01 - Homeostasis and Transport

physiology

study of function of organisms, how organism works to maintain “normal” life

homeostasis

self-regulating process by which biological systems maintain stability while adjusting to changing external conditions

• involves coordination of the body

• cells → tissues → organs → systems → body

how are cells provided life supporting factors?

all cells are surrounded by an internal environment for life-supporting nutrients and are maintained by systems

• environment surrounding cells maintains compartments that promote well-being of species

• systems maintain variables within a confined range via control systems

compartments

body is at a steady state, not an equilibrium

• compartments and compositions are varied and require complicated and redundant mechanisms to adjust components to achieve homeostasis

• steady state: no change in system properties with time, requires constant input of energy

• equilibrium: all opposing forces are counter-balanced, maintained without energy expenditure

where is steady state maintained?

extracellular and intracellular fluids

• multiple systems are used to balance homeostatic variables, using a constant supply of energy to achieve balance

• hormones maintain extracellular concentrations of nutrients and deliver them into cells

organs and tissue types

organs are formed from tissues made of a primary cell type that have a major effect on organ function

• there are four major tissue types:

  • epithelium: generally covers and lines body surface

    • polarized, cell surfaces are physiologically different and is ideal for unidirectional transport

  • connective tissue: most abundant and widely distributed

  • muscular tissue: contain long cells that are highly specialized to actively shorten

  • nervous tissue: responsible for rapid communication throughout body

• tissue types form into specific organs that are organized into organ systems to maintain homeostatic variables within defined ranges

  • adjustment of variable levels through negative feedback mechanism

epithelium

generally cover and line body surfaces to mark “inside” from “outside”

• polarized cell surface that are physiologically different but ideal for unidirectional transport

• cells fit closely together to form membranes, or sheets of cells

  • membranes have one free surface and on surface attached to basement membrane

  • basement membrane secreted partly by epithelial cells and partly connective tissue

• avascular → nutrients move from adjacent connective tissue

• classified by cell shape and arrangement

connective tissue

most abundant and widely distributed

• vary in appearance and perform a wide variety of functions

• bind tighter, protect, and support other tissues

• distinguishable by presence of extracellular matrix in bones, cartilage, plasma, and adipose tissue

muscular tissue

contain long cells that are highly specialized to actively shorten

• striated muscle: very ordered contractile proteins with comparative rapid shortening

  • excited by electrical signals within muscles or nerves

  • skeletal (type I, type IIa, type IIb), cardiac

• smooth muscle: no visible pattern to contractile proteins, relatively slow contractions

  • excited by electrical or ligand signals

nervous tissue

responsible for rapid communication throughout body

• neurons act through electrical impulses delivered to specialized endings

• neuroglia support nervous systems by protecting and nourishing neurons

• difference in membrane potential for electrical signaling

regulatory systems

control homeostasis by monitoring levels of various substances in fluid compartments and correcting any disturbance that deviates from “normal”

• intrinsic control: controls within organism itself

• extrinsic control: controls that originate outside of organ system, generally are neural or endocrine

• feedforward system: adjustments made before disturbance, involving behavioral adjustments

• feedback system: adjustments made in response to disturbance

  • positive: relatively rare, outcome is episodic event

  • negative: most prevalent, outcome is state of constancy that is primarily responsible for homeostasis

• closed loop: stimulus directly affected by response

• open loop: stimulus not affected by response, not able to correct stimulus

negative feedback

maintains homeostasis by triggering a response that opposes change

• most prevalent

• outcome is a state of constancy

• error-driven

• responds to detected error signal on the difference between current and desired state

  • fast response can lead to oscillations

  • mulitple systems can control the same variable

  • fast is not sensitive, slow is sensitive

• stimulus is detected by sensor that is used to compare with set point

  • if values are not the same, it creates an error signal that generates a physiological response to move stimulus in the opposite direction

gain

amount of deviation in variable that is prevented by presence of a control system

• gain = (immediate level - steady state) / (steady state - control set point)

• gain = correction / error

• high gain: slow (minutes to days), typical of endocrine systems, >30

• low gain: fast (seconds to minutes), typical of neural systems, <15

• tells how effective feedback the control is

transport vs uptake

• transport: specific mechanism to cross a barrier

• uptake: total entry from one compartment into another

• when barrier is rate-limiting, transport is essentially equal to uptake

• transport rate varies with different modes of transport

  • highest rate of transport via channels

  • rate is concentration-dependent, with maximum rate dependent on number of carriers

physiological barrier

cell membrane is a major physiological barrier

• has many transport mechanisms to facilitate movement

• molecules are forced through membrane by some form of diffusion due to electrochemical gradient across the barrier

