Biochemistry: Body and Biological Fluids

Models of Human Body Composition

Body Composition Models

Two-Compartment Model:
- Fat Mass: Accounts for $15\text{--}25\%$ of total body mass.
- Fat-Free Mass: Accounts for $75\text{--}85\%$ of total body mass.
Three-Compartment Model:
- Fat Mass: Accounts for $15\text{--}25\%$ of total body mass.
- Body Water: Accounts for $60\%$ of total body mass.
- Protein and Minerals: Combined to account for $20\%$ of total body mass.
Four-Compartment Model:
- Fat Mass: Accounts for $15\text{--}25\%$ of total body mass.
- Body Water: Accounts for $60\%$ of total body mass.
- Protein: Accounts for $15\%$ of total body mass.
- Minerals: Accounts for $5\%$ of total body mass.
General Estimation Rule:
- As the number of measured body compartments increases, the margin of error in total body composition estimates decreases.

Quantitative Mass Breakdown of the Adult Human Body

Overall Mass Distribution:
- The adult human body is composed of approximately $40\text{--}50\%$ solid materials by mass.
- The remaining $50\text{--}60\%$ of body mass consists of water and gases.
- Primary chemical elements constituting the solid portion include oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.
- Solid structures are predominantly partitioned into bones, muscles, and organs.
Male Adult Body Composition Defaults:
- Body Water: $60\%$ of total body mass.
- Solid Mass: $40\%$ of total body mass.
  - Fat: $20\%$ of total body mass.
  - Protein: $15\%$ of total body mass.
  - Minerals: $5\%$ of total body mass.
Female Adult Body Composition Defaults:
- Body Water: Approximately $50\%$ of total body mass.
- Solid Mass: Approximately $50\%$ of total body mass.
  - Fat: Approximately $30\%$ of total body mass.
  - Protein: Approximately $15\%$ of total body mass.
  - Minerals: Approximately $5\%$ of total body mass.

Calculated Example Problems

Problem 1: Calculate the mass composition of body water and solids in kilograms for an adult male (Jeremy) weighing $176\,\text{lbs}$ .
- Step 1: Conversion of mass from pounds to kilograms $\text{Mass in kg} = \frac{176\,\text{lbs}}{2.20462\,\text{lbs/kg}} = 79.832\,\text{kg}$ (Using the standard conversion factor of $2.2\,\text{lbs/kg}$ gives $176 \div 2.2 = 80\,\text{kg}$ )
- Step 2: Calculate Body Water Mass ( $60\%$ of total mass) $\text{Body Water Mass} = 0.60 \times 80\,\text{kg} = 48\,\text{kg}$ (Or $0.60 \times 79.832\,\text{kg} = 47.90\,\text{kg}$ )
- Step 3: Calculate Solid Body Mass ( $40\%$ of total mass) $\text{Solid Mass} = 0.40 \times 80\,\text{kg} = 32\,\text{kg}$ (Or $0.40 \times 79.832\,\text{kg} = 31.93\,\text{kg}$ )
- Step 4: Detailed Breakdown of Solid Components $\text{Fat Mass (20\%)} = 0.20 \times 80\,\text{kg} = 16\,\text{kg}$ $\text{Protein Mass (15\%)} = 0.15 \times 80\,\text{kg} = 12\,\text{kg}$ $\text{Mineral Mass (5\%)} = 0.05 \times 80\,\text{kg} = 4\,\text{kg}$
Problem 2: Determine the percentage composition and individual component masses for an adult female (Pia) with a $60\,\text{kg}$ body frame.
- Step 1: Determine percentage composition parameters for adult females $\text{Body Water} = 50\%$ $\text{Total Solid Mass} = 50\%$ $\text{Fat Mass} = 30\%$ $\text{Protein Mass} = 15\%$ $\text{Mineral Mass} = 5\%$
- Step 2: Calculate individual component masses for $60\,\text{kg}$ body mass $\text{Body Water Mass} = 0.50 \times 60\,\text{kg} = 30\,\text{kg}$ $\text{Fat Mass} = 0.30 \times 60\,\text{kg} = 18\,\text{kg}$ $\text{Protein Mass} = 0.15 \times 60\,\text{kg} = 9\,\text{kg}$ $\text{Mineral Mass} = 0.05 \times 60\,\text{kg} = 3\,\text{kg}$

