Starling forces

In arterioles, the hydrostatic pressure is about 37 mm Hg, with an interstitial (tissue) pressure of 1 mm Hg opposing it. The osmotic pressure (oncotic pressure) exerted by the plasma proteins is approximately 25 mm Hg. Thus, a net outward force of about 11 mm Hg drives fluid out into the interstitial spaces. In venules, the hydrostatic pressure is about 17 mm Hg, with the oncotic and interstitial pressures as described above thus, a net force of about 9 mm Hg attracts water back into the circulation. The above pressures are often referred to as the Starling forces. If the concentration of plasma proteins is markedly diminished (eg, due to severe protein malnutrition), fluid is not attracted back into the intravascular compartment and accumulates in the extravascular tissue spaces, a condition known as edema. Edema has many causes protein deficiency is one of them. 

Osmotic diuretics alter Starling forces so that water leaves cells and reduces intracellular volume. This effect is used to reduce intracranial pressure in neurologic conditions and to reduce intraocular pressure before ophthalmologic procedures. A dose of 1-2 g/kg mannitol is administered intravenously. Intracranial pressure, which must be monitored, should fall in 60-90 minutes. 

However, the significance of the decreased colloidosmotic (oncotic) pressure is not as great as has been hitherto assumed. Nevertheless, when the hydrostatic pressure is raised at the same time, incongruity between these two Starling forces is created, and fluid escapes into the abdominal cavity. This process is greatly furthered by the disparity between lymph production and lymph transport. Tliese mechanical factors (s. tab. 16.3) may effect the formation of ascites, yet they cannot produce large quantities of ascitic fluid. Such a development, however, can be expected if the capillary permeability is additionally heightened due to toxic or inflammatory causes. 

Underfill theory According to the underfill theory (S. Sherlock et al., 1963), the development of ascites is set off by mechanical factors and physical mechanisms ( imbalance of the Starling forces ). As a result, the effective plasma volume is reduced (so-called volume deficiency concept). 

Strandhoy JW, Ott CE, Schneider EG et al. Effects of prostaglandins El and E2 on renal sodium reabsorption and Starling forces. American Journal of Physiology 1974 226 1015-1021. 

Taylor AE. Capillary fluid filtration Starling forces and lymph flow. Circ Res 1981 49 557-575. 

Eaton SA, Allen D, Eades SC et al. Digital Starling forces aid hemodynamics during early laminitis induced by an aqueous extract of black walnut (Juglans nigra) in horses. Am J Vet Res 56 1338-1343, 1995. 

Frank-Starling mechanism The ability of the heart to change its force of contraction and therefore stroke volume in response to changes in venous return. 

Figure 15.7 Starling principle a summary of forces determining the bulk flow of fluid across the wall of a capillary. Hydrostatic forces include capillary pressure (Pc) and interstitial fluid pressure (PJ. Capillary pressure pushes fluid out of the capillary. Interstitial fluid pressure is negative and acts as a suction pulling fluid out of the capillary. Osmotic forces include plasma colloid osmotic pressure (np) and interstitial fluid colloid osmotic pressure (n,). These forces are caused by proteins that pull fluid toward them. The sum of these four forces results in net filtration of fluid at the arteriolar end of the capillary (where Pc is high) and net reabsorption of fluid at the venular end of the capillary (where Pc is low).

VV. C. Thomas. Note on the Heal Transfer Equation for Forced Conveclion Flow over a Flat Plate with an Unheated Starling Length. Mechanica] Engineering News, 9, no. 1 (1977), p. 361. 

Several factors, such as preload, afterload, heart rate and contractility, determine normal cardiovascular function. Contractility is affected both by preload and afterload, as well as the inotropic state of the ventricular myocardium. Preload is the diastolic load placed upon the ventricle by the venous return of blood and affects contractility as described by Starling s law of the heart, where increased diastolic volume leads to a greater force of contraction. Preload is affected by the venous tone and by the circulating blood volume. Afterload opposes contraction throughout systole and is determined by the arterial resistance and by the circulating blood volume. 

