Human Physiology: Body Compartments

• Physiology: The study of the functions of living things.
• Human physiology: The study of the functions of organs and systems and the way these functions are integrated.
• The human body is made up of 75-100 trillion cells.
• A collection of cells with similar properties forms tissues.
• Different tissues combine to form organs.
• Organs of complementary functions constitute the different systems.

Body Compartments

• The body of normal adult males is composed of 18% proteins, 15% fat, 7% minerals, and 60% water which is called Total Body Water (TBW).
• TBW is 75% in infants.
• TBW is less than 60% in adult females and obese people.
• TBW is present in two compartments: the intracellular fluid compartment (ICF) and the extracellular fluid compartment (ECF).

Intracellular Fluid Compartment (ICF)

• Constitutes 2/3 of TBW.
• Present inside the cells.
• Equals 40% of Total Body Weight.
• Contains more K+ and less Na+, Cl- and HCO3-.

Extracellular Fluid Compartment (ECF)

• Constitutes 1/3 of TBW.
• Present outside the cells.
• Equals 20% of Total Body Weight.
• Contains less K+ and more Na+, Cl- and HCO3-.
• Extracellular fluid distribution: 5% inside blood vessels (intravascular fluid/plasma) and 15% outside blood vessels (interstitial fluid).

Homeostasis

• Definition: Keeping the conditions in the internal environment constant.
• Examples of conditions to keep constant are O2, CO2, glucose, different ions, amino acids and fatty acids concentrations.
• Most systems in the body work to maintain homeostasis.

The Cell

• The cell is the structural unit of various tissues and organs.
• It consists of a mass of protoplasm surrounded by the cell membrane.
• Protoplasm comprises the cytoplasm, cell organelles, and the nucleus.

The Cell Membrane

• The thickness of the cell membrane is 7.5 nm (10^-9 meter).
• The cell membrane is made up of 55% proteins, 29% lipids (phospholipids & glycolipids), 13% cholesterol and 3% carbohydrates.
• Under an electron microscope, it is formed of a lipid bilayer with globular proteins interspersed in the thin lipid film.
• Phospholipid molecules are shaped like a clothespin, formed of a polar head and two non-polar tails.

Phospholipid Head

• Formed of phosphate charged group.
• Hydrophilic (relatively soluble in water).
• Polarized.

Phospholipid Tail

• Formed of 2 fatty acid chains.
• Hydrophobic (relatively insoluble in water).
• Non-polar.
• The phospholipid molecule is amphipathic, meaning it has one part hydrophilic and the other hydrophobic.
• In the membrane, hydrophilic ends of the molecule are exposed to the aqueous environment, while hydrophobic ends meet in the water-poor interior of the membrane.
• The lipid bilayer provides flexibility and selective permeability to the cell membrane.

Cell Membrane Proteins

• Cell membrane proteins are present as globular masses floating in the lipid bilayer.
• They have a hydrophobic, uncharged portion in the interior of the membrane, and a hydrophilic, charged part directed to the surface.
• Two types of cell membrane proteins: integral proteins and peripheral proteins.
• Integral proteins pass all the way through the membrane.
• Peripheral proteins attach to only one surface of the membrane.
• Cell membrane proteins form complexes with glycolipids, which are important as recognition sites and act as antigens for cell self-recognition, the immune system, and differentiation.
• Cholesterol molecules are completely hydrophobic and embedded in the phospholipid bilayer, affecting permeability and giving toughness to the membrane.

Functions of Cell Membrane Proteins

• Structural proteins: They keep the integrity of the membrane and give it strength.
• Form passive channels: Protein channels with various diameters, shapes, and electric charges along their surfaces.
• Channels are classified, according to the presence or absence of gates, into gated channels and non-gated channels.
• Gated channels are closed during rest.
• Non-gated channels are open all the time.
• Gated channels can be voltage gated or ligand gated.
• Voltage gated channels open or close when the membrane potential changes.
• Ligand gated channels open or close when a ligand binds a specific receptor on the cell membrane.
• Carriers in facilitated diffusion: They bind molecules and change configuration to move them across the membrane.
• Carriers in active transport (= Pumps): This requires energy to transport substances against electrical and chemical gradients.
• There are three types of carriers.
• Uniport: transports one substance in one direction (e.g., Ca++).
• Symport: transports two substances simultaneously in the same direction (e.g., glucose and Na+ from the intestinal lumen to inside the cell).
• Antiport: transports one substance in one direction and another substance in the opposite direction (e.g., Na+ - K+ pump).
• Symport and Antiport are called Cotransport carriers.
• Receptors: Proteins or glycoproteins present on the outer cell membrane, inactive during rest.
• When receptors combine with a specific ligand (neurotransmitter, drug, or hormone), they become active and stimulate or inhibit cellular functions.

Receptor Regulation

• Downregulation: When a ligand is present in excess, the number of receptors decreases, and sensitivity to the ligand also decreases.