Exam 4 - Fluid Balance and pH Flashcards
Major fluid compartments of the body and their subdivisions
The body’s fluids are divided into two main compartments: the intracellular fluid compartment and the extracellular fluid compartment.
Intracellular fluid compartment: Includes all the fluid inside the body’s cells, which is part of the cytoplasm. This compartment is consistent in composition across all cells and accounts for the majority of the body’s water—about 70%, which equals approximately 40% of total body weight.
Extracellular fluid compartment: Makes up the remaining 30% of body water, or about 20% of total body weight, and includes all fluid found outside the cells. It is further divided into several sub compartments: interstitial fluid (fluid between cells), plasma (the liquid portion of blood inside vessels), lymph (within lymphatic vessels), cerebrospinal fluid (in the brain and spinal cord), and synovial fluid (in joints). Among these, interstitial fluid and plasma represent the majority of the extracellular fluid.
List the dominant cations and anions in the major fluid compartments
Cations:
Na+
K+
Ca2+
Mg2+
Anions:
Cl-
HCO3-
HPO4 2- and HPO4-
Protein
Causes of edema
During an inflammatory response (e.g., from infection), the capillary walls become more permeable. This increased permeability allows proteins to leak out of the plasma into the interstitial fluid. The presence of proteins in the interstitial space increases the osmotic pressure there, causing water to move out of the plasma and into the interstitial fluid by osmosis.
This movement of water into the interstitial space results in swelling, which is defined as edema
Hypernatremia is caused by high plasma Na+ levels. Hypernatremia, such that caused by excess aldosterone secretion, can cause pulmonary edema
How water content of the body is maintained
Maintained through a balance of fluid intake and fluid output, regulated by thirst mechanisms, hormonal control, and kidney function.
When the body loses water (through sweating, urination, or breathing), osmoreceptors in the hypothalamus detect an increase in the concentration of solutes in the extracellular fluid. This triggers the thirst response, making the person feel the urge to drink water. Simultaneously, baroreceptors in the blood vessels sense changes in blood pressure and send signals to activate hormonal systems, such as the renin-angiotensin-aldosterone system (RAAS) and antidiuretic hormone (ADH). These hormones increase water reabsorption in the kidneys and help conserve fluid in the body.
On the other hand, when there is excess water in the body, thirst is suppressed, and hormones like ADH are reduced. This leads to increased urine output, allowing the kidneys to eliminate the extra water and bring fluid levels back to normal.
How body fluid osmolality is achieved and held in homeostatic balance
The body maintains fluid osmolality—the concentration of solutes like salts in body fluids—through a careful balance between hydrostatic pressure and osmotic pressure, allowing water to move freely between compartments like the blood, interstitial fluid, and intracellular fluid.
Osmosis plays a major role in this regulation. Water moves toward areas with higher solute concentrations. For example, during inflammation, capillaries become more permeable, allowing proteins to leak out into the interstitial fluid. This increases the osmotic pressure in that space, drawing water out of the blood and causing swelling (edema). In contrast, during dehydration, blood becomes more concentrated (higher osmolarity), and water moves from the interstitial space back into the bloodstream to balance the concentrations.
The intracellular fluid is kept distinct from extracellular fluid due to the selective permeability of cell membranes and the action of transport proteins like the sodium-potassium (Na⁺-K⁺) pump. This pump moves Na⁺ out of the cell and K⁺ into the cell using ATP, maintaining ion gradients that are essential for normal cell function and fluid balance. These ionic gradients also affect the movement of water and contribute to osmotic balance across membranes.
Together, these mechanisms ensure a homeostatic equilibrium where the total osmotic pressure between compartments remains relatively equal, allowing efficient exchange of water and solutes while maintaining stable internal conditions.
Explain the mechanisms that regulate extracellular fluid volume
The composition of intracellular fluid (ICF) is maintained by the plasma membrane’s selective permeability and active ion transport. The membrane allows small, nonpolar molecules to pass while blocking large or polar ones, helping the cell control its internal environment. A key component in this process is the sodium-potassium pump, which uses energy to move Na⁺ out of the cell and K⁺ in, creating a concentration and charge difference essential for cell function.
Large molecules like proteins remain inside the cell, further contributing to the unique composition of ICF. Together, selective transport and molecular retention help maintain osmotic balance, proper cell shape, and overall cellular stability.