Heart rate is controlled by the autonomic nervous system. Stroke volume, or the volume of blood ejected during systole, depends on preload, afterload, and contractility. As defined by the Frank-Starling mechanism, the ability of the heart to alter the force of contraction... 

Stokes (Starling, 1935) showed that the force exerted by gravity upon a falling body could be given by ... 

The balance of forces across the capillary wall is expressed succinctly by the Starling equation ... 

Carnahan and Starling (14) have suggested using the correct hard-sphere equation of state together with the RK expression for the contribution of the attractive forces. Thus... 

Unfortunately, almost no empirical equations of state contain an adequate description of the hard-sphere fluid. The VDW term, (V — Nb) 1, seems to be a part of most, if not all, the popular empirical equations of state. Carnahan and Starling (14), Gubbins (30), Prausnitz (31), and no doubt, others have called for the replacement of this term in chemical-engineering practice. It is time that their advice was followed. The VDW term (V — Nb)"1 is an inadequate description of a singlecomponent, hard-sphere fluid. It is even less suitable to describe mixtures or fluids composed of nonspherical molecules. Fortunately, the replacement of (V — Nb)"1 is easily accomplished since, in all the popular empirical equations of state, the contributions of the repulsive and attractive forces are separated clearly. Moreover, the form of the term representing the contribution of the attractive forces generally is written in a theoretically reasonable form. The replacement of (V — Nb)"1 by a more reliable expression results not only in an intellectually more satisfying equation but usually in a more reliable one also. 

These drugs cause dilation of veins. Venodilation results in a decrease in preload, and decreased ventricular filling, which decreases the load to the myocardium and increases myocardial efficiency (i.e., by Starling s law). The decrease in preload contributes to a decreased fiber stretch, and optimal actin-myosin interaction, which results in increased contractile force and increased myocardial efficiency. 

As the arterial blood enters the capillaries, fluid moves from the intravascular space into the interstitial space (that surrounding the capillaries) because of what are known as Starling s forces. The hydrostatic pressure in the arteriolar end of the capillaries ( 37 mm Hg) exceeds the sum of the tissue pressure ( 1 mm Hg) and the osmotic pressure of the plasma proteins ( 25 mm Hg). Thus, water tends to leave the capillaries and enter extravascular spaces. At the venous end of the capillaries, the hydrostatic pressure falls to approximately 17 mm Hg while the osmotic pressure and the tissue pressure remain constant, resulting in movement of fluid back from the extravascular (interstitial) spaces and into the blood. Thus, most of the force bringing water back from the tissues is the osmotic pressure mediated by the presence of proteins in the plasma. 

We shall now illustrate the use of dimensional analysis with the appHcation of Laplace s law to mammalian hearts. The beat-to-beat pumping ability of the mammalian heart is determined by its force-generating capability and the lengths of its constituent muscle fibers, as governed by the Starling s experimental observations on the heart. The formula for calculating force or tension, however, has been based on the law of Laplace... 

Myocardial tissue exerts a more forceful contraction if it is stretched before contraction begins. This property (known as Starling s law of the heart) serves to equalize the flows between the two hearts by causing a more powerful ejection from the heart in which more blood accumulates during diastole. The amount of initial stretching of the cardiac muscle is known as preload. 

In both previously mentioned examples, a fundamental property of the myocardium, described by the Frank-Starting mechanism, comes into play. The Frank-Starling mechanism holds that the force of contraction generated by the heart is increased as the amount of blood filling the LV is increased and the myocardium is stretched (Moss and Fitzsimons, 2002 Vincent, 2008). In the example of valvular... 

Because part of the force couple of the biceps femoris and the multifidus has been weakened, the sacrum is able to rotate posteriorly, as the multifidus can no longer prevent the top fo the sacrum being drawn posteriorly, as it was weakened by stretching (Starling s law). The gluteal mus-... 

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