Water is the universal body solvent, providing an aqueous medium for biochemical reactions.
It is the most abundant compound in the body, making up 60% to 70% of body weight, a percentage that decreases with age.
Water is present in all body fluid compartments.
Total body water (TBW) is homeostatically regulated; oral water intake replaces water lost through urinary, gastrointestinal, cutaneous (skin), and respiratory systems.
Functions of Body Fluids
Dispersion medium for body solutes, providing an optimal environment for cellular biochemical reactions.
Maintenance of cellular and organelle structural integrity, as intracellular and extracellular fluids impart spatial structure to cells and tissues.
Thermoregulation in terrestrial animals, primarily through sweat.
Body's transport medium, carrying nutrients from absorption/storage sites to cells and conveying metabolic byproducts to excretory organs like the kidneys.
Body Fluid Classification
Fluids are classified by the anatomical compartment they occupy.
Total Body Water (TBW): The entire volume of water in all body fluid compartments (approximately 60-70% of body weight).
Intracellular Fluid Compartment (ICF): Total fluid within all cells (total cytoplasm). Accounts for about 65% of total body water, making it the largest compartment.
Extracellular Fluid Compartment (ECF): Fluid in all spaces outside the cytoplasm, about 35% of TBW.
Intravascular fluid: Found in blood and lymphatic vessels (e.g., plasma and lymph).
Extravascular fluid: ECF occupying spaces outside the vascular system.
Interstitial fluid compartment: Fluid in the intercellular (interstitial) spaces that directly bathes cells; it is considered the true internal environment in homeostasis.
Transcellular fluid: A group of ECF compartments containing special fluids with specific functions, such as synovial fluid, ocular fluids, etc.
Composition of Body Fluids
Composition of ECF: Water is the major constituent. Cations: Na+ is the most abundant. Anions: Main ones are Cl- and HCO3-.
Composition of Intracellular Fluid (ICF): Cations: K+ and Mg2+ are the main intracellular cations. Anions: PO4 2-, SO4 2-, and Prot- are the chief ICF anions.
The Na-K pump is crucial for maintaining these concentration differences, transporting 3 Na+ ions out for every 2 K+ ions in.
Formation of ECF at a Capillary Bed
Hydrostatic pressure: Generated by the pumping heart, this pressure causes water and small ions to filter from blood into the surrounding interstitium.
Diffusion: Substances diffuse down a concentration gradient (e.g., O2 from blood to tissues).
Osmotic Pressure (Oncotic Pressure): Large plasma proteins confined to the capillary raise osmotic pressure, causing water to flow back into the capillaries.
Regulation of Body Fluid Volume and Osmolality (Homeostasis)
Response to Water Loss (Increased Plasma Osmolality/Hypovolemia):
Stimulates osmoreceptors in arteries and hypothalamus.
Conserve water: Antidiuretic hormone (ADH) is secreted, increasing water reabsorption in the kidneys.
Initiate thirst: The thirst center in the hypothalamus is stimulated.
Response to Higher than Normal Total Body Water Volume (Decreased Plasma Osmolality):
Inhibits thirst and ADH secretion.
Stimulates aldosterone secretion, which increases Na+ reabsorption, leading to the excretion of a large volume of dilute urine.
II. Nervous Physiology
Resting Membrane Potential (RMP)
Definition: An electrical potential difference across the cell membrane of any cell at rest.
RMP is always negative inside the cell (average –70mV) due to:
Higher permeability of cell membranes to K+ than Na+.
Na+-K+ ATPase pump activity.
Confinement of large negatively charged ions within the cytoplasm.
Nerve Cell (Neuron) Structure and Function
Nerves transmit electrical activity called impulses.
Myelinated nerve fibers: Impulse conduction spreads through nodes of Ranvier (saltatory conduction), which is faster.
Excitation of the Cell Membrane – Action Potential (AP)
Definition: A brief and rapid change in resting membrane potential.
Phases: Depolarization and Repolarization.
Stimulation of Nerve Cell Membrane
A stimulus initiates a homeostatic mechanism or causes depolarization.
Receptor: A special cell that responds to a specific stimulus.
Adaptation: Many receptors decrease the frequency of action potentials with continuous stimulation.
Ionic Basis of Action Potential Generation
Depolarization: A stimulus opens sodium channels, causing Na+ to rapidly diffuse into the cell, changing the membrane polarity to positive.
Repolarization: Voltage-gated K+ channels open, leading to rapid diffusion of K+ out of the cytoplasm, restoring the negative charge inside. The Na-K pump also contributes.
Nerve Impulse Propagation and All-or-None Law
Saltatory conduction: AP jumps between nodes of Ranvier (myelinated nerves).
All-or-none law: An AP will only occur if the stimulus reaches a threshold level. The AP's intensity is not affected by the stimulus strength; only its frequency changes.
III. Cardiovascular Physiology
Heart Structure
The heart consists of left and right muscle pumps, each with an atrium and a ventricle.
Myocardial cells are separated by intercalated discs, allowing electrical charge to spread, and the heart functions as a syncytium.
Myocardial Action Potential
Unique feature: a prolonged plateau phase (about 0.2 seconds) due to slow Na-Ca channels opening and inhibited K+ efflux.
Ca2+ and Na+ are actively pumped out to restore the RMP of -85mV.
Cardiac Contraction Mechanism
Ca2+ released from the sarcoplasmic reticulum during the plateau phase initiates muscle contraction.