Overview and Definition of Body Fluids

Definition: Body fluids are specialized aqueous solutions located inside the body, composed of water acting as a solvent and various dissolved solute substances.
Major Solutes: Key dissolved constituents include electrolytes (such as sodium, potassium, chloride, and bicarbonate) and organic macromolecules (such as proteins, enzymes, and hormones).
Primary Functions:
- Facilitating intercellular nutrient transport.
- Enabling metabolic waste extraction and excretion.
- Sustaining systemic homeostatic equilibrium.

Primary Fluid Compartments: ICF vs. ECF

Fluid Compartments Diagram

Intracellular Fluid (ICF) Compartment:
- Includes all biological fluid enclosed within cell bodies by plasma membranes.
- Contains approximately two-thirds ( $66.7\%$ ) of total body water.
- Represents about $40\%$ of total body mass in an individual with $60\%$ body water content.
- Chemical Profile: Primarily composed of potassium ions ( $\text{K}^+$ ), organic anions, and proteins.
- Regulation: Composition is actively maintained and regulated by selective cell membrane permeability and active cellular metabolism.
Extracellular Fluid (ECF) Compartment:
- Encompasses all biological fluids located outside of cell membranes.
- Contains the remaining one-third ( $33.3\%$ ) of total body water.
- Represents approximately $20\%$ of total body mass.
- Chemical Profile: Consists predominantly of sodium chloride ( $\text{NaCl}$ ) and sodium bicarbonate ( $\text{NaHCO}_3$ ) solutions.

Intracellular and Extracellular Fluid Distribution

Sub-Compartments of Extracellular Fluid (ECF)

Interstitial Fluid (ISF):
- Surrounds tissue cells within intercellular space (tissue spaces) without circulating actively through blood vessels.
- Constitutes the largest fraction of ECF, making up approximately three-fourths ( $75\%$ ) of total ECF volume.
- Components: Water solvent, amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, inorganic salts, and cellular waste products.
- Physiological Roles: Serves as a medium for intercellular communication, bathes tissue cells directly, and collects metabolic waste products for removal.
Plasma:
- The non-cellular, yellow liquid fraction of blood in which erythrocytes, leukocytes, and platelets are suspended within blood vessels.
- Forms the intravascular fluid component of ECF, constituting approximately one-fourth ( $25\%$ ) of total ECF volume.
- Makes up $55\%$ of total whole blood volume.
- Components: Water solvent, dissolved plasma proteins (albumin, globulins, fibrinogen), glucose, carbon dioxide, clotting factors, mineral ions, and hormones.
- Physiological Roles: Serves as the primary intravascular transport medium; carries blood cells, nutrients, signaling molecules, and excretory products; regulates systemic blood pressure, intravascular volume, and acid-base pH balance; assists in body temperature regulation.
Transcellular Fluid:
- The portion of total body water contained within specialized epithelial-lined space compartments.
- Represents the smallest fluid compartment, comprising approximately $2.5\%$ of total body water.
- Representative Examples: Cerebrospinal fluid (CSF), aqueous humor, synovial fluid, urine, and serous cavity secretions.

