Cellular Fluids and Acid-Base Homeostasis

Fluid Compartments: Intracellular and Extracellular Fluids

• Intracellular Fluid (ICF): Fluid that is located inside of the cells. The prefix "intra" denotes "within."   - Intracellular fluid makes up the majority of our total body fluid.
• Extracellular Fluid (ECF): Fluid that is located outside of the cells.   - Extracellular fluid is further categorized into several distinct sub-categories:     - Interstitial Fluid: This is fluid located within the tissue. While it is normal for fluid to be present in the tissue, an imbalance or increase in the percentage of interstitial fluid leads to a specific condition.     - Edema: This is the clinical term for the buildup of excessive interstitial fluid in the tissue.     - Plasma: The fluid component of the blood. It includes the non-cellular components such as serum.       - When blood in a tube is placed in a centrifuge and spun, the clear fluid that separates is the serum or plasma.       - Blood is composed of plasma (the fluid) and non-fluid elements including red blood cells, white blood cells, and platelets.     - Lymph: The lymphatic fluid that circulates through the lymph nodes and the lymphatic vessels.     - Transcellular Fluid: A catch-all category for various fluids that do not fit neatly into the other extracellular categories.

Indicators of Fluid Imbalance

• Vital Signs: Fluid imbalance is often first noticed through changes in vital signs, particularly blood pressure.   - Hypotension: A drop in blood pressure often indicates dehydration.   - Patient Variation: Older patients may show a drop in vital signs much more quickly than younger individuals.
• Physical and Visual Examination: Sometimes dehydration can be identified physically before it appears on a monitor.   - Dry Lips: A visual sign of dehydration.   - Tenting of the Skin: During a visual exam, if a patient's skin is pulled up and stays up rather than "snapping back" into place, it is a sign of dehydration.

Mechanisms of Fluid Movement

• Diffusion: The movement of particles from an area of high concentration to an area of low concentration.
• Osmosis (Osmolality): The movement of water from an area of high concentration to an area of low concentration.
• Osmolarity: The concentration of particles, specifically electrolytes, within the fluid.   - High Osmolarity: Indicates an increased concentration of electrolytes.   - Low Osmolarity: Indicates a decreased concentration of electrolytes.
• Fluid Homeostasis and Balance: Fluid moves to create balance between the inside and outside of the cell.   - Scenario A (High Tissue Osmolarity): If the concentration of electrolytes in the tissue (outside the cell) is higher than inside the cell, water will move from inside the cell into the tissue to dilute the high electrolyte level.     - Result: The cell will shrink. This shrinking can lead to lysis or cellular destruction.   - Scenario B (Low Tissue Osmolarity): If the concentration of electrolytes in the tissue is lower than inside the cell, water will move from the tissue into the cell to create balance.     - Result: The cell will swell. If the swelling is excessive, the cell can burst.

Fluid Intake and Output (I&O)

• Equilibrium: To maintain homeostasis, the amount of water taken into the body daily should equal the amount of water put out.
• Standard Volume: This balance is approximately $2500\,ml$ per day ($2.5\,L$).
• Methods of Intake:   - Liquids/Beverages: The primary source of intake.   - Food: Provides a portion of daily fluid.   - Metabolic Byproduct: Approximately $200\,ml$ of fluid is gained naturally as a byproduct of metabolism (the building and breaking of molecules) without needing to consume extra fluid.
• Methods of Output:   - Urine: The primary/major source of fluid loss.   - Feces: Fluid lost during bowel movements.   - Skin (Sweat): Fluid lost through perspiration.   - Exhaled Vapors: Fluid vapors lost during breathing. This is also how germs are spread, as they are carried on exhaled vapors.
• Activity and Loss:   - Increased physical activity/exercise leads to more sweating, requiring increased intake.   - Illnesses like diarrhea cause excessive fluid loss through feces, necessitating increased intake to maintain balance.

Overhydration and Water Toxicity

• Water Intoxication: Also known as "internal drowning." This occurs when someone consumes large amounts of water faster than the kidneys can process it.
• Mechanism: If water is consumed rapidly (e.g., in "water challenges"), the cells take up the water before the kidneys can excrete it as urine, causing the cells to burst.
• Symptoms:   - Feeling of lack of oxygen (identical to drowning underwater).   - Electrolyte flushing, which can cause cardiac arrhythmia.   - Feelings of anxiety.
• Management: In patients with impaired kidney function, diuretics or dialysis may be required to remove excess fluid.

