1625915643-5d53d156c9525bd62bd0d3434ecdc231 (Netters - Essential Physiology (на английском)), страница 4

Thus, bulk movement in the liver sinusoidsis controlled by hydrostatic pressure. In contrast, capillariesin most tissues have low permeability to proteins, and σ = ~1,so the Starling equation can easily be viewed as the pressuresgoverning filtration minus those favoring absorption:( Filtration ) − ( Absorption )Net Filtration = K f [( HPc + πi ) − ( Pi + πc )]Although Kf is a “constant,” it differs between systemic,cerebral, and renal glomerular capillaries, with cerebralThe Cell and Fluid HomeostasisHydrostaticpressureSemipermeablemembraneOsmoticpressureOsmoticpressureA. Initial state of unopposedB.osmotic pressure.Equilibrium state in which remainingosmotic pressure is opposed by equaland opposite hydrostatic pressure.Figure 1.7 Osmosis and Osmotic Pressure When a semipermeable membrane separates twocompartments in a “V tube,” fluid will move through the membrane toward the higher solute concentration(A), until near equilibrium of solutes is reached and the remaining osmotic pressure is opposed by the hydrostatic pressure difference between the two sides of the tube (B).

In blood vessels, hydrostatic pressure isgenerated by gravity and the pumping of the heart, and the osmotic pressure is measured as the force neededto oppose the hydrostatic pressure. Osmotic pressure works toward equalization of solute concentrations oneither side of the membrane. Oncotic pressure (π) is the osmotic pressure produced by impermeable proteins.In the plasma, oncotic pressure is considered the effective osmotic pressure of the capillary.ArterioleVenuleCapillaryPi = –3 mm HgPc = 30 mm HgPc = 10 mm Hg␲c = 28 mm Hg␲i = 8 mm Hgmm Hg4030Pc – Pi20␴(␲c – ␲i)10␴ = 0.9Distance along capillaryFigure 1.8 Starling Forces across the Capillary The Starlingforces (hydrostatic and oncotic pressures) allow for bulk flow of fluid andnutrients across the capillary wall.

The permeability of the membranesof the capillary wall to proteins is usually very low in most tissues and isreflected by a protein reflection coefficient (σ) of ~1. The inset panelillustrates that as fluid moves though the capillaries and diffusion intotissues occurs, the Starling forces change, and the forces favoring netfiltration (especially Pc [HPc, capillary hydrostatic pressure]) decrease(dotted blue line).910Cell Physiology, Fluid Homeostasis, and Membrane TransportAdd H2ONormal300Osmolality(mosm/L)300Intracellularfluid(ICF)Extracellularfluid(ECF)ICF00A. Under normal conditions, ICF is 2/3and ECF is 1/3 of TBW. The osmolalityof both compartments is ~300 mosm/L.B.

If water was added to the plasma (thus, ECF), ECF osmolality wouldbe diluted initially (become hypotonic) compared to ICF. Water wouldthen enter the ICF space (cells would swell) to equilibrate the osmolalitybetween compartments. The overall effect would be to reduce theosmolality of ICF and ECF and expand the compartment size.Add isotonic salineAdd hypertonic saline300Osmolality(mosm/L)ECF300ICFECFICF0ECF0C. If isotonic saline (solution with NaClconcentration of ~300 mosm/L) was addedto the plasma (ECF), the fluid will stay inthe ECF because it is isotonic, expandingthat compartment.D. If hypertonic saline was added to the plasma, ECF osmolality wouldincrease greatly initially and fluid would be drawn out of the cells andinto the ECF to lower the tonicity of the ECF.

This would contract thevolume of the ICF compartment (cells would shrink) and increase thevolume of the ECF compartment, as well as increase overall osmolality.Figure 1.9 Effect of Adding Solutes to the Extracellular Fluid on Compartment Size If therewere not compensatory mechanisms in place to sense and regulate plasma volume and osmolarity, additionof water and hypertonic solutions would have a profound effect on ICF volume and tonicity. A illustratesthe intracellular fluid (ICF) and extracellular fluid (ECF) compartment sizes under normal osmolar conditions(300 mosm/L). B through D illustrate the effect of changing the tonicity (the osmolarity relative to plasma)of the extracellular fluid on cells. B, Addition of pure water to the ECF will increase both ECF and ICF volume,and reduce overall osmolarity. C, Addition of isotonic (same osmolarity as plasma) saline (NaCl) to the ECFwill expand only the ECF compartment size, because NaCl is mainly ECF electrolytes.

