Tubular systems

For ultrafiltration appHcations, hollow-fine fibers have never been seriously considered because of their susceptibiUty to fouling. If the feed solution is extremely fouling, tubular or plate-and-frame systems ate still used. Recentiy, however, spiral-wound modules with improved resistance to fouling have been developed, and these modules are increasingly displacing the more expensive plate-and-frame and tubular systems. Capillary systems are also used in some ultrafiltration appHcations. 

At any instant, because gas-phase reactions are often carried out in tubular systems, the mass flowrate G and C- the concentration of i in moles per unit mass is used. The mass flowrate G does not change with position when fluid density changes as is the case with u, the volumetric flowrate (Figure 5-29). 

Polymer properties can be varied during polymerization. The basic chemical process is carried during their manufacture the polymer is formed under the influence of heat, pressure, catalyst, or combination inside vessels or tubular systems called reactors. One special form of property variation involves the use of two or more different monomers as comonomers, copolymerizing them to produce copolymers (two comonomers) or ter-polymers (three monomers). Their properties are usually intermediate between those... 

Voluntary muscle contraction is initiated in the brain-eliciting action potentials which are transmitted via motor nerves to the neuromuscular junction where acetylcholine is released causing a depolarization of the muscle cell membrane. An action potential is formed which is spread over the surface membrane and into the transverse (T) tubular system. The action potential in the T-tubular system triggers Ca " release from the sarcoplasmic reticulum (SR) into the myoplasm where Ca " binds to troponin C and activates actin. This results in crossbridge formation between actin and myosin and muscle contraction. 

Four different localizations of fatigue can be identified (a) decreased central command (b) decreased activation of the muscle membrane and the T-tubular system (c) decreased Ca release from the SR and (d) decreased response to the Ca release by the contractile proteins. The first two are partly extra-muscular while c and d are intramuscular responses to the excitation of the muscle membrane and often defined as excitation-contraction coupling. 

Possible mechanisms responsible for the decreased Ca release are changes in the sensitivity of the voltage sensor in the T-tubular system or in the SR Ca channel to the sensor stimulus. A third possibility would be a decreased availability... 

High frequency fatigue or fatigue during continuous high frequency stimulation seems to be mainly due to impaired propagation of the stimulating impulse in the T-tubular system. 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

A systematic, rational analysis of both isothermal and nonisothermal tubular systems in which two fluids are flowing must be carried out, if optimal design and economic operation of these pipeline devices is to be achieved. The design of all two-phase contactors must be based on a firm knowledge of two-phase hydrodynamics. In addition, a mathematical description is needed of the heat and mass transfer and of the chemical reaction occurring within a particular system. 

Martinelli and Nelson (M7) developed a procedure for calculating the pressure drop in tubular systems with forced-circulation boiling. The procedure, which includes the accelerative effects due to phase change while assuming each phase is an incompressible fluid, is an extrapolation of the Lockhart and Martinelli x parameter correlation. Other pressure drop calculation procedures have been proposed for forced-circulation phase-change systems however, these suffer severe shortcomings, and have not proved more accurate than the Martinelli and Nelson method. 

This regime is characterized by the presence of one continuous fluid phase and one discrete fluid phase in tubular systems. The existence of the discrete phase generates a large interfacial area per unit tube volume for all flow configurations included in this regime. For that reason, Regime IV is of pragmatic interest when interphase heat and mass transfer are of key importance. 

The model equations in Section II,A have been formulated to describe the energy and mass transfer processes occurring in two-phase tubular systems. The accuracy of these model equations in representing the physical processes depends on the parameters of the equations being correctly evaluated. Constitutive equations that relate each of the parameters to the physical properties, system properties, and dependent variables of the system are discussed in the following sections. 

The model equations in Section III,C, have been formulated to describe those energy and mass-transfer processes in two-phase tubular systems for which one cannot neglect phase change. Constitutive equations for the parameters in these model equations are discussed in this section. 

Sanders There are discrete sites of innervation of smooth muscles. Therefore, it is unlikely you ever get store depletion with neurotransmission. The SR is all connected it is a continuous tubular system. Releasing Ca2+ at one site is not going to empty the store in any way. 

MAHMA NONOate spontaneously dissociates in a pH-dependent, first-order process with a half-life of 1 min at 37 °C. In vivo, examination in anesthetised rats showed, that MAHMA NONOate had both platelet inhibitory and vasodepressor effects [50]. Like other NONOates, MAHMA NONOate inhibited collagen-induced and ADP-induced rat platelet aggregation in a concentration-dependent manner [51]. ODQ, a soluble GS inhibitor, caused only small influence on the concentration-response curve to MAHMA NONOate, indicating that cGMP-independent mechanisms play a crucial role. A potential target of MAHMA NONOate induced inhibition was the sarco-endoplasmic reticulum calcium-ATPase of the platelet dense tubular system... 

The dominant mercury pool in the body is the kidney [23, 31]. The kidneys contained over 85% of the body burden of mercury 15 days or more after a single injection of mercuric chloride into rats [32]. Maximum levels in the rat kidney were attained in less than a day after doses of mercuric chloride [33], The renal cortex contained the highest levels [34-36], the maximum concentrations being found in the proximal tubular system, while mercury was close to background levels in the glomeruli and collecting ducts. 

In humans, it has been calculated that about 65% of the Ni2+ in the glomerular filtrate is reabsorbed by the renal tubular system [263], and after absorption, urine is the major route for excretion. 

Tubular systems can also be converted to thin-channel devices with the use of "volume displacement rods". In one such design ( 7), manufactured by Amlcon and Romicon, a splined core has the membrane wrapped around it, sealed, and braided to form thin-channels between the core and the membrane (Figure 17). These braided tubes are then potted in a shell and tube module where the permeate is collected on the shell side (Figure 18). 

Spherical turbulence promoters have also been used in tubular systems (Figure 31). In the author s experience, these spherical promoters are not as effective as detached spiral wire promoters (9). Particles collect in the dead stagnant areas between the spheres and foul the system. The spheres serve more effectively as volume displacement spheres than as turbulence promoters. 

Fig. 2. Effect of calcium antagonists (CA) on a cardiac cell. Top typical cardiac action potential. The calcium (slow) inward current flows during the characteristic plateau phase (phase 2) of the action potential. This calcium influx is selectively inhibited by CA. Activation of the sarcoplasmic reticulum (SR) and other cellular calcium pools occurs via Ca + and Na+ ions which flow into the cell. The SR and other pools donate activator Ca + ions which stimulate the contractile proteins. The presence of tubular systems (invaginations), which are characteristic of cardiac tissues, results in considerable enlargement of the cellular surface, thus enabling an effective influx of Na+ and Ca + ions. Inhibition of the calcium inward flux by a CA causes diminished activation of the contractile proteins.

D. While the other parts of the renal tubular system do contain active transport systems, these systems do not have an affinity of urate transport. 

Histochemical evidence has indicated that the enzyme resides in the sarcoplasmic reticulum and transverse tubular system of rat myocardium (69). However, other evidence indicates that enzymic activity is also localized within the walls of the coronary blood vessels (70-72). Recent histochemical studies (73) have shown that the enzyme resides not... 

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