  • passive, facilitated, active transport

flow

determined by force and physical constraints

• force: pressure gradient, electrochemical gradient

• physical constraint: media constraints, resistance

• flow = net force x conductance = energy / physical constraint

bulk flow

volume displacement determined by hydraulic permeability due to pressure difference

• pressure gradient provides force → gravitational, osmotic, mechanical

• mechanism for water transport

• constrained by hydraulic permeability

• Q (flow) = pressure difference / resistance

  • flow and resistance are inversely related

  • flow and pressure gradient are directly related

solute flux

particles dissolved within fluid determined by physical properties of solvent and solute

• electrochemical gradient provides force → viscosity, charge, size

• mechanism for nutrient and ion transport

• achieved by forcing molecules through membrane by diffusion due to electrochemical gradient

  • J(flow) = -(diffusion coefficient x area) x (change in concentration / distance)

  • force = concentration / distance

  • big molecules have slower diffusion than smaller molecules

  • more viscous solutions have slower diffusion

• constrained by physical properties (diffusion coefficient and area) of solvent and solute

hydrostatic pressure

net pressure from beginning to end, net driving force for flow

• force on liquid due to compressive forces

• total fluid energy made of potential (gravity, compressive) and kinetic (inertia) energies

• pressure dissipates as flow occurs through a resistance

osmotic pressure

due to number of particles in a solution (not size)

• oncotic force: osmotic force exerted on one side of a barrier due to semi-permeable membrane

• result of number of free particles in a solution

  • number of particles formed x number of moles

• if separated by semi-permeable membrane, differnece in particle concentration of either side produces a net force called oncotic pressure or colloid pressure

  • hydrostatic: pressure pushed out on capillary walls

  • oncotic: pressure pulling water into capillary

• flow = change in pressure gradient / hydraulic resistance

Starling force

balance of pressures across capillary walls

• trans-capillary flow = pemeability surface area product x [(capillary - interstitial hydrostatic pressure) - (capillary - interstitial oncotic pressure)]

Poiseuille’s Law

flow = hydrostatic pressure / [(8 x viscosity x length) / (# of vessels in parallel x pi x r⁴)]

• small changes to capillary radius can elicit a large change in a system’s flow

resistor circuits

arrangement of resistors in circuit is important to determine total resistance

• series: total resistance is sum of individual resistances

  • every additional resistance increases total resistance and decreases flow

  • flow through each resistance unit is the same and equal to total flow

  • R_T = R_a + R_bcd + R_e

• parallel: total resistance is inversely related to sum of the reciprocal of individual ersistances

  • every additional resistance decreases total resistance and increases flow

  • sum of the flows through each resistance is equal to total flow

  • 1/R_bcd = 1/R_b + 1/R_c + 1/R_d

permeability

ease with which substance crosses a barrier

• small lipid-soluble molecules (gases) can enter cells by diffusion

• carriers and channels increase permeability by facilitating transport

• J(flow) = -(permeability coefficient x area) x (concentration outside - inside)

carriers vs channels

• carriers: faster than diffusion but slower than channels

  • dependent on electrochemical gradient

  • demonstrates specificity and maximum rate of saturation

• channels: promote movement of ions across membrane like a pore

  • selective → only one or a few charged substances can enter, where selectivity is determined by pore

  • force acting on movement is controlled by electrochemical gradient

  • gated → only active under certain conditions

pump-leak model

movement of ions across cell membrane is regulated by active pumping mechanisms and passive leakage channels

• “normal” levels are kept at steady state by pumping ions to restore gradient

active transport

requires input of metabolic energy that alters affinity on one side of barrier

• can create gradient

• may be electrogeneic → primary or secondary transport

extracellular fluid

fluid outside of cell

• ions → sodium, chloride, bicarbonate

• nutrients → oxygen, glucose, fatty acids, amino acids

• waste products → carbon dioxide, water

intracellular fluid

fluid inside of cell

• ions → potassium, magnesium, phosphate

• proteins

• nutrients → oxygen, glucose, fatty acids, amino acids

• metabolic products → carbon dioxide, water

• compartments