Editing Anatomy and Physiology (section)

== Levels of Organization and the Cell ==

The body is organized from chemicals to cells to tissues to organs to organ systems to the whole organism. The '''cell''' is the smallest living unit.

Key cellular structures and their nursing relevance:

* '''Cell membrane''': a phospholipid bilayer that is selectively permeable. It controls what enters and leaves the cell and maintains the electrical gradient that makes nerve and muscle function possible.
* '''Mitochondria''': the site of aerobic ATP production. When oxygen delivery fails, cells shift to anaerobic metabolism, produce lactic acid, and the patient develops metabolic acidosis. This is the chemistry behind shock.
* '''Nucleus''': contains DNA. Cells that divide rapidly (bone marrow, gastrointestinal mucosa, hair follicles) are the cells most damaged by chemotherapy and radiation, which explains myelosuppression, mucositis, and alopecia.
* '''Lysosomes''': digestive enzyme sacs involved in cellular cleanup and in the inflammatory response.
* '''Ribosomes and endoplasmic reticulum''': protein synthesis. The liver's abundant endoplasmic reticulum underlies its role in making albumin and clotting factors.

=== Transport Across Membranes ===

* '''Diffusion''': solutes move from higher to lower concentration. Gas exchange in the alveoli is diffusion.
* '''Osmosis''': water moves across a semipermeable membrane toward the higher solute concentration. This is the mechanism behind every IV fluid decision.
* '''Facilitated diffusion''': carrier-assisted movement down a gradient, such as glucose entry into most cells with insulin's help.
* '''Active transport''': movement against a gradient using ATP. The sodium-potassium pump moves three sodium ions out for every two potassium ions in, maintaining the resting membrane potential.
* '''Filtration''': movement driven by hydrostatic pressure, as in the glomerulus and at the arterial end of capillaries.

'''Nursing application:''' IV fluid tonicity follows directly from osmosis.
* '''Isotonic''' (0.9% sodium chloride, lactated Ringer's): stays in the extracellular space, expands vascular volume, used for dehydration and hypovolemia. Risk is fluid overload.
* '''Hypotonic''' (0.45% sodium chloride): water shifts into cells; used for cellular dehydration. Risk is cerebral edema and worsening hypotension, so it is avoided in head injury and in hypovolemic shock.
* '''Hypertonic''' (3% sodium chloride, dextrose 5% in 0.9% sodium chloride): pulls water out of cells into the vascular space. Used cautiously for severe hyponatremia and cerebral edema. Risk is fluid overload and phlebitis, so infuse slowly and monitor closely.