Explain the mechanisms that regulate extracellular fluid volume
The body regulates extracellular fluid (ECF) volume through a combination of thirst mechanisms, hormonal control, and kidney function. When fluid loss occurs—due to factors like excessive sweating, alcohol intake, vomiting, or diarrhea—the volume of ECF decreases and its osmolarity increases. This change triggers the thirst response and activates hormones such as antidiuretic hormone (ADH) and aldosterone. ADH increases water reabsorption in the kidneys, while aldosterone promotes sodium retention, which in turn helps the body retain water. Additionally, the renin-angiotensin system becomes active when blood pressure drops, further encouraging fluid retention and stimulating thirst. These mechanisms help restore ECF volume and maintain proper hydration.
In cases of dehydration, water moves by osmosis from the cells into the extracellular space, causing cells to shrink and potentially impairing their function. If left untreated, this can lead to serious complications. Rehydration with fluids containing electrolytes is essential to restore normal fluid balance and prevent over-dilution of body fluids. On the other hand, hyperhydration, or water intoxication, occurs when too much water is consumed without adequate electrolyte intake. This dilutes sodium levels in the blood, causing water to move into cells by osmosis and leading them to swell. In the brain, this can increase intracranial pressure, resulting in symptoms like headache, confusion, seizures, or even coma. To prevent such imbalances, the body uses the same regulatory mechanisms to adjust fluid intake and output, ensuring ECF volume stays within a healthy range.
How sodium ion concentration is achieved
Sodium ion concentration in the body is regulated mainly by the kidneys and several hormones to maintain fluid balance and blood pressure. Sodium is the most abundant extracellular cation and plays a key role in creating osmotic pressure, which drives water movement. The kidneys filter sodium and reabsorb it based on the body’s needs—more is excreted when sodium intake is high, and more is reabsorbed when levels are low.
Hormones such as aldosterone increase sodium reabsorption, reducing sodium loss in urine, while ADH helps conserve water when sodium levels rise, preventing dehydration. ANH promotes sodium and water excretion when blood pressure is high, counteracting the effects of aldosterone and ADH. Sodium can also be lost through sweat, especially during heat or exercise, but the body adjusts by reducing urinary sodium loss. These mechanisms work together to maintain sodium levels within a narrow, healthy range, ensuring stable blood volume and pressure.
Regulation of chloride ions
Chloride ions (Cl⁻) are the main anions in the extracellular fluid and play a key role in maintaining electrical neutrality and inhibiting nerve and muscle activity. Their movement and concentration are closely linked to the regulation of positively charged ions, especially sodium (Na⁺). Because Cl⁻ and Na⁺ are electrostatically attracted, mechanisms that control Na⁺ concentration also affect Cl⁻ levels.
As Na⁺ moves across cell membranes, Cl⁻ often follows to balance the electrical charge. Therefore, sodium regulation is the primary factor in chloride regulation.
Potassium ions in homeostasis
Potassium ion (K⁺) homeostasis is crucial for maintaining the electrical activity of excitable cells like neurons and muscle fibers. K⁺ concentration in the extracellular fluid is tightly regulated because it directly affects the resting membrane potential. If K⁺ levels rise (hyperkalemia), cells depolarize and become more excitable. If levels fall (hypokalemia), cells hyperpolarize and become less responsive.
Calcium ion homeostasis
Calcium ion (Ca²⁺) homeostasis is tightly regulated to ensure proper muscle contraction, neurotransmitter release, and bone strength. Most calcium in the body is stored in bones, while the small amount in extracellular fluid must be kept within a narrow range. Calcium regulation involves three main organs: the bones, kidneys, and digestive tract, and is controlled by three hormones: parathyroid hormone (PTH), vitamin D₃, and calcitonin.
When blood calcium levels drop, PTH is secreted to raise them. It acts in three ways: (1) stimulating osteoclasts to release calcium from bone, (2) increasing calcium reabsorption in the kidneys, and (3) promoting the formation of active vitamin D₃, which enhances calcium absorption from the digestive tract. Without sufficient PTH or vitamin D₃, calcium levels fall quickly, which can lead to life-threatening conditions like muscle tetany.
Vitamin D₃, obtained from diet or sunlight exposure, is essential for calcium absorption in the intestines. A lack of sunlight can lead to reduced vitamin D₃ and impaired calcium uptake, even with adequate calcium intake.
