L2 human phys

Body fluid compartments

Body water is divided into intracellular fluid or ICF inside cells and extracellular fluid or ECF outside cells. ECF includes interstitial fluid and blood plasma

Distribution of body water

In the standard 70 kg man shown in the slides, about two thirds of body water is ICF at 28 L and one third is ECF at 14 L. About 75 percent of ECF is interstitial fluid and 25 percent is plasma

Age and sex effects on body water

Age and sex influence total body water content. The slides identify these as important factors but do not specify the direction of their effects

Role of extracellular fluid

ECF surrounds cells and acts as a buffer between cells and the outside world, so its composition must remain relatively stable

Osmotic equilibrium between ICF and ECF

Water moves freely between ICF and ECF so the compartments reach equal overall fluid concentration and have no net water movement

Chemical and electrical disequilibrium

Individual solute and ion concentrations remain unequal between ICF and ECF because the cell membrane is selectively permeable

How osmotic equilibrium can exist with chemical and electrical disequilibrium

Water can move across the membrane to equalize osmotic concentration while selective permeability and membrane transport maintain unequal concentrations of specific ions and solutes

Osmosis

Passive movement of water across a membrane in response to a solute concentration gradient. Water moves toward the more concentrated solution until there is no net water movement

Osmotic pressure

Pressure that opposes the movement of water by osmosis

How water crosses cell membranes

Water moves across membranes primarily through aquaporins and water filled ion channels

Molarity

Concentration based on the number of solute molecules in a given volume of solution

Osmolarity

Concentration based on the total number of osmotically active particles. Dissociation matters because one molecule can produce multiple osmotically active ions

Osmolality

Osmoles of solute per kilogram of water and is commonly used clinically for body water

Osmolarity versus osmolality

Osmolarity describes osmotically active particles relative to solution volume while osmolality describes osmoles per kilogram of water

Tonicity

Describes how a solution affects cell volume and depends only on the relative concentrations of nonpenetrating solutes inside and outside the cell

Why tonicity depends on nonpenetrating solutes

Penetrating solutes can cross the membrane and dissipate their gradients. Nonpenetrating solutes remain separated and therefore cause sustained water movement

Hypertonic isotonic and hypotonic solutions

Hypertonic means more nonpenetrating solute outside so water leaves and the cell shrinks. Isotonic means equal effective concentrations so cell volume stays constant. Hypotonic means less nonpenetrating solute outside so water enters and the cell swells

Rule for determining osmolarity

Count the total number of osmotically active particles in solution and account for solutes that dissociate into multiple ions

Rule for determining tonicity

Determine which solutes are nonpenetrating and compare their concentrations inside and outside the cell. Greater outside is hypertonic, equal is isotonic, and greater inside is hypotonic

Osmolarity versus tonicity problem solving

Osmolarity considers all osmotically active particles while tonicity considers only nonpenetrating solutes and predicts cell volume change

Bulk flow

Movement of liquids or gases from high pressure to low pressure due to a pressure gradient

Bulk flow versus solute diffusion

Bulk flow is driven by pressure gradients while diffusion of individual solutes is driven by concentration or electrochemical gradients

Diffusion

Passive net movement of molecules from high concentration to low concentration until equilibrium. Molecules continue moving at equilibrium but there is no net movement

General factors affecting diffusion

Diffusion is faster over short distances, at higher temperatures, with larger concentration gradients, and for smaller and lighter molecules

Simple diffusion

Passive movement directly through the membrane down a concentration gradient without assistance from a transport protein

What crosses the lipid bilayer most easily

Small nonpolar molecules such as oxygen, carbon dioxide, and nitrogen cross easily. Polar molecules, large molecules, and charged ions have difficulty because of the lipid bilayer

Fick law of diffusion

Diffusion rate increases as membrane surface area, concentration gradient, or membrane permeability increases

Membrane permeability in Fick law

Permeability depends on lipid solubility, molecular size, and membrane lipid composition. Greater lipid solubility and smaller molecular size generally increase permeability