Quantitative Fluid Dynamics in a Standard 70 kg Adult Male

Body Composition Flowchart for 70 kg Male Adult

Total Body Mass: $70\,\text{kg}$
Solid Body Fraction ( $40\%$ ): $28\,\text{kg}$
- Fat Component ( $20\%$ ): $14\,\text{kg}$
- Protein Component ( $15\%$ ): $10.5\,\text{kg}$
- Mineral Component ( $5\%$ ): $3.5\,\text{kg}$
Body Water Fraction ( $60\%$ ): $42\,\text{kg}$ ( $42\,\text{L}$ )
- Intracellular Fluid (ICF) ( $\frac{2}{3}$ of body water): $28\,\text{L}$
- Extracellular Fluid (ECF) ( $\frac{1}{3}$ of body water): $14\,\text{L}$
  - Interstitial Fluid (ISF) ( $\frac{3}{4}$ of ECF): $10.5\,\text{L}$
  - Blood Plasma ( $\frac{1}{4}$ of ECF): $3.5\,\text{L}$
  - Transcellular Fluid ( $2.5\%$ of total body water): $1.05\,\text{L}$

Physiological Distribution and Systems of Biological Fluids

Cardiovascular System: Blood flows through a continuous vascular network of arteries, capillaries, and veins to deliver oxygen and essential nutrients to peripheral tissues while retrieving metabolic waste products.
Lymphatic System: Lymphatic vessels collect excess interstitial fluid (lymph) from tissue spaces and transport it back into systemic venous circulation, passing through lymph nodes for filtration and immune monitoring.
Central Nervous System (CNS): Cerebrospinal fluid circulates through the subarachnoid space and ventricular system around the brain and spinal cord, providing mechanical cushioning, buoyancy, nutrient distribution, and waste clearance.
Digestive System: Saliva in the oral cavity and specialized gastric/intestinal secretions along the gastrointestinal tract facilitate mechanical lubrication, chemical digestion, and nutrient absorption.
Joint Cavities: Synovial fluid resides inside synovial joint capsules to lubricate articular cartilage surfaces and prevent mechanical wear.

Essential Functions of Biological Fluids

Transportation: Serves as a medium to convey gases (oxygen and carbon dioxide), nutrients, metabolic substrates, hormones, and metabolic wastes throughout the body.
Protection: Contains specialized immune defense elements, including white blood cells, immunoglobulins (antibodies), and complement proteins to combat pathogens.
Regulation: Controls environmental variables such as extracellular pH balance, ionic osmolality, and targeted central nervous system homeostasis.
Lubrication: Reduces mechanical friction across sliding anatomical surfaces, including articular joints (synovial fluid), heart membranes (pericardial fluid), lung linings (pleural fluid), and abdominal cavities (peritoneal fluid).
Thermoregulation: Controls core temperature through evaporative cooling mediated by sweat secretion onto skin surfaces.

Detailed Catalog of Specific Biological Fluids

Anatomical Distribution of Common Body Fluids

Amniotic Fluid: Clear, yellowish fluid contained within the amniotic sac that surrounds and cushions the developing fetus during pregnancy.
Aqueous Humor: Clear intraocular fluid produced by the ciliary body in the eye that supplies nourishment to avascular tissues (cornea and lens) and maintains intraocular pressure.
Bile:
- Synthesized in the liver and stored/concentrated in the gallbladder.
- Contains bile salts, phospholipids, cholesterol, and bile pigments (bilirubin).
- Functions to emulsify and solubilize dietary fats for digestion and absorption, as well as serve as an excretory pathway for bilirubin and lipophilic metabolic wastes.

Bile Synthesis, Storage, and Functions

Blood Plasma:
- Liquid fraction of whole blood remaining after cellular components are separated.
- Composes $55\%$ of whole blood volume.
- Contains dissolved proteins, salts, glucose, clotting factors, and hormones.

Centrifugation of Whole Blood into Plasma, Buffy Coat, and Erythrocytes

Breast Milk (Mother's Milk): Secretion produced by female human mammary glands serving as the complete primary nutritional source for newborns; contains fats, proteins, carbohydrates (lactose), minerals, vitamins, and maternal antibodies.
Cerebrospinal Fluid (CSF): Clear, colorless fluid filling the brain ventricles and spinal subarachnoid space; functions as a hydraulic shock absorber, provides physical buoyancy, delivers nutrients, and extracts metabolic wastes from neural tissue.

