Distensibility of small arteries of the dog lung.

To obtain in situ measurements of the distensibility of small (100- to 1,000-microns-diam) pulmonary arterial vessels of the dog lung, X-ray angiograms were obtained from isolated lung lobes with the vascular pressure adjusted to various levels. The in situ diameter-pressure relationships were compared with the diameter-pressure relationships for small arteries that were dissected free from the lungs and cannulated with small glass pipettes for the measurement of diameter and transmural pressure. The diameter-vascular or diameter-transmural pressure curves from both in situ and cannulated vessels were sufficiently linear in the pressure range studied (0-30 Torr) that they could be characterized by linear regression to obtain estimates of D0, the diameter at zero vascular pressure, and beta, the change in diameter (micron) per Torr change in pressure. The vessel distensibility coefficient (alpha) was defined as alpha = beta/D0. The mean values of alpha were approximately 2.0 +/- 0.8%/Torr (SD) for the in situ vessels and 1.7 +/- 0.6%/Torr for the cannulated vessels, with no statistically significant difference between the two methods. The influence of vasoconstriction elicited by serotonin was evaluated in the in situ vessels. Serotonin-induced vasoconstriction caused a decrease in D0 and little change in alpha.     

Ventilation and circulation during exercise inOctopus vulgaris

Summary Ventilation frequency, volume, oxygen uptake, and oxygen transport by the blood have been studied in unrestrained octopus,Octopus vulgaris before, during and after recovery from 20 min of enforced activity. Exercise increased oxygen consumption 2.8 fold. The percentage utilisation of oxygen from the branchial water is maintained or increased at around 35% during activity and the calculated ventilation volume increases by 3 times. Prior to exercise the hemocyanin in arterial blood is 98% saturated and there is 83% utilisation of the oxygen in the blood. During activity there is remarkably little change in blood parameters so that the hemocyanin in the arterial blood remains at 96% saturation and oxygen utilisation is 90%. Cardiac output was calculated to have risen 2.5 fold during activity. As theP _{O 2} gradients across the gill do not change significantly during exercise the major adaptation which can account for an increase in oxygen consumption must be a 3 fold increase in the transfer factor. At rest 22% of the total CO₂ present in the blood is excreted during its passage through the gills and this rises to 32% during activity. There is no accumulation of CO₂ and only a slight acidification of the blood during activity. A significant respiratory and metabolic acidosis is avoided and the hemocyanin continues to function normally.     

Distensibility and pressure-flow relationship of the pulmonary circulation. II. Multibranched model

The contribution of distensibility and recruitment to the distinctive behavior of the pulmonary circulation is not known. To examine this question we developed a multibranched model in which an arterial vascular bed bifurcates sequentially up to 8 parallel channels that converge and reunite at the venous side to end in the left atrium. Eight resistors representing the capillary bed separate the arterial and venous beds. The elastic behavior of capillaries and extra-alveolar vessels was modeled after Fung and Sobin (Circ. Res. 30: 451-490, 1972) and Smith and Mitzner (J. Appl. Physiol. 48: 450-467, 1980), respectively. Forces acting on each component are modified and calculated individually, thus enabling the user to explore the effects of parallel and longitudinal heterogeneities in applied forces (e.g., gravity, vasomotor tone). Model predictions indicate that the contribution of distensibility to nonlinearities in the pressure-flow (P-F) and atrial-pulmonary arterial pressure (Pla-Ppa) relationships is substantial, whereas gravity-related recruitment contributes very little to these relationships. In addition, Pla-Ppa relationships, obtained at a constant flow, have no discriminating ability in identifying the presence or absence of a waterfall along the circulation. The P-F relationship is routinely shifted in a parallel fashion, within the physiological flow range, whenever extra forces (e.g., lung volume, tone) are applied uniformly at one or more branching levels, regardless of whether a waterfall is created. For a given applied force, the magnitude of parallel shift varies with proportion of the circulation subjected to the added force and with Pla.     

Distensibility and pressure-flow relationship of the pulmonary circulation. I. Single-vessel model

To ascertain the relative contributions of vascular distensibility and nonhomogeneous behavior within the pulmonary circulation to the distinctive nonlinear relationship between inflow pressure (Pin) and flow [pressure-flow (P-F) relationship] and between Pin and outflow pressure (Pout) at constant flow (Pin-Pout relationship), we developed a multibranched model in which the elastic behavior of, and forces acting on, individual branches can be varied independently. The response of the multibranched model is described in the companion article (J. Appl. Physiol. 68: 1514-1527, 1990). Here we describe the methods used and the responses of single components of the larger model. Perivascular pressure is modeled as a function of intravascular and transpulmonary pressures (Pv and Ptp, respectively) and vessel length as a function of lung volume. These and the relationship between vascular area (A) and transmural pressure (Ptm) were modeled primarily from the dog data of Smith and Mitzner (J. Appl. Physiol. 48: 450-467, 1980). Vasomotor tone is modeled as a radial collapsing pressure (Pt) in the same plane as Ptm. In view of lack of information about the relationship between Pt and A for a given active state, different patterns were assumed that span a wide range of possible relationships. The P-F and Pin-Pout relationships of single vessels were very similar to those reported for the entire intact circulation. Of note, the slope of the Pin-Pout relationship in the low Pout range (0-5 Torr) was very low (less than 0.25) and increased gradually with Pout toward unity. Vasomotor tone caused an apparent parallel shift in the P-F relationship in the physiological flow range of the dog (2-8 l/min) regardless of the pattern used to model the Pt vs. A relationship; different patterns affected the P-F relationship only over the low flow range before the parallel shift was established.     

