Understanding Body Fluids: Composition and Distribution

• Body fluids are liquids originating from inside living humans, including excreted or secreted fluids like blood.
• The total amount of fluids in the human body is approximately 70% of body weight.
• Body fluid is divided into two compartments: intracellular fluid (ICF) and extracellular fluid.

Intracellular Fluid (ICF)

• ICF is inside the cells, comprising 55% of total body water.

Extracellular Fluid

• Outside the cells, extracellular fluid makes up 45% of total body water.

Composition of Body Fluids

• Organic substances include glucose, amino acids, fatty acids, hormones, and enzymes.
• Inorganic substances include sodium, potassium, calcium, magnesium, chloride, phosphate, and sulphate.

Extracellular Fluid Details:

• Interstitial fluid is present between the cells, making up approximately 80% of the extracellular fluid.
• Plasma is present in the blood, approximately 20% of the extracellular fluid.
• Extracellular fluid also includes lymph, synovial fluid, cerebrospinal fluid, and pleural, pericardial, and peritoneal fluids.

Interstitial Fluid

• Interstitial fluid, also known as intercellular fluid or tissue fluid, is fluid between the cells of multi-cellular organisms.
• It delivers materials to the cells, aids intercellular communication, and removes metabolic waste, representing the largest portion of the ECF compartment.
• It consists of a water solvent containing amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, salts, and waste products from the cells.
• This fluid presents as a gel-like extracellular matrix.
• Plasma and interstitial fluid integrate through pores in blood capillaries, allowing water and most dissolved substances except protein to diffuse.
• Oxygen, nutrients, and regulatory molecules must pass into the interstitial fluid before reaching body cells.
• Waste products and hormone secretions from cells must first pass into the interstitial fluid before reaching the blood plasma.
• The exchange of material across capillaries occurs at a high rate by diffusion in both directions.
• Tissue fluid composition depends upon the exchanges between cells in biological tissue and blood.
• Tissue fluid has a different composition in different tissues and areas of the body and acts as a medium for intercellular communication.

Barriers Separating ICF, Interstitial Fluid, and Plasma

• Plasma membrane separates ICF from surrounding interstitial fluid.
• The blood vessel wall separates interstitial fluid from plasma.

Blood as a Fluid

• Blood is the fluid portion of the body.
• Blood transports oxygen from the lungs to body cells and carbon dioxide from the body cells to the lungs.
• Blood transports nutrients derived from food in the intestine to body cells, and other nutrients between organs.

Lymph

• Lymph is a clear and colorless fluid composed of 96% water and 4% solids.
• It is composed 2-6% of proteins like albumin, globulin, fibrinogen, prothrombin, clotting factors, antibodies, enzymes,
• Lipids, 5-15% and include chylomicrons and lipoproteins
• Carbohydrates mainly consist of glucose.
• Includes Non Protein Nitrogen like urea and creatinine
• Electrolytes present are sodium, calcium, potassium, chloride, and bicarbonates.

Functions of Lymph

• Lymph returns protein from tissue spaces into blood.
• It helps in the removal of bacteria, toxins, and other foreign bodies from tissues.
• Lymph maintains structural and functional integrity of tissue.
• It is a mean for intestinal fat absorption and helps to transport lymphocytes.

Intracellular Fluid Details

• Cytosol or intracellular fluid is the liquid found inside the cells with no single function.
• The cytosol is the site of multiple cell processes like glycolysis, gluconeogenesis, and the PPP, involved in signal transduction from the cell membrane to sites within the cell.

Body Fluid Composition Factors

• Tissue type: lean tissues have higher fluid content than fat tissues.
• Gender: males have more lean tissue and therefore more body fluid.
• Age: lean tissue and body fluid are lost with age.

Electrolytes of Body Fluids

• Electrolytes are salts and minerals that conduct electrical impulses in the body; common ones are sodium, chloride, potassium, calcium, and bicarbonate.
• Electrolytes in body fluids are charged, capable of being cations (positively charged, like Na+, K+, Ca+2) or anions (negatively charged, like Cl-, HCO3-, PO4-3).
• The chief extracellular cation is Na+, and the chief intracellular cation is K+.
• Cl- is the predominant anion outside cells, while phosphates constitute the bulk of intracellular anions.

Electrolyte Importance

• Control of the fluid balance of the body.
• They are important in muscle contraction, energy generation, and almost every major biochemical reaction in the body.

Movement of Body Fluids

• Movement occurs through membrane transport processes, including:
  • Passive transport, where substances move across a membrane from higher to lower concentration (down a concentration gradient) and does not require metabolic energy.
    • Simple diffusion involves the movement of substances from a region of high concentration to a of low concentration.
    • Facilitated diffusion transports substances across a biological membrane from an area of higher concentration to an area of lower concentration via carrier proteins.
  • Active transport, where substances move across a membrane against a gradient (from low concentration to high concentration), requiring energy and specific carrier proteins.

Movement of Fluids

• Hydrostatic pressure: regulates fluid intake and output as well as movement of water and substances dissolved in it between body compartments.
• Osmotic pressure: the pressure that drives water to move from areas of low solute concentration to areas of high solute concentration.

Osmotic Pressure Details

• Is driven by the number of particles per unit volume of fluids, with the amount of osmotic pressure exerted by a solute proportional to the number of molecules or ions.
• Osmoles are the unit used to express the concentration in terms of the numbers of particles.

Water Balance and Regulation

• Water balance exists when water intake equals water output.
• Daily water intake varies, with about 60% gained from drinking, 30% from moist foods, and 10% from metabolism.
• Thethirst mechanism is the primary regulator of water intake
• Thirst mechanism originates from the osmotic pressure of extracellular fluids and a thirst center in the hypothalamus will drive the need to drink
• Water output occurs through urine, feces, perspiration, evaporation from skin, and breathing.
• Distal convoluted tubules and collecting ducts of the nephrons regulate water output.
• Antidiuretic hormone from the posterior pituitary reduces water loss in urine.
• Inhibiting the ADH mechanism, when drinking adequate water, will lead to more water being released in urine.

Disorders of Water Balance

• Dehydration: water loss exceeds water intake.
• Hypotonic hydration: ECF is diluted, causing ECF sodium levels to lower (hyponatremia), increased osmosis occurs and tissue cells swell (oedema).

Electrolytes

• Electrolytes are important for maintaining fluid balance, contributing to acid-base regulation, facilitating enzyme reactions, and transmitting neuromuscular reactions.
• Electrolyte Balance:
  • Electrolyte balance exisits when the quantities of electrolytes gained equals the amount lost.
  • Charged ions capable of conducting electricity, are present in all body fluids and fluid compartments.
• Although the concentration of specific electrolytes differ between fluid compartments, a balance of cations and anions always exists.

Regulation of Fluid Volume in Body

• Includes antidiuretic hormone, Renin/angiotensin/aldosterone system, baroreceptors in carotid arteries and aorta , stretch receptors in atrium and juxtaglomerular apparatus, and cortisol.

Explore the composition and distribution of body fluids, including intracellular and extracellular fluids. Learn about the organic and inorganic substances found in body fluids. Understand the difference between interstitial fluid and plasma.