Human Body Organization and Cell Structure

Human Body Organization

• Human body is composed of coordinated systems like the digestive system
• Systems consist of organs, which are comprised of tissues with complementary functions
• Tissues consist of numerous cells
• Cells are the fundamental units of body structure and function
• Human cells, as well as multi-cellular animals and plants, are eukaryotic containing a nucleus surrounded by a membrane and other membrane-bound structures
• These membrane-bound structures are called organelles, such as mitochondria and ribosomes, and perform specialized functions
• Prokaryotic cells, for example bacteria, lack these membranous structures
• The eukaryotic cell is enclosed by a plasma membrane, which controls communication between the inside and outside of the cell and facilitates cellular movement
• Organelles are essential for cell survival by carrying out specific functions, such as protein synthesis by ribosomes and energy production by mitochondria

Body Fluids

• Body fluids are aqueous solutions containing various organic molecules (like carbohydrates, lipids, proteins, and nucleic acids) and inorganic molecules and ions
• They constitute about 65% of total body weight, or about 40-42 liters in a 70kg adult
• Body fluids are classified into two main compartments:
  • Intracellular Fluid (ICF): The fluid inside the cell, making up two-thirds of total body fluids (about 25-28 liters)
  • Extracellular Fluid (ECF): The fluid outside the cell, constituting one-third of total body fluids (about 14-15 liters), separated from ICF by the cell membrane

Extracellular Fluid (ECF)

• ECF is termed the internal environment providing nutrients and other essential substances for cellular function
• It includes:
  • Plasma: Found inside blood vessels (3-3.5 liters)
  • Interstitial Fluid: Located in spaces between cells and lymphatic vessels (10-12 liters)
  • Transcellular Fluid: Present in body cavities (1 liter), including digestive juices and cerebrospinal fluid
• Plasma and interstitial fluid are separated by capillary walls

Composition of Body Fluids

• ECF has high concentrations of sodium, chloride, and bicarbonate ions, whereas ICF contains large amounts of potassium, magnesium, and phosphate ions
• These differences are crucial for sustaining cellular life

Water Balance

• Total body water represents about 60% of body weight in adult men and 50% in women due to higher fat content in women
• Fat percentage negatively affects water percentage
• Children have a higher water content (70%) than adults, making them more vulnerable to dehydration
• Water content decreases in old age

Functions of Body Water

• Crucial for all chemical reactions within the body
• Acts as a powerful solvent dissolving various substances for body access and transportation of nutrients, gasses, and waste
• Important for regulating body temperature due to its high latent heat of evaporation, requiring 0.58°C to evaporate per gram of water
• Moistens tissues like the mouth, eyes, and nose, and lubricates joints
• Facilitates osmotic equilibrium between body compartments due to its ease of passage through cell membranes
• Contributes to digestive secretions and enzyme activity
• Involved in absorption at the venous end of capillaries, gastrointestinal tract (GIT), and kidneys
• Facilitates filtration at the arterial end of capillaries and glomeruli
• Participates in excretion through the GIT, kidneys, and skin
• Mediates exchange at capillary membranes, alveolar membranes, and cell membranes
• Serves as a refractive medium in the eye

Water Balance: Input & Output

• Body water is stabilized by adjusting water intake and loss
• Water input comes from:
  • Exogenous Water: Ingested through water or liquids (about 2200 ml/day)
  • Endogenous Water: Synthesized during metabolism (about 200 ml/day)
• Water output occurs through:
  • Insensible Water Loss: From lungs as water vapor and skin through insensible perspiration (about 700 ml)
  • Sweating: Typically 100 ml, but increases during exercise
  • Feces: Approximately 100 ml, increasing with diarrhea
  • Urine: About 1500 ml

Control of Water Balance

• Water intake is regulated by thirst sensation stimulated by:
  • Increased plasma osmolarity (hypertonicity)
  • Decreased blood volume (hypovolemia)
  • Angiotensin II
• Water loss is primarily controlled by adjusting urine volume, regulated by antidiuretic hormone (ADH), secreted by the posterior pituitary gland
• ADH secretion is also stimulated by hypertonicity and hypovolemia

Homeostasis

• Refers to mechanisms maintaining a stable internal environment
• Crucial for normal cellular function, including:
  • Body water
  • Ions
  • Body temperature
  • Blood glucose
  • pH
  • Arterial blood pressure
• Deviations beyond a narrow range greatly affect cell function
• All body systems work to maintain homeostasis, including:

  • Nervous system (rapid)

  • Endocrine system (slow)

  • Feedback mechanisms regulate the system

  • Negative feedback mechanisms maintain system stability by inhibiting the stimulus. For example:

    • Thermoregulation: When body temperature increases, the body responds by sweating and vasodilation, which cools the body down. When body temperature decreases, the body responds by shivering and vasoconstriction, which warms the body up.