2 - Intro to physiology

Why is movement of substances important in physiology?

Movement within and between body fluids (often across membranes) is essential for normal physiological function

What is a concentration gradient?

A difference in concentration between two areas

Define equilibrium

Even distribution of molecules

Passive vs active transport

Passive = down gradient (no energy); Active = against gradient (requires ATP)

TRANSPORT ACROSS CELL MEMBRANES

What are the main types of membrane transport?

Passive (diffusion, facilitated diffusion, osmosis), Active (primary, secondary), Bulk transport

DIFFUSION

Define diffusion

Passive movement of molecules from high to low concentration

Where does diffusion occur?

Gases, liquids, solutions, across semi-permeable membranes

What affects diffusion?

Temperature, concentration gradient, distance, molecule size/shape, charge, solvent

What is the electrochemical gradient?

Combination of chemical gradient (concentration) and electrical gradient (charge)

Why is inside of cells negative?

Due to negatively charged proteins

State Fick’s Law of diffusion

J = -DA (ΔC/Δx)

What does J represent in Fick’s Law?

Rate of diffusion

What does D represent?

Diffusion coefficient

What does A represent?

Surface area

What does ΔC represent?

Concentration gradient

What does Δx represent?

Distance

Key takeaway from Fick’s Law

Diffusion rate is proportional to concentration gradient

How does distance affect diffusion?

Fast over short distances (µm), slow over long distances (> mm)

SIMPLE DIFFUSION

What is simple diffusion?

Direct passive movement across membrane without proteins

Which substances use simple diffusion?

Lipid-soluble molecules, O2, CO2, steroids, fatty acids, small ions via channels

FACILITATED DIFFUSION

What is facilitated diffusion?

Passive transport via carrier proteins

Characteristics of facilitated diffusion

Specific, rate-limited, no ATP required

Examples of facilitated diffusion

Glucose, amino acids

Difference between symporters and antiporters

Symporters move same direction; antiporters swap molecules

OSMOSIS

Define osmosis

Diffusion of water down its concentration gradient

When does osmosis occur?

When solute cannot cross membrane

Requirements for osmosis

Semi-permeable membrane and solute concentration difference

Direction of water movement in osmosis

From high water concentration to low water concentration

ACTIVE TRANSPORT

Define active transport

Movement of substances against concentration gradient using ATP

Features of active transport

Requires energy, uses carrier proteins, rate-limited

SODIUM POTASSIUM PUMP

What does the Na+/K+ pump do?

Pumps 3 Na+ out and 2 K+ into the cell

Why is the Na+/K+ pump important?

Maintains ion gradients, prevents cell swelling, maintains electrical gradient

How much ATP does Na+/K+ pump use?

Up to 30% of cellular ATP

PRIMARY VS SECONDARY ACTIVE TRANSPORT

Define primary active transport

Direct use of ATP to move substances

Example of primary active transport

Na+/K+ pump

Define secondary active transport

Uses gradient created by primary transport

Example of secondary active transport

Na+/glucose co-transporter (SGLT)

BULK TRANSPORT

What is bulk transport?

Movement of large substances across membrane

Types of bulk transport

Pinocytosis (fluid), phagocytosis (particles), exocytosis (release)

OSMOLARITY + OSMOTIC PRESSURE

Define osmolarity

Number of osmoles per litre of solution

Define an osmole

1 mole of dissolved particles (6.02 × 10^23)

VAN’T HOFF EQUATION

State van’t Hoff equation

π = CRT

What does osmotic pressure depend on?

Mainly solute concentration

MOLARITY VS OSMOLARITY

Osmolarity of non-electrolytes

Osmolarity = molarity

Osmolarity of electrolytes

Molarity × number of ions (NaCl = 2 osmoles)

0.9% SALINE EQUATION

Mass of NaCl in 1L of 0.9% saline

9 g

Molarity of 0.9% saline

0.154 mol/L

Osmolarity of 0.9% saline

≈ 283 mOsm/L (corrected)

Why is 0.9% saline used clinically

It is isotonic with plasma

BODY FLUID COMPOSITION

Major ions in extracellular fluid (ECF)

Na+ and Cl-

Major ion in intracellular fluid (ICF)

K+

Difference between plasma and interstitial fluid

Plasma contains more proteins → oncotic pressure

TONICITY

Define tonicity

Effect of a solution on cell volume

What determines tonicity

Osmolarity and whether solutes can cross membrane

TYPES OF SOLUTIONS

Define isotonic solution

Same osmolarity as plasma → no cell change

Define hypotonic solution

Lower osmolarity → water enters cell → swelling/lysis

Define hypertonic solution

Higher osmolarity → water leaves cell → shrinkage (crenation)

IV SOLUTIONS

Examples of isotonic IV fluids

0.9% NaCl, Lactated Ringer’s

Use of isotonic fluids

Expand blood volume (e.g. blood loss)

Examples of hypotonic IV fluids

0.45% NaCl

Use of hypotonic fluids

Treat dehydration (push water into cells)

Examples of hypertonic IV fluids

D5NS

Use of hypertonic fluids

Treat hyponatraemia (draw water into blood)

What are the three body fluid compartments?

Plasma, interstitial fluid, intracellular fluid

What is osmosis in one line?

Diffusion of water across a membrane

Why is tonicity important clinically

Incorrect IV fluids can cause cells to burst or shrink

Does glucose cross by active transport?

No, mainly facilitated diffusion (can be secondary active)

Do hypertonic solutions cause cells to burst?

No, they cause cells to shrink

Does Na+/K+ pump use 70% ATP?

No, about 30%