Calcitonin, produced by the thyroid gland, helps lower blood calcium when levels are high by inhibiting bone breakdown. However, its role is less critical than PTH in daily calcium regulation.
Mechanism of the renin-angiotensin-aldosterone system
The renin-angiotensin-aldosterone system (RAAS) is a hormonal mechanism that helps regulate blood pressure and extracellular fluid volume. It is activated when blood pressure drops, especially in the afferent arterioles of the kidneys. Specialized cells called juxtaglomerular cells detect this drop and release the enzyme renin.
Renin initiates a cascade by converting angiotensinogen (produced by the liver) into angiotensin I. Angiotensin I is then converted into angiotensin II by an enzyme in the lungs. Angiotensin II has three main effects:
1) Vasoconstriction – it narrows blood vessels, increasing resistance and raising blood pressure.
2) Stimulates thirst – prompting water intake to boost blood volume.
3) Stimulates aldosterone secretion from the adrenal cortex – aldosterone promotes sodium reabsorption in the kidneys.
As sodium is reabsorbed, water follows due to osmosis, which increases the volume of fluid retained in the body and reduces urine output. This raises extracellular fluid volume and helps restore normal blood pressure.
Mechanism of ANH
The atrial natriuretic hormone (ANH) mechanism helps reduce blood volume when it becomes too high. When the atria of the heart are stretched due to increased blood volume, they release ANH. This hormone reduces sodium reabsorption in the kidneys, especially in the distal tubules and collecting ducts. As more sodium is lost in the urine, water follows by osmosis, increasing urine volume and decreasing extracellular fluid and blood volume.
In short, ANH lowers blood volume and pressure by promoting sodium and water excretion. It is primarily a response to high blood volume and does not significantly activate during low blood pressure. Instead, low atrial pressure inhibits ANH secretion.
Mechanism of ADH
The antidiuretic hormone (ADH) mechanism helps regulate extracellular fluid volume, especially during significant drops in blood pressure (5–10%). When blood pressure falls, ADH is released, prompting the kidneys to reabsorb water from the distal convoluted tubules and collecting ducts. This leads to less urine output and more concentrated urine.
By retaining more water, ADH increases extracellular fluid volume, which helps restore blood pressure. In short, ADH helps the body conserve water and maintain stable blood volume and pressure during times of dehydration or low blood pressure.
Acids and bases and their relationship with buffers
Acids and bases play a crucial role in maintaining the pH balance of body fluids. Acids are substances that release hydrogen ions (H⁺) into a solution, while bases remove H⁺. Strong acids, such as hydrochloric acid, completely dissociate into their ions, releasing a large amount of H⁺. Strong bases like sodium hydroxide also fully dissociate, producing hydroxide ions (OH⁻) that bind with H⁺ to form water. In contrast, weak acids and bases only partially dissociate. Weak acids release H⁺ until equilibrium is reached, and weak bases, like ammonia, bind to free H⁺ to reduce its concentration.
Buffers are essential for preventing drastic changes in pH. They function by binding excess hydrogen ions when the environment becomes too acidic or releasing H⁺ when it becomes too basic. This stabilizes the pH of body fluids, keeping it within a narrow, healthy range. Regulation of acid-base balance in the body is managed by both chemical and physiological buffer systems. Chemical buffers act quickly to resist pH changes, while physiological systems, including the respiratory and renal systems, provide more sustained regulation. The respiratory system responds within minutes by adjusting carbon dioxide levels, which affect carbonic acid concentration, while the renal system makes longer-term adjustments by controlling the excretion or reabsorption of H⁺ and bicarbonate. Together, these systems maintain the acid-base balance vital for normal cellular function.
Actions of the three buffer systems: Carbonic Acid/Bicarbonate Buffer System
The carbonic acid/bicarbonate buffer system is one of the most important regulators of extracellular pH. It works through a reversible reaction where carbonic acid (H₂CO₃) breaks down into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺). When excess H⁺ is present (making the fluid acidic), it binds with bicarbonate to form carbonic acid, reducing free H⁺ and preventing a sharp pH drop. When H⁺ is low (making the fluid more basic), carbonic acid dissociates to release H⁺, restoring balance. This system helps stabilize pH during metabolic conditions such as intense exercise, fat metabolism, or ingestion of alkaline substances.
