Water Properties and Body Fluids

Water Properties

• Water molecules are attracted to other substances (adhesion) and to other water molecules (cohesion).
• Capillary action is the combined force of cohesion and adhesion.
• "Wettable" surfaces cause a film of water to cling to the surface due to adhesion.
• Capillary action is essential for water transport in soil and plants.

Specific Heat

• Water has a large heat capacity, helping to regulate body temperature.
• Sweating allows the body to release heat through water evaporation when the ambient temperature is higher than body temperature.

Universal Solvent

• Water is a polar molecule that dissolves substances by breaking them into smaller pieces.
• This ability makes water an excellent solvent for many compounds.

Body Fluid Compartments

• The human body contains two main fluid compartments: intracellular fluid (ICF) and extracellular fluid (ECF).
• ICF is the fluid inside cells, representing 40% of body weight.
• ECF is the fluid outside cells, representing 60% of body weight.
• ECF can be further divided into:
  • Intravascular fluid (blood plasma): 5% of body fluid.
  • Interstitial fluid: between cells and in lymph vessels, 15% of body fluid.
  • Transcellular fluid: cerebrospinal, bile, pericardial, synovial fluid, 20% of body fluid.

Extracellular Fluid (ECF)

• ECF is comprised of extravascular and intravascular compartments.
• Extravascular fluid includes interstitial fluid and lymph fluid, as well as transcellular fluid.
• Intravascular fluid is represented by blood plasma.

Composition of Compartments

• Extracellular fluids are high in sodium (Na+), chloride (Cl-), calcium (Ca++), and bicarbonate (HCO3-). Blood plasma has more protein than interstitial fluid and lymph.
• Intracellular fluids are high in potassium (K+), phosphate, magnesium (Mg++), and have more protein than plasma.

Extracellular Fluid - Blood Plasma

• Blood plasma is the yellow liquid part of blood containing:
  • Dissolved proteins (albumins, globulins, fibrinogen)
  • Glucose
  • Clotting factors
  • Electrolytes (Na+, Ca2+, Mg2+, HCO3-, Cl-)
  • Hormones
  • Carbon dioxide
• Blood plasma also serves as the primary medium for excretory product transportation.

Extracellular Fluid - Interstitial Fluid

• Interstitial fluid bathes cells in tissues, contributing to:
  • Delivery of materials to cells
  • Intercellular communication
  • Removal of metabolic waste

Extracellular Fluid - Lymph

• Lymph circulates throughout the lymphatic system, collecting interstitial fluid from tissues and transporting it back to the blood.
• Lymph nodes filter microorganisms and foreign substances, contributing to the body's defense.
• Lymph plays a role in fat absorption from the small intestine and transports large molecules (enzymes, hormones) to the bloodstream.

Extracellular Fluid - Transcellular Fluid

• Transcellular fluid is a small component of ECF, residing in:
  • Cerebrospinal fluid
  • Gastrointestinal tract (saliva, gastric juice, pancreatic juice, intestinal juice, bile)
  • Urinary tract (urine)
  • Ducts of glands (salivary glands, sweat glands)
  • Serous cavities (pleural fluid, cerebrospinal fluid, synovial fluid, seminal fluid, amniotic fluid)

Functions of Water

• Water acts as a medium and reactant for chemical reactions.
• Water transports nutrients, electrolytes, blood gases, and metabolic wastes.
• Water regulates body temperature through its specific heat and enthalpy of vaporization.
• Water serves as a lubricant for internal organs.
• Water acts as a cushion (cerebrospinal fluid, amniotic fluid).

Taking Nutrients and Oxygen, Flushing Waste

• The body requires organic and inorganic molecules, including:
  • Amino acids
  • Carbohydrates (glucose, fructose, galactose)
  • Fatty acids
  • Vitamins
  • Minerals (Ca, Fe, K, Mg, etc.)
• These molecules are obtained through food.

Diffusion and Osmosis

• Diffusion is the movement of substances from areas of high concentration to areas of low concentration.
• Osmosis is the movement of water across a semi-permeable membrane from an area of high water concentration to an area of low water concentration.
• Osmotic pressure is the pressure that needs to be applied to prevent the inward flow of water across a semipermeable membrane.
• Hydrostatic pressure is the pressure exerted by a fluid at rest.

Dehydration

• A 1% decrease in body water can lead to:
  • Decreased saliva production
  • Increased blood osmotic pressure, stimulating osmoreceptors in the hypothalamus.
  • Decreased blood volume, leading to renin production.

Causes of Thirst

• Increased extracellular fluid osmolarity (1-2%) stimulates the thirst center via osmoreceptors in the hypothalamus.
• Decreased effective blood volume (hypovolemia) stimulates the thirst center via volume receptors in the vena cavae and atrium.
• Elevated levels of angiotensin II stimulate the thirst center.
• Dryness of mouth can also stimulate the thirst center.

Regulation of Fluid Output

• Hormones regulate fluid output:
  • Antidiuretic hormone (ADH) from the neurohypophysis
  • Aldosterone from the adrenal cortex
  • Atrial natriuretic peptide (ANP) from the heart atrial walls

Physiologic Fluid Imbalances

• Dehydration: decreased blood pressure, decreased glomerular filtration rate.
• Overhydration: increased blood pressure, increased GFR.
• Hyperventilation: water loss through lungs.
• Vomiting and diarrhea: excessive water loss.
• Fever: heavy perspiration.
• Burns: initial fluid loss persisting in severe burns.
• Hemorrhage: severe blood loss.