Cerebrospinal Fluid System and Ventricles

Cerumen (Earwax): Waxy lipid-rich secretion (varying in color from brown, orange, red, yellow, to gray) produced by ceruminous and sebaceous glands in the human external auditory canal.
Chyle: Milky bodily fluid comprising lymph fluid and emulsified free fatty acids/triglycerides formed in the small intestine during fatty food digestion and absorbed via specialized lymphatic capillaries called lacteals.
Exudate: High-protein inflammatory fluid containing white blood cells and cellular debris that leaks out of damaged blood vessels into surrounding tissue during infection, injury, or wound healing.
Gastric Juice: Highly acidic, variable digestive fluid secreted by stomach mucosal glands; contains water, hydrochloric acid ( $\text{HCl}$ ), electrolytes ( $\text{Na}^+$ , $\text{K}^+$ , $\text{Ca}^{2+}$ , $\text{PO}_4^{3-}$ , $\text{SO}_4^{2-}$ , $\text{HCO}_3^-$ ), and functional proteins/enzymes (pepsins and intrinsic factor).
Lymph: Clear-to-translucent fluid collected from interstitial spaces that flows through lymphatic vessels and lymph nodes to return tissue fluid and immune cell surveillance products back into venous circulation.
Mucus: Viscous, slippery fluid rich in mucin glycoproteins produced by epithelial lining membranes; acts as a protective barrier to prevent tissue desiccation and traps airborne particulate irritants, micro-organisms, or dust.
Pericardial Fluid: Serous fluid secreted between the parietal and visceral layers of the pericardial sac surrounding the heart to minimize surface friction during myocardial contraction cycles.
Peritoneal Fluid: Lubricating fluid located in small physiological volumes ( $5\text{--}20\,\text{mL}$ ) between the visceral and parietal peritoneal membranes lining the abdominal wall.
Pleural Fluid / Pleural Effusion: Serous fluid existing between lung pleural membranes (visceral and parietal pleura) that reduces respiratory friction; excessive fluid accumulation within this space is termed pleural effusion.
Pus: Opaque, thick inflammatory exudate rich in dead neutrophils, necrotic tissue cells, and bacterial microorganisms formed in response to pyogenic bacterial infections; displays variable coloration (white, yellow, green, or brown).
Saliva: Complex aqueous oral fluid produced by salivary glands; contains water, mucin, salivary amylase enzyme, immunoglobulins, inorganic mineral salts, desquamated epithelial cells, and leukocytes.
Sebum: Oily, lipid-dense substance secreted by cutaneous sebaceous glands into hair follicles to moisturize, coat, and waterproof skin surfaces.
Serous Fluid: Watery fluid derived from blood serum ultrafiltration that fills closed body cavities (pleural, pericardial, peritoneal) to attenuate surface friction.
Semen (Seminal Fluid): Viscous fluid ejaculated from the male urethra containing spermatozoa suspended in secretions from the seminal vesicles, prostate, and bulbourethral glands.
Sputum (Phlegm): Abnormally thick mucus produced and coughed up from diseased or inflamed lower respiratory airways (trachea and lungs), distinctly separate from oral saliva.
Synovial Fluid: Viscous, interstitial-like fluid containing hyaluronic acid located inside diarthrodial joint cavities; functions to cushion articular bone ends and eliminate kinetic friction during joint articulation.
Sweat: Hypotonic aqueous solution containing sodium chloride and minor metabolic solute wastes produced by dermal eccrine and apocrine glands; acts as the chief effector mechanism for evaporative thermoregulation.
Tears: Aqueous fluid secreted by lacrimal glands to clean, lubricate, and protect ocular surface tissues (basal tears, reflex tears from irritants, and emotional tears) with antimicrobial enzymes like lysozyme.
Urine: Liquid excretory waste product generated by renal nephrons through glomerular filtration, tubular reabsorption, and secretion; travels via ureters to the urinary bladder for temporary storage prior to micturition.
Vaginal Fluids: Cervicovaginal mucosal secretions produced by the vaginal epithelium that maintain physiological moisture, maintain acidic microenvironment flora, and participate in innate immune defense.