Osmosis and Dehydration Mechanics

• Osmosis: The movement of water following particles (solutes). Water moves toward the more concentrated area.

• Dehydration Sequence:

  1. Fluid is lost first from the plasma and interstitial fluid via sweat or respiration.

  2. The plasma becomes concentrated, increasing the concentration of particles and decreasing blood volume/pressure.

  3. Osmosis pulls water from the interstitial fluid into the plasma.

  4. As interstitial fluid becomes concentrated, water is pulled from the intracellular fluid (the cells).

• The Thirst Lag: By the time the hypothalamus triggers the thirst sensation, the body is already approximately $1$ to $2$ liters dehydrated. Urine output is a more reliable indicator of fluid balance than thirst.

Electrolyte and Hormonal Regulation

• Principal Electrolytes:

  • Intracellular: The main cation is Potassium ($K^+$). Magnesium ($Mg^{2+}+$) is also more prevalent inside cells.

  • Extracellular: The main cation is Sodium ($Na^+$). Calcium ($Ca^{2+}+$) is also higher in the ECF.

• Hormones:

  • Antidiuretic Hormone (ADH): Secreted in response to high plasma osmolarity; it adds water canals to the collecting ducts to retain water.

  • Aldosterone: Causes the body to hang on to sodium ($Na^+$) and excrete potassium ($K^+$); increases blood pressure and volume.

  • Atrial Natriuretic Peptide (ANP): Secreted when blood volume is too high. It causes the kidneys to waste sodium, and water follows the sodium out of the body.

Edema

• Edema is the accumulation of excess water in the interstitial spaces. It is a sign, not a disease itself. Causes include:

  • High blood pressure (pushing fluid out of capillaries).

  • Low albumin/protein (failing to pull fluid back into capillaries).

  • Leaky vessels (due to histamine release in anaphylaxis).

  • Blocked lymphatic system (due to cancer or parasites).

Questions & Discussion

• Question: A decrease in respiratory rate will result in what?

• Answer: A decreased blood pH (Acidosis) because $CO_2$ is being retained.

• Question: Why do children become "monsters" when dehydrated?

• Answer: Children have high surface-area-to-mass ratios, making them very susceptible to dehydration, which manifests as irritability and crankiness.

• Question: Is pregnancy a true respiratory alkalosis?

• Answer: No. While pregnant women breathe differently to blow off $CO_2$, it is usually a compensatory mechanism for the metabolic acidosis caused by high fetal and maternal metabolic activity. The rib cage physically expands to facilitate this.