Rules for diffusion of uncharged molecules

Diffusion is passive and proceeds down a concentration gradient. It becomes faster with a larger gradient, higher temperature, smaller molecules, larger membrane area, and greater membrane permeability

Three major ways substances cross membranes

Substances can move by simple diffusion through the lipid bilayer, protein mediated transport through channels or carriers, or vesicular transport using membrane bound vesicles

Protein mediated transport

Transport requiring a membrane protein. Channel proteins provide passages while carrier proteins bind solutes and change conformation

Channel proteins

Channels directly connect intracellular and extracellular compartments and allow substances such as water or ions to move through. Channels may be open or gated

Types of gated channels

Gated channels can be chemically gated, voltage gated, or mechanically gated depending on the signal that opens or closes them

Carrier proteins

Carriers bind specific molecules and change conformation to move them across the membrane. They can function as uniporters, symporters, or antiporters

Facilitated diffusion

Passive carrier mediated transport down a concentration gradient with no outside energy input. GLUT glucose transporters are an example

Active transport

Carrier mediated movement against a concentration gradient that requires energy either directly or indirectly

Channels versus facilitated diffusion versus active transport

Channels and facilitated diffusion are passive and move solutes down their gradients. Channels provide a pore while facilitated diffusion uses a conformationally changing carrier. Active transport also uses carriers but moves substances against gradients using energy

Primary active transport

Directly uses ATP to move a substance against its concentration gradient

Sodium potassium pump

Primary active transporter that uses one ATP to pump 3 Na positive out of the cell and 2 K positive into the cell, maintaining Na positive and K positive gradients

Secondary active transport

Uses potential energy stored in the concentration gradient of one molecule to move another molecule against its gradient. The sodium gradient commonly provides this energy

Symport versus antiport

Symport moves cotransported substances in the same direction while antiport moves them in opposite directions

SGLT sodium glucose transporter

Secondary active symporter in which Na positive moves down its gradient and provides the energy needed to transport glucose. Sodium binds first, then glucose, and both are moved into the cell

Specificity of carrier mediated transport

A carrier recognizes particular molecules because its binding site has specificity for certain substrates

Competition in carrier mediated transport

Similar molecules can compete for the same carrier binding site. For example, maltose can bind GLUT and competitively inhibit glucose transport without being transported itself

Saturation and transport maximum

As substrate concentration rises, carrier transport increases until all carriers are occupied. At this transport maximum, adding more substrate cannot substantially increase the transport rate

Phagocytosis

Active engulfment of a large particle such as a bacterium. The membrane extends around the particle using the cytoskeleton and forms a large vesicle called a phagosome that can fuse with a lysosome

Pinocytosis

Nonselective endocytosis in which extracellular material is taken into the cell in small vesicles

Receptor mediated endocytosis

Highly selective endocytosis in which extracellular ligands bind specific membrane receptors before entering the cell. Clathrin is commonly associated with the coated pits

Exocytosis

Vesicles fuse with the cell membrane to release substances outside the cell. It exports large lipophobic molecules and removes waste, with Rabs and SNAREs helping vesicles dock at the membrane

Endocytosis versus exocytosis

Endocytosis brings material into the cell by forming vesicles while exocytosis releases material by fusing vesicles with the membrane. Together they contribute to membrane recycling

Epithelial polarity

Transporting epithelial cells have an apical or mucosal membrane facing the lumen and a basolateral or serosal membrane facing the ECF. Different transport proteins on these surfaces allow directional transport

Absorption versus secretion across epithelia

Absorption moves substances from the lumen toward the ECF while secretion moves substances from the ECF toward the lumen

Paracellular versus transcellular transport

Paracellular transport moves substances through junctions between adjacent cells. Transcellular transport moves through the cells themselves and requires crossing both the apical and basolateral membranes

Energy use in transcellular transport

Transcellular transport can combine active and passive mechanisms with one uphill step requiring energy and one downhill step requiring no energy

Transcytosis

Vesicular transport across a cell in which material enters one side by vesicle formation, travels through the cell, and is released from the opposite side. Unlike paracellular transport it moves through the cell rather than between cells