The Thirst Mechanism and Dehydration Response

• Hypothalamus: The thirst center of the brain located in the hypothalamus.
• Process 1: Increasing Intake:   1. Dehydration occurs (physical change).   2. The hypothalamus picks up on the dehydration.   3. The hypothalamus signals to decrease salivation.   4. Decreased saliva causes a dry mouth.   5. Dry mouth stimulates the sensation of thirst, prompting the individual to consume water.
• Process 2: Decreasing Output (Antidiuretic Hormone):   1. Simultaneously, the hypothalamus stimulates the posterior pituitary gland (also in the brain).   2. The posterior pituitary secretes Antidiuretic Hormone (ADH).   3. ADH (as the name "antidiuretic" implies) stops diuresis (urine output).   4. ADH signals the kidneys to retain water.   5. This results in a decrease in urine output to conserve the fluid already in the body.

Sodium's Role in Fluid Balance

• Sodium: A primary electrolyte critical for cellular osmosis and diffusion, as well as the nervous system.
• Salty Foods: Consuming salty foods (like french fries) increases serum sodium. Because the body desires homeostasis, it triggers thirst to intake more water to balance out the high electrolyte levels.

Disorders of Water Imbalance

• Disorders can occur due to:   - Volume Issues: Too much fluid or too little fluid.   - Concentration Issues: Too many electrolytes or too few electrolytes.   - Distribution Issues: An imbalance in where the fluid is located, even if the total amount of fluid or particles in the body is correct.     - Example of Distribution Issue: Edema (fluid moves into the interstitial space rather than staying in circulation). This can be painful, limit mobility (especially in the ankles), and make the patient feel "heavy."

Fluid Deficiency: Dehydration vs. Volume Depletion

• Dehydration: More water is being lost than sodium. The primary concern is water loss.   - Causes: Diuretics, insufficient intake, or excessive sweating.
• Volume Depletion: Water and sodium (electrolytes) are lost equitably or in significant amounts together.   - Examples: Hemorrhage, vomiting, and diarrhea.

The Mechanism of Sweat Loss

• Fluid leaves the body via sweat in a specific "up and out" sequence:   1. Intracellular: Fluid starts inside the cells.   2. Interstitial: Fluid shifts from the cells into the tissue.   3. Plasma: Fluid is then absorbed from the tissue into the bloodstream.   4. Pores: Fluid is expelled out of the pores as sweat.

Acid-Base Balance and Buffers

• pH Range: The normal blood pH range is $7.35$ to $7.45$. Variations outside this range can be fatal.
• ABGs (Arterial Blood Gases): Monitored closely, especially in patients on ventilators.
• Chemical Buffers: Substances that bind to acids to neutralize them and bring a low pH back up to a neutral level.   - Bicarbonate ($HCO_3$): Considered the primary extracellular chemical buffer. It is a key value in ABGs.   - Phosphate: An electrolyte that also acts as a chemical buffer.
• Acid-Base Components:   - Lungs (Respiratory): Responsible for Carbon Dioxide ($CO_2$). $CO_2$ is an acid and lowers pH.   - Kidneys (Metabolic): Responsible for Bicarbonate ($HCO_3$). The kidneys also expel free Hydrogen ($H^+$) ions, which lowers pH.
• Homeostasis Goal: If pH is low (acidic), a buffer (like bicarb) will bind to the acid to stabilize and raise the pH.

Chemoreceptors and Respiratory Control of pH

• Chemoreceptors: Receptors that detect chemical changes in the body, including changes in $CO_2$ levels and pH.
• The Mechanism of Compensation:   1. Chemoreceptors detect that pH is decreasing (becoming more acidic).   2. The body recognizes this is because Carbon Dioxide ($CO_2$) is high (since $CO_2$ is an acid).   3. The brain signals the body to increase the respiratory rate.   4. Increasing the respiratory rate helps the body expel more $CO_2$ through breathing.   5. As $CO_2$ levels go down, the pH levels go back up, restoring balance.

Questions & Discussion

• Q: Is fluid imbalance usually noticed first through vital signs?   - A: Usually it is seen first in blood pressure as hypotension (dehydration drop), but physical signs like dry lips and skin tenting can sometimes be picked up visually before anything shows on a monitor. Older patients' vitals drop quicker than younger people.
• Q: Does the one on that side (Scenario B) mean swelling?   - A: Yes. If there are fewer electrolytes in the tissue than the cell, water moves from the tissue into the cell, making it swell. In Scenario A, where the tissue has too many electrolytes, water leaves the cell, making it shrink.
• Q: What about diarrhea? Is it volume depletion or dehydration?   - A: It is volume depletion because you lose not only fluid but a significant amount of electrolytes with it. Everything comes out. Similarly, vomiting and having your stomach pumped are volume depletion.
• Q: Is bicarbonate considered the primary extracellular buffer?   - A: Yes, especially regarding ABGs, where $CO_2$ and $HCO_3$ are the primary focuses.