D, Addition of pureNaCl will increase overall osmolarity of both compartments to a new level. ICF will shrink, as water is drawninto the ECF toward the higher osmolar concentration, and the volume of the ECF will expand by additionof the ICF fluid.capillaries having a lower Kf (limiting filtration), and glomerular capillaries having a greater Kf (promoting filtration)compared with systemic capillaries. Thus, filtration will bedetermined by the difference in hydrostatic pressure betweenthe capillary and interstitium, minus the difference betweencapillary and interstitial oncotic pressure (corrected for theprotein reflection coefficient).

It should be clear that undernormal conditions, the forces that are most variable are theHPc and the πc, because those can reflect changes in plasmavolume.The Cell and Fluid HomeostasisExamples of the effects of Starling forces on fluid shiftscan be illustrated by changes in fluid volume as well aschanges in physical factors. A severely dehydrated person willhave a decreased blood volume, which would potentially lowerblood pressure (e.g., HPc) and increase πc.

Looking at the Starling equation, those changes will decrease filtration force andincrease the absorptive force, causing an overall decrease in netfiltration. This will keep fluid in the vascular space.When physical attributes of the membrane change, Starlingforces are also affected: Kf can change if the capillary membraneis damaged, such as by toxins or disease. If the clefts betweenendothelial cells or fenestrations expand (as seen in diseasedrenal glomeruli), plasma proteins may pass into the interstitialspace and alter the Starling forces by increasing πi: In peripheralcapillaries, this causes edema. Starling forces are also affected incongestive heart failure (CHF), cirrhosis, and sepsis.HomeostasisFrench physiologist Claude Bernard first articulated theconcept that maintaining a constant internal environment, ormilieu intérieur, was essential to good health.

In multicellularorganisms, the balance between internal and external envi-ronments is critical, and the ability to maintain a constantinternal function during changes in the external environmentis termed homeostasis. This is accomplished through integrated regulation of the internal environment by the multipleorgan systems (see Fig. 1.2).On the cellular level, homeostasis is possible due to expandable semipermeable membranes, which can accommodatesmall changes in osmolarity via osmosis. However, for propercellular function, the intracellular fluid, and thus osmolarity,must be kept under tight control.The plasma is the interface between the internal and the external environment; therefore, maintaining plasma osmolarity isan important key to cell homeostasis. Because of this, manysystems play a role in controlling plasma osmolarity.

Boththirst and the salt appetite are behavioral responses that canbe stimulated by dehydration and/or blood loss. These serveto stimulate specific ingestive behaviors (drinking, or eatingsalty foods that will also stimulate drinking) that will increasethe input of fluid and salt to the system. On a minute-tominute basis, the endocrine and sympathetic nervous systemswork to regulate the amount of sodium and water retained bythe kidneys, thus controlling plasma osmolarity (Fig. 1.9).Fluid balanceIncreasedurine outputExcessfluidFluiddeficitIntake(~2.5 L/day)BeveragesIncreasedthirstOutput(~2.5 L/day)1.3 L1.5 LFood0.9 LOxidation0.3 L110.9 LUrineSweat andrespirationExcreted infeces (0.1 L)Figure 1.10 Net Fluid Balance To maintain fluid balance, fluid input needs to match fluid output.

Ifintake (from food and beverages) exceeds output (urine, fluid in feces, insensible losses), the organism isin positive balance, and urine volume will increase to eliminate excess fluid. Negative fluid balance occurswhen intake is less than output. In this case, integrated responses will increase thirst and decrease additionalfluid losses, until homeostasis is restored.12Cell Physiology, Fluid Homeostasis, and Membrane TransportNormally, changes in plasma osmolarity are well controlledand homeostasis is maintained as a result of hypothalamicosmoreceptors and the kidneys sensing fluid composition;carotid and aortic baroreceptors sensing pressure; release ofhormones in response to altered pressure and osmolarity; andthe actions of the kidney to regulate sodium and water reabsorption.

This integrated control is the key to fluid homeostasis. Control of renal fluid and electrolyte handling will becovered in Section 5.Fluid intake and output must be in balance (Fig. 1.10). Ifwater intake (through food and fluids) is greater than theoutput (urine and insensible losses from sweat, breathing, andfeces), the organism has a surplus of fluid, which will decreaseplasma osmolarity, and the kidneys will excrete the excessfluid (see Section 5).

Conversely, if the intake is less thanoutput, the organism has a deficit of fluid, and plasma osmolarity will increase. In this situation, the thirst response will beactivated and the kidneys will conserve fluid, producing lessurine. This idea of balance is expanded on in the followingsections, and the integration of the endocrine, cardiovascular,and renal systems in regulation of fluid and electrolyte homeostasis is discussed more fully.CLINICAL CORRELATEMeasurement of Fluid Compartment SizeBecause blood volume = plasma volume + red blood cell volume(see earlier), it can be calculated by the formula:The indicator-dilution method is used to determine the volumeof fluid in the different fluid compartments.