Body Fluids and Homeostasis

Body Fluids

• Extracellular fluid (ECF) makes up 1/3 of total body fluid and includes plasma (inside blood cells) and interstitial fluid (around cells).
• Intracellular fluid (ICF) makes up 2/3 of total body fluid and is found within all body cells.
• ICF has higher K+ levels and less Na+ compared to ECF. It also contains protein carrying a negative charge.
• Total body water = 60% of body weight.
• For a 70kg male, ICF is approximately 28L and ECF is approximately 14L.
• Plasma makes up 1/4 of ECF (3.5L), while interstitial fluid makes up 3/4 of ECF (10.5L).

Balancing the Internal and External Environment

• Cells constantly exchange nutrients and waste products with their surroundings.
• The internal environment (extracellular fluids) is directly influenced by the organ systems it passes through, creating a feedback loop: ICF - ISF - plasma - organs - external environment.

Homeostasis

• Refers to the maintenance of a relatively constant and stable internal environment (extracellular fluids).
• It involves maintaining a dynamic steady state where internal conditions are kept within a narrow range, despite changes in the external environment.
• Each organ contributes to the maintenance of homeostasis.
• Homeostasis is crucial for the survival and function of all cells.

Factors Homeostatically Regulated

• Concentration of nutrients (glucose, O2, CO2, and waste products)
• Concentration of water, salt, and other electrolytes
• pH
• Blood volume and pressure
• Body temperature

Homeostasis and Controls

• Successful compensation leads to re-establishment of homeostasis.
• Failure to compensate results in pathophysiology, potentially leading to illness or death.

Homeostatic Control Mechanisms

• Involve a receptor, a control center, and an effector.
• The receptor detects changes in the internal environment.
• The control center determines the set point for the variable being regulated.
• The effector produces a response that changes the internal conditions.

Regulation of Internal Constancy

• Homeostasis is not a static state, but rather a dynamic equilibrium around a set point.
• Negative feedback loops are crucial for maintaining homeostasis.
• Sensors monitor internal conditions and detect changes.
• Integrating centers receive and integrate information from the sensors.
• Effectors respond to changes and return conditions to normal levels.

Basic Cell Functions

• Obtaining nutrients and oxygen from the environment.
• Performing chemical reactions to provide energy (metabolism).
• Eliminating waste products (CO2 etc.).
• Synthesizing cellular components (e.g., proteins).
• Controlling material movement in and out of cells.
• Sensing and responding to environmental changes.
• Reproduction (except for nerve and muscle cells which lose this ability during development).

Specialized Cell Functions

• Each cell type has specialized functions (e.g., white blood cells phagocytize foreign materials while red blood cells transport oxygen).

Cell Membrane

• Thin and mechanically weak membrane surrounding each cell.
• Functions include: maintaining cell structure, controlling substance movement in and out (selective permeability), regulating cell-cell interactions, and acting as an interface between the cytoplasm and the external environment.

The Importance of the Cell Membrane

• Creates a barrier between the ICF and ECF.
• Cell death occurs if the membrane breaks.
• Maintains concentration differences between intracellular and extracellular solutions:
  • Higher K+ concentration inside than outside.
  • Higher Na+ concentration outside than inside.
  • Higher Cl- concentration outside than inside.
  • Higher Ca++ concentration outside than inside.
  • Higher protein concentration inside than outside.
  • The extracellular fluid is slightly alkaline (pH=7.4), while the intracellular fluid is neutral (pH=7).

Composition of the Cell Membrane

• Proteins: 55%
• Phospholipids: 25%
• Cholesterol: 13%

Osmosis

• Movement of water across a selectively permeable membrane from areas of higher concentration to areas of lower concentration.
• Important at the cellular level, affecting cell shrinkage or swelling.
• Also occurs in tissues like the gastrointestinal tract, renal system, and capillary walls.

Cell Membrane Physiology

• Osmotic pressure (OP) is determined by the concentration of solutes inside the cell.
• Human cells have an osmotic pressure equivalent to that of a 0.9% solution of sodium chloride (normal saline).
• Solutions can be classified based on their osmotic pressure relative to the intracellular fluid:
  • Hypotonic: OP outside is less than OP inside, causing water to enter the cell.
  • Hypertonic: OP outside is greater than OP inside, causing the cell to lose water.
  • Isotonic: OP outside is equal to OP inside, resulting in hydrostatic equilibrium.

Secondary Active Transport

• Requires energy indirectly from the movement of another molecule, typically Na+.
• Co-transporters move molecules in the same direction (e.g., glucose transport).
• Counter-transporters move molecules in opposite directions (e.g., Na+/Ca++ exchange).

Movements Into and Out of the Cell

• Simple diffusion: Passive movement across the membrane, does not require energy, not highly selective.
• Facilitated diffusion: Requires membrane proteins, still passive and does not require energy, highly selective.
• Active transport: Requires membrane proteins and energy, can move molecules against their concentration gradient.

Membrane Resting Potential

• A constant potential difference across the resting cell membrane.
• Allows cells to fire action potentials.
• The resting potential is typically around -70mV.

