Body Composition and Fluids

• The human body can be organized from cells to tissues, then organs, and finally systems.

Body Composition

• Water makes up about 60% of body weight.
• Other components, about 40% of body weight, include proteins (18%), fats (15%), and electrolytes (7%).

Body Fluid Compartments

• A normal, young adult male who weighs 70kg has 60% of body weight as water.
• Body water is mainly distributed in two major compartments.
• Intracellular Fluid (ICF) comprises 2/3 of the body's TBW (Total Body Water).
• Extracellular Fluid (ECF) makes up 1/3 of TBW.
• Interstitial Fluid (IF) is 3/4 of ECF.
• Plasma accounts for 1/4 of ECF and 8% of TBW.

Factors Affecting Total Body Water (TBW)

• Physiological factors: age, sex, and body fat influence TBW.
• Pathological factors: vomiting, diarrhea, diseases with excessive water loss (like diabetes mellitus and excessive sweating), blood loss, and burns affect TBW.
• Other factors: climate, habits, and physical activity can also alter TBW, leading to disturbances in fluid balance.
• A baby presents low fat and bone mass, 73% or more of their mass is water.
• A woman presents a higher body fat percentage, smaller amount of skeletal muscles and 40-50% water.
• An "average" person has ~60% of their body weight represented by water.
• An elder has ~45% of their body weight represented by water.

Importance of Body Water

• Water is essential for life, constituting not less than 40% of TBW in any living organism.
• It acts as a solvent for digestion, absorption, secretion, exertion, and ion mobility.
• Water has an ionizing property, dissociating into OH- and H+, which is important in many aspects.
• Water is essential for all sensations except tactile sensation.
• It is necessary for body temperature regulation.
• Water makes tissues pliable and flexible.

Body Electrolytes

• Represents about 7% of the body composition
• Normal electrolyte value is 7% of the body composition.
• Electrolytes maintain the osmolarity of body fluids (number of osmoles/kg of solvent).
• Electrolytes have specific functions, such as calcium (Ca++) in bone mineralization.
• They are involved in determining cell membrane potential.
• Affect the water content and distribution in the body.
• Electrolytes depend on acid-base balance in the body.

Types of Solutions

• True solution: particle size is less than 1nm, stable and homogenous, passes freely through semi-permeable filter paper; e.g., NaCl glucose solution.
• Colloidal solution: particle size ranges from 1-200 nm, stable and homogenous, cannot pass through a biological filter paper; e.g., egg albumin and starch solution.
• Suspension: particle size is greater than 200nm, unstable and precipitates, cannot pass through biological filter paper; e.g., blood, sand, and charcoal solution.

Other Definitions

• Cells are the smallest independent unit with microscopic structure.
• Tissues are cells with similar structure and function.
• Organs are different types of tissues that carry out specific functions.
• Systems are groups of organs that carry out specific functions.

Organization of the human body from cells to systems with a focus on body composition, including water, proteins, fats, and electrolytes. Discusses the distribution of body water in intracellular and extracellular fluid compartments and factors affecting total body water.