Overview
This lecture reviews how fluids and solutes move within the body, focusing on cellular transport mechanisms and the roles of hydrostatic and oncotic pressure in fluid balance.
Cellular Membrane Structure
- The cell membrane is composed of a phospholipid bilayer with hydrophilic heads and hydrophobic tails.
- Selective permeability of the membrane allows only certain substances to pass based on size and charge.
- Protein channels and carrier proteins within the membrane aid in transport.
Types of Transport Across Cell Membranes
- Simple diffusion moves small, non-charged molecules (like Oâ‚‚ and COâ‚‚) from high to low concentration without energy.
- Facilitated diffusion uses carrier proteins to move larger or charged molecules (like glucose and ions) from high to low concentration; no energy required.
- Active transport moves molecules from low to high concentration using energy (ATP) and carrier proteins; this goes against the concentration gradient.
- Osmosis is the passive movement of water through a semi-permeable membrane from areas of low solute concentration to high solute concentration to achieve equilibrium.
Osmosis in Cells
- Water moves into or out of cells depending on solute concentration inside and outside the cell.
- High intracellular solute concentration draws water in, potentially causing cell swelling or rupture.
- High extracellular solute concentration draws water out, potentially dehydrating the cell.
Clinical Application of Osmosis
- IV fluids can manipulate osmosis to treat fluid volume deficit or overload in patients.
Pressures Regulating Capillary Fluid Movement
- Oncotic (colloidal osmotic) pressure pulls water into capillaries due to plasma proteins, mainly albumin.
- Hydrostatic pressure pushes water out of capillaries into the interstitial space and is created by heart contractions.
- Arterial end of capillaries has high hydrostatic pressure; venous end has lower.
- Filtration is the process where hydrostatic pressure moves water and solutes out of capillaries.
- Low albumin levels decrease oncotic pressure and can cause swelling due to fluid accumulation in tissues.
Key Terms & Definitions
- Phospholipid bilayer — the double-layered structure forming the cell membrane.
- Simple diffusion — passive movement of small, non-charged molecules from high to low concentration.
- Facilitated diffusion — passive, protein-assisted movement of larger/charged molecules from high to low concentration.
- Active transport — energy-requiring movement of substances from low to high concentration.
- Osmosis — passive movement of water across a membrane from low to high solute concentration.
- Oncotic pressure — the pulling force exerted by plasma proteins to retain water in blood vessels.
- Hydrostatic pressure — the pushing force exerted by fluid within blood vessels.
Action Items / Next Steps
- Review videos on IV fluids and osmosis for further understanding.
- Check lecture links for additional resources on fluids and electrolytes.