Membrane Potential

• The difference in electrical potential between the inside and outside of the cell membrane.
• Established and maintained by the distribution of ions across the membrane.
• Key contributors to membrane potential:
  • Sodium-Potassium pump: Actively transports Na+ out and K+ in, contributing to the Na+ gradient.
  • Potassium leak channels: Allow K+ to move down its concentration gradient, contributing to the membrane potential.
  • Negative charges inside the cell: Due to large, negatively charged proteins and other molecules.

Body Fluids

• Extracellular fluid (ECF): The fluid environment surrounding cells, including plasma (inside blood cells) and interstitial fluid (around cells)
• Intracellular fluid (ICF): The fluid inside body cells, with a higher concentration of potassium (K+) and lower sodium (Na+) compared to ECF
• Total body water: 60% of body weight
• ICF: 2/3 of total body water
• ECF: 1/3 of total body water
• Plasma is 1/4 of ECF
• Interstitial fluid is 3/4 of ECF

Balancing the Internal and External Environments

• Cells exchange nutrients and waste with their surroundings
• ICF is conditioned by interstitial fluid, which is conditioned by plasma
• Plasma is conditioned by the organs it passes through and the external environment
• Internal environment refers to the fluid surrounding cells (extracellular fluids)

Homeostasis

• Homeostasis is the maintenance of a relatively constant internal environment.
• It is essential for survival and function of all cells.
• Each cell contributes to maintaining a stable internal environment.

Factors Homeostatically Regulated

• Concentration of nutrients (glucose, oxygen, carbon dioxide, and waste products)
• Concentration of water, salt, and other electrolytes
• pH (acidity/alkalinity)
• Blood volume and pressure
• Body temperature

Homeostasis & Controls

• Successful compensation leads to reestablishment of homeostasis
• Failure to compensate leads to pathophysiology, illness, and death

Homeostatic Control Mechanisms

• These mechanisms monitor and correct internal environmental variations within limits
• They consist of:
  • Receptor: Provides information about stimuli
  • Control Center: Determines the set point for a particular value
  • Effector: Elicits responses to change internal environment conditions

Regulation of Internal Constancy

• Homeostasis is not a static state, it is dynamic with changes around set points
• Relies on negative feedback loops
  • Sensor: Monitors internal conditions and detects changes
  • Integrating Center: Receives and integrates information
  • Effector: Responds to changes and returns the condition to normal levels

Cell Membrane

• The phospholipid bilayer is the major component of the plasma membrane
• Phospholipids are amphipathic, meaning they have both water-soluble and water-insoluble parts
• Phospholipid heads are water-soluble (hydrophilic) and form the surfaces of the membrane
• Phospholipid tails are water-insoluble (hydrophobic) and form the interior of the membrane
• Membrane permeability is influenced by the lipid-soluble nature of substances

Movements Into and Out of the Cell

• Passive processes: Net movement down a concentration gradient (high to low concentration)
  • Does not require energy (ATP)
  • Examples: Simple diffusion, facilitated diffusion, osmosis
• Active processes: Net movement against a concentration gradient
  • Requires energy (ATP)
  • Examples: Primary active transport, Secondary active transport

1- Simple Diffusion

• Movement of molecules from an area of high concentration to an area of low concentration
• Does not require a carrier protein

2- Facilitated Diffusion

• Movement of molecules across membranes with the aid of carrier proteins
• Carrier proteins facilitate the movement of molecules down a concentration gradient
• Does not require energy (ATP)

3- Osmosis

• Movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration
• Water moves down its concentration gradient
• Does not require a carrier protein

4- Active Transport

• Carrier molecules transport substances across membranes from regions of lower concentration to regions of higher concentration
• Requires energy (ATP)
• Carrier proteins facilitate the movement of molecules against their concentration gradient
• Examples:
  • Primary active transport: Uses energy directly from ATP hydrolysis
  • Secondary active transport: Uses energy stored in ionic concentration differences created by primary active transport

4.1 Primary Active Transport

• Energy is derived directly from ATP breakdown
• The carrier protein is phosphorylated (ATP breakdown)
• This causes a conformational change that releases the transported molecule to the other side of the membrane
• Example: Na+/K+ pump (Found in all cells) It pumps three sodium (Na+) ions out of the cell and two potassium (K+) ions into the cell

4.2 Secondary Active Transport

• Energy is derived indirectly from stored energy by the concentration gradient
• The movement of one molecule is coupled with the movement of another molecule down its concentration gradient
• Example: Na+/Glucose co-transport Na+ moves down its concentration gradient (energy source), providing the energy for Glucose to move against its concentration gradient

Factors Affecting Body Fluids

• Osmolarity: The concentration of solutes in a solution
• Permeability: The ability of a membrane to allow substances to pass through it
• Pressure: The force exerted by a fluid on a membrane
• Temperature: Higher temperature increases the rate of diffusion

Membrane Potential

• The difference in electrical charge between the inside and outside of a cell membrane
• Created by the uneven distribution of ions across the membrane
• Essential for nerve impulse transmission, muscle contraction, and other cellular processes

Studying Physiology

• Understanding the relationship between structure and function requires knowledge of anatomy and physiology
• Homeostasis is maintained by dynamic changes around a set point, not a static state

This quiz explores the concepts of body fluids, including intracellular and extracellular fluids, their compositions, and their roles in maintaining homeostasis. It covers the distribution and importance of these fluids in the body, as well as the feedback loops that regulate the internal environment. Test your knowledge on these fundamental physiological principles.