Reflex control of the circulation during exercise: chemoreflexes and mechanoreflexes

The overall scheme for control is as follows: central command sets basic patterns of cardiovascular effector activity, which is modulated via muscle chemo- and mechanoreflexes and arterial mechanoreflexes (baroreflexes) as appropriate error signals develop. A key question is whether the primary error corrected is a mismatch between blood flow and metabolism (a flow error that accumulates muscle metabolites that activate group III and IV chemosensitive muscle afferents) or a mismatch between cardiac output (CO) and vascular conductance [a blood pressure (BP) error] that activates the arterial baroreflex and raises BP. Reduction in muscle blood flow to a threshold for the muscle chemoreflex raises muscle metabolite concentration and reflexly raises BP by activating chemosensitive muscle afferents. In isometric exercise, sympathetic nervous activity (SNA) is increased mainly by muscle chemoreflex whereas central command raises heart rate (HR) and CO by vagal withdrawal. Cardiovascular control changes for dynamic exercise with large muscles. At exercise onset, central command increases HR by vagal withdrawal and "resets" the baroreflex to a higher BP. As long as vagal withdrawal can raise HR and CO rapidly so that BP rises quickly to its higher operating point, there is no mismatch between CO and vascular conductance (no BP error) and SNA does not increase. Increased SNA occurs at whatever HR (depending on species) exceeds the range of vagal withdrawal; the additional sympathetically mediated rise in CO needed to raise BP to its new operating point is slower and leads to a BP error. Sympathetic vasoconstriction is needed to complete the rise in BP. The baroreflex is essential for BP elevation at onset of exercise and for BP stabilization during mild exercise (subthreshold for chemoreflex), and it can oppose or magnify the chemoreflex when it is activated at higher work rates. Ultimately, when vascular conductance exceeds cardiac pumping capacity in the most severe exercise both chemoreflex and baroreflex must maintain BP by vasoconstricting active muscle.     

Regulation of end-expiratory lung volume during exercise

We determined the effects of exercise on active expiration and end-expiratory lung volume (EELV) during steady-state exercise in 13 healthy subjects. We also addressed the questions of what affects active expiration during exercise. Exercise effects on EELV were determined by a He-dilution technique and verified by changes in end-expiratory esophageal pressure. We also used abdominal pressure-volume loops to determine active expiration. EELV was reduced with increasing exercise intensity. EELV was reduced significantly during even mild steady-state exercise and during heavy exercise decreased an average of 0.71 +/- 0.3 liter. Dynamic lung compliance was reduced 30-50%; EELV remained greater than closing volume. Changing the resistance to airflow (via SF6-O2 or He-O2 breathing) during steady-state exercise changed the peak gastric and esophageal pressure generation during expiration but did not alter EELV; breathing through the mouthpiece produced similar effects during exercise. EELV was significantly reduced in the supine position. With supine exercise active expiration was not elicited, and EELV remained the same as in supine rest. With CO2-driven hyperpnea (7-70 l/min), EELV remained unchanged from resting levels, whereas during exercise, at similar minute ventilation (VE) values EELV was consistently decreased. At the same VE, treadmill running caused an increase in tonic gastric pressure and greater reductions in EELV than either walking or cycling. We conclude that both the exercise stimulus and the resultant hyperpnea stimulate active expiration and a reduced FRC. This new EELV is preserved in the face of moderate changes in mechanical time constants of the lung. This reduced EELV during exercise aids inspiration by optimizing diaphragmatic length and permitting elastic recoil of the chest wall.     

Influence of central command and ergoreceptors on the splanchnic circulation during isometric exercise

The splanchnic circulation can make a major contribution to blood flow changes. However, the role of the splanchnic circulation in the reflex adjustments to the blood pressure increase during isometric exercise is not well documented. The central command and the muscle chemoreflex are the two major mechanisms involved in the blood pressure response to isometric exercise. This study aimed to examine the behaviour of the superior mesenteric artery during isometric handgrip (IHG) at 30% maximal voluntary contraction (MVC). The pulsatility index (PI) of the blood velocity waveform of the superior mesenteric artery was taken as the study parameter. A total of ten healthy subjects [mean age, 21.1 (SEM 0.3) years] performed an IHG at 30% MVC for 90 s. At 5 s prior to the end of the exercise, muscle circulation was arrested for 90 s to study the effect of the muscle chemoreflex (post exercise arterial occlusion, PEAO). The IHG at 30% MVC caused a decrease in superior mesenteric artery PI, from 4.84 (SEM 1.57) at control level to 3.90 (SEM 1.07) (P = 0.015). The PI further decreased to 3.17 (SEM 0.70) (P = 0.01) during PEAO. Our results indicated that ergoreceptors may be involved in the superior mesenteric artery vasodilatation during isometric exercise.     

Plasma renin system during exercise in normal men

The exercise-related increase in plasma renin activity (PRA) and in the plasma concentration of angiotensin II (ANG II) and aldosterone (Aldo) was studied in 43 healthy volunteers whose 24-h urinary sodium excretion (UVNa) ranged from 10 to 250 mmol. Arterial blood samples were obtained at rest and during bicycle ergometry. Compared with rest, PRA, ANG II, and Aldo rose to a similar extent during light and moderate exercise. However, at peak exercise ANG II increased significantly more (P less than 0.001) than PRA and Aldo. Thus, with increasing intensity of exercise, the slope of the linear regression of ANG II on PRA became significantly (P less than 0.001) steeper, whereas at maximal exercise the Aldo response did not follow the acute rise in ANG II. At rest as well as during exercise, Aldo rose with increasing ANG II, but the stimulatory effect of ANG II on Aldo was attenuated with higher sodium intake, as estimated from UVNa. Finally, independent of the level of physical activity, UVNa was negatively correlated with PRA, ANG II, and Aldo.