Actions of the three buffer systems: Protein Buffer System
The protein buffer system is the most significant buffer within cells and plasma, accounting for about three-fourths of the body’s buffering capacity. Proteins act as buffers due to the presence of amino acid functional groups like carboxyl (–COOH) and amino (–NH₂), which can accept or donate H⁺. As H⁺ levels rise, these groups bind excess hydrogen ions, and when H⁺ levels fall, they release hydrogen ions to maintain pH stability. Hemoglobin in red blood cells is a major contributor to this system.
Phosphate Buffer System
The phosphate buffer system mainly operates inside cells and involves molecules like HPO₄²⁻ and H₂PO₄⁻. When pH drops, HPO₄²⁻ binds with H⁺ to form H₂PO₄⁻, reducing acidity. When pH rises, H₂PO₄⁻ releases H⁺ to increase acidity. This system is especially important for buffering pH in the intracellular fluid and within kidney tubules.
Mechanisms of acid-base balance
The body maintains acid-base balance through the respiratory and renal systems. The respiratory system adjusts blood pH by regulating CO₂ levels. Increased CO₂ forms more H⁺, lowering pH, while deeper breathing expels CO₂ and raises pH. Chemoreceptors sense pH changes and adjust breathing accordingly.
The renal system contributes by secreting H⁺ and reabsorbing bicarbonate (HCO₃⁻). This reabsorption neutralizes excess H⁺. If acidosis persists, kidneys produce ammonia (NH₃), which binds H⁺ to form ammonium (NH₄⁺), allowing its excretion in urine. Aldosterone also plays a role by enhancing H⁺ secretion. Together, these systems maintain blood pH within a narrow, healthy range.
Cause and effects of acid-base titrations: Acidosis
Acidosis arises when the pH of body fluids drops below 7.35 and can be either respiratory or metabolic in origin. Respiratory acidosis results from impaired CO₂ elimination due to poor ventilation, while metabolic acidosis stems from the accumulation of acidic byproducts of metabolism, such as in diabetes or kidney dysfunction. The primary effect of acidosis is central nervous system depression, which can lead to confusion, disorientation, and coma. The body attempts to maintain pH through buffer systems, respiratory adjustments, and renal compensation. The respiratory system responds quickly by altering the breathing rate to regulate CO₂, while the kidneys take 1–2 days to enhance H⁺ excretion and bicarbonate reabsorption. If these mechanisms fail or are overwhelmed, acidosis symptoms become more severe.
Cause and effects of acid-base titrations: Alkalosis
Alkalosis is a condition where blood pH rises above 7.45, resulting from either respiratory or metabolic causes. Respiratory alkalosis is typically due to hyperventilation, which removes too much CO₂, reducing hydrogen ion concentration and raising pH. Metabolic alkalosis can occur from excess antacid intake or loss of stomach acids. The primary effect of alkalosis is heightened nervous system activity, leading to muscle spasms and, in severe cases, convulsions or even respiratory failure. The body compensates through buffer systems, slowed respiration, and renal adjustments. If alkalosis is sustained, the kidneys help by reducing hydrogen ion secretion and bicarbonate reabsorption to restore pH balance.
Body water distribution (NOTES)
Water in the body is compartmentalized into two main areas: intracellular fluid (ICF) and extracellular fluid (ECF). About two-thirds of total body water (roughly 25 out of 40 liters in an average adult) is found in the ICF, inside cells. The remaining one-third is in the ECF, which includes plasma (the fluid part of blood) and interstitial fluid (the fluid between cells).
Factors affecting body water distribution (NOTES)
Total body water content is influenced by factors such as age, body mass, gender, and fat composition. Infants have the highest water content—about 73%—due to low body fat and bone mass. Adult males generally have 60% water content, while females have around 50%, largely because females tend to have higher body fat and less skeletal muscle. In old age, water content decreases to about 45%. Throughout life, a gradual decline in body water occurs. Skeletal muscle is 65% water, whereas adipose tissue is only about 20%, making leaner individuals more hydrated overall.
Water balance and hydration (NOTES)
For proper hydration, water intake must equal water output. On average, water intake consists of 60% from ingested fluids, 30% from solid foods, and 10% from metabolic processes. Water loss occurs primarily through urine (60%), followed by insensible losses through skin and lungs (28%), sweat (8%), and feces (4%). An increase in plasma osmolality triggers the thirst mechanism and the release of antidiuretic hormone (ADH), both of which work to retain and restore body water.
