Test: Homeostasis of Body Fluids- 1 - NEET PG MCQ

ICF constitutes 40% of the overall body weight.

ECF constitutes 20% of overall body weight, which means it represents 1/5th of body mass and accounts for 1/3rd of total body water (33%).

• ICF constitutes 40% of total body mass.
• An adult weighing 70 kg has a total body water content of 42 litres.
• The volume of ICF, which is two-thirds of the total body water, amounts to 28 litres.

In the extracellular fluid (ECF), the principal cation is Na⁺ and the anion is Cl⁻.

Within the intracellular fluid (ICF), the predominant cation is K⁺ and the anion is phosphate.

• Since the concentration of K⁺ is highest within the cell,
• the most osmotically active particle is K⁺.

The Donnan effect refers to the distribution of all permeable ions as a result of the presence of non-permeable ions. K⁺ plays a key role in the resting membrane potential (RMP). Thus, it is not solely Na⁺ that contributes; other ions also play a significant part.

Almost all of the body's K⁺ exists within the exchangeable pool. In contrast, only 65–70% of the body's Na⁺ is exchangeable.

Plasma proteins contribute to colloid osmotic pressure, also known as oncotic pressure. The average colloid osmotic pressure of healthy human plasma is approximately 28 mm Hg. This pressure consists of:

• 19 mm resulting from the molecular effects of the dissolved proteins
• 9 mm attributed to the Donnan effect, which refers to the additional osmotic pressure produced by sodium, potassium, and other cations retained in the plasma due to the negative charge of proteins.

Ma²⁺ is primarily found within cells.

Water loss through sweat production from the skin can fluctuate significantly:

• Approximately 100 mL/day under normal conditions at a room temperature of 23°C
• Up to 1400 mL in hot weather
• As much as 5000 mL during extended periods of exercise

The total sodium concentration in the body ranges from 3500 to 5000 mEq (mMol), which equates to approximately 58 mEq/kg of body weight. The distribution of this sodium is as follows:

• 2100 mMols are found in ECF
• 2500 mMols are located in bone
• 270 mMol is present in ICF

Potassium is predominantly found in cells, with an intracellular concentration (ICF) of 98%. It is mainly located in:

• Muscle
• Skin
• Subcutaneous tissue
• Red blood cells

In contrast, the volume of cytoplasm in platelets is significantly smaller than in other cells, resulting in low potassium content within platelets.

A person's total blood volume (TBV) is associated with their body weight. The TBV for a child is approximately 75–80 mL/kg and is elevated during the neonatal stage:

• Starting at 85 mL/kg, it peaks at 105 mL/kg by the end of the first month.
• Following this peak, it gradually decreases over the subsequent months.

Consequently, the TBV of a 3.5-kg infant who is 2 weeks old will be around 350 mL, whereas a 10-kg toddler aged 15 months will have a TBV of about 800 mL. Blood volume can be roughly estimated as:

• 70 mL/kg for adults,
• 80 mL/kg for children,
• 100 mL/kg for neonates.

The concentration of plasma proteins in plasma is approximately 14 mEq/L. However, due to the fact that these proteins typically possess numerous negative charges per molecule, only a small number of particles (around 1 mM) are required to represent these milliequivalents. Furthermore, although the protein concentration, when assessed in grams per litre, may appear substantial, the high molecular weight of the average protein indicates that the protein concentration—when evaluated in moles per litre (molar concentration)—is quite low. Therefore, proteins contribute only minimally to the overall count of osmotically active particles (about 1 mOsm).

The contribution of plasma protein to plasma osmolality is low, at around 1 mOsm/litre. This is primarily due to:

• Low molar concentration: Plasma proteins are present in smaller amounts compared to other solutes.
• High molecular weight: Plasma proteins have larger sizes, which means fewer particles are available to exert osmotic pressure.

These factors combined lead to a minimal impact on overall plasma osmolality.

Edema can be characterised as noticeable swelling resulting from the increase in interstitial fluid volume. It does not become clinically apparent until the volume of interstitial fluid has risen by between 2.5 L and 3.5 L.

• Thus, if we take into account that the typical interstitial fluid volume is approximately 10 L,
• the most accurate response to the question would be 1.5 times the normal volume.

There are three primary safety mechanisms that inhibit excessive fluid build-up in the interstitial areas:

• Low compliance of the interstitium within the negative pressure range is equal to 3 mm Hg.
• Safety factor resulting from enhanced lymphatic flow is 7 mm Hg.
• Safety factor due to the washout of proteins from the interstitial regions is also 7 mm Hg.

Consequently, the overall safety factor against oedema is approximately 17 mm Hg. This indicates that the capillary pressure in peripheral tissues could theoretically increase by 17 mm Hg, or nearly twice the normal level, before significant oedema manifests.

In this context, two distinct substances are necessary:

• one for estimating the extracellular fluid (ECF)
• another for determining plasma volume

The interstitial fluid (ISF) is calculated using the formula ISF = (ECF – Plasma). The ECF can be assessed using Inulin (considered the best), 22Na, 125I-iothalamate, thiosulfate, mannitol, or sucrose. Plasma volume is evaluated with the help of 125I-albumin or Evans blue dye (T-1824).

Total body potassium is roughly 55 mEq/kg of body mass. Of this total:

• 98% resides in the intracellular compartment (mainly within the muscle, skin, subcutaneous tissue, and red blood cells).
• 2% is located in the extracellular compartment.

Some potassium is actively secreted in the most distal section of the distal convoluted tubules; however, potassium excretion mainly occurs through a passive mechanism. The Na-K-ATPase pump is responsible for maintaining the distribution of K⁺ ions.

Potassium is predominantly found in the cell, with an intracellular concentration (ICF) of 98%. Its highest levels are located in:

• Muscle
• Skin
• Subcutaneous tissue
• Red blood cells

The greatest amount of sodium is stored in the bone.

The total sodium content in the body ranges from 3500 to 5000 mEq (mMol), which is approximately 58 mEq/kg of body weight. Of this total:

• 90% is located in the ECF compartment
• 10% is found in the ICF compartment, primarily within the bones

The total potassium levels in the body range from 3000 to 3750 mEq (mMol), averaging approximately 53 mEq/kg of body weight. Of this total:

• 98% is found within the intracellular fluid (ICF) compartment, mainly in muscle tissue.
• 2% is located in the extracellular fluid (ECF) compartment.

Since plasma makes up one-quarter of the ECF, the concentration of K⁺ in plasma will be 0.5% of the overall body potassium percentage.

• Extracellular fluid (ECF) = 14 L
• Intracellular fluid (ICF) = 28 L

Plasma osmolarity refers to the concentration of solutes in the blood. It is an important measure that helps in understanding various bodily functions.

The normal range for plasma osmolarity is typically:

• 285 to 295 mOsm/L

Values outside this range may indicate certain health issues. For instance:

• Lower values could suggest conditions like hypoosmolarity.
• Higher values might indicate hyperosmolarity or dehydration.

Understanding plasma osmolarity is crucial for diagnosing and managing various medical conditions.