No abstract

1. Lymph from the lungs of lambs and sheep was found to enter both the right lymph duct and the thoracic duct. Right lymph duct flow was collected by constructing a venous sac, the venous tributaries of which were ligated but which the right lymph duct entered; thoracic duct flow was collected by cannulating the duct. Lymph from sites other than the lungs was excluded from the collections.2. Measurements were made of the surface tension characteristics of lung extracts and of the liquid present in foetal lungs. These values were used together with gestational age and crown-rump length to designate the foetal lambs into mature and immature groups.3. Lymph flow from the lungs averaged 0.99 ml./kg body wt./hr in immature foetal lambs, and 1.81 ml./kg/hr in mature foetal lambs before the start of ventilation. Lymph flow from the lungs of spontaneously delivered new-born lambs (mean age 51 hr) averaged 0.86 ml./kg/hr. In adult ewes right lymph duct flow averaged 0.11 ml./kg/hr and total lung lymph flow was estimated indirectly to be 0.33 ml./kg/hr. Calculated rates of protein flow in lung lymph (flow x protein concentration) were greater in foetal lambs than in adult sheep.4. Total thoracic duct flow averaged 2.48 ml./kg/hr in immature foetal lambs, 5.30 ml./kg/hr in mature foetal lambs, 3.65 ml./kg/hr in new-born lambs, and 2.92 ml./kg/hr in adult ewes.5. At the start of ventilation there was an increase in lymph flow from the lungs, which at 15-30 min reached a mean of 6.4 ml./kg/hr in mature lambs and 2.6 ml./kg/hr in immature lambs. At the same time the protein concentration of lymph decreased but the calculated protein flow increased.6. The lungs of foetal lambs weighed more than the lungs of spontaneously delivered new-born lambs