# 4 Mass balance

Learning Objectives

After reading this section you should be able to-

• Define mass balance and identify components that affect mass balance (i.e., inputs, outputs)
• Define compartment
• Identify different kinds of matter that can be contained in body compartments
• Describe factors that can affect mass balance
• Calculate mass balance
• Explain how feedback loops are used to maintain mass balance

Mass balance is a fundamental concept in physiology that refers to the balance between the input and output of substances in the body to maintain homeostasis. In the context of human physiology, mass balance ensures that the total amount of a substance within the body remains constant over time, unless it is intentionally altered. This concept is crucial because it helps to regulate the body’s internal environment, ensuring that cells and organs function optimally.

Mass balance can be expressed with a simple equation:

where “Mass In” refers to the amount of a substance entering the body, “Mass Out” refers to the amount of a substance exiting the body, and “Δ Storage” represents any changes in the amount of a substance stored within the body compartments.

This equation can be taken a step further by stating that the mass of some substance X in a compartment equals the initial mass of substance X + Mass In – Mass Out. For example, if a person goes for a run and drinks 2 L of water during their run, but they lose 3 L of water to sweat, this would mean they have lost a total of 1 L of body water.

## Body Compartments

In physiology, a compartment refers to any distinct region within the body where specific substances are stored or transported. These compartments can be physical, like the various organs and tissues, or they can be functional, such as intracellular and extracellular spaces. Compartmentalization is essential for organizing and managing the different biochemical processes occurring in the body.

### Types of Body Compartments

The body has several types of compartments where different kinds of matter are contained:

• Intracellular Fluid (ICF) Compartment: This compartment includes all the fluid within the cells, constituting about 60-65% of the total body water. It contains ions like potassium (K+), magnesium (Mg2+), and phosphate (PO43-), as well as proteins and other organic molecules necessary for cellular function.
• Extracellular Fluid (ECF) Compartment: This compartment is outside of the cells and includes interstitial fluid, plasma, and transcellular fluid. It accounts for about 35-40% of the total body water. The ECF is rich in sodium (Na+), chloride (Cl), and bicarbonate (HCO3).
• Interstitial Fluid: This fluid bathes the cells and provides a medium for the exchange of substances between the blood and cells. It forms a significant part of the ECF.
• Plasma: The liquid component of blood, plasma contains water, electrolytes, nutrients, hormones, and waste products. It is crucial for transporting these substances throughout the body.
• Transcellular Fluid: This includes specialized fluids such as cerebrospinal fluid, synovial fluid, and digestive fluids. Though they make up a small percentage of total body fluid, they play specialized roles in their respective systems.
• Intracellular Compartments of Organelles: Within cells, organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus form additional compartments that segregate various biochemical processes.

## Different Kinds of Matter in Body Compartments

The body compartments contain various types of matter, including:

• Water: The most abundant substance, essential for maintaining cellular structure and function.
• Ions and Electrolytes: Such as sodium, potassium, calcium, and chloride, crucial for nerve transmission, muscle contraction, and fluid balance.
• Proteins: Including enzymes, hormones, and structural proteins that are vital for numerous bodily functions.
• Lipids: Stored in adipose tissue and used as an energy source, they also form cell membranes.
• Carbohydrates: Such as glucose, which provides energy for cellular processes.
• Gases: Including oxygen and carbon dioxide, essential for cellular respiration and metabolic waste removal.

## Factors Affecting Mass Balance

Several factors can affect mass balance in the body:

1. Dietary Intake: The quantity and quality of food and beverages consumed can significantly influence the mass balance of nutrients, electrolytes, and water. For example, a high-sodium diet can disrupt electrolyte balance and lead to fluid retention.
2. Excretion: The removal of waste products through urine, feces, sweat, and exhaled air is crucial for maintaining mass balance. Kidneys play a central role in excreting excess ions and waste products while conserving necessary substances.
3. Metabolism: Metabolic processes convert nutrients into energy and building blocks for cells. These processes can either consume or produce substances that affect mass balance. For instance, during exercise, the body consumes more glucose and oxygen, producing carbon dioxide and water as by-products.
4. Absorption and Secretion: The absorption of nutrients in the gastrointestinal tract and the secretion of substances by glands (e.g., digestive enzymes, hormones) impact mass balance. Conditions affecting these processes, like malabsorption syndromes, can disrupt balance.
5. Homeostatic Mechanisms: The body employs various feedback mechanisms to regulate mass balance. For example, antidiuretic hormone (ADH) regulates water balance by controlling the amount of water reabsorbed by the kidneys. Similarly, aldosterone helps maintain sodium and potassium balance.
6. Environmental Factors: Temperature, humidity, and physical activity levels can affect fluid and electrolyte balance. For instance, in hot weather or during intense exercise, the body loses more water and electrolytes through sweat, necessitating adjustments in intake and excretion.

## Mass balance and feedback loops

Homeostatic mechanisms, such as negative feedback loops, play a critical role in maintaining mass balance. For example:

• Antidiuretic Hormone (ADH): Regulates water balance by increasing water reabsorption in the kidneys, thus reducing urine output.
• Aldosterone: Helps regulate sodium and potassium balance by increasing sodium reabsorption and potassium excretion in the kidneys.

Maintenance of mass balance and the factors influencing it is essential for maintaining homeostasis and overall health. Disruptions in mass balance can lead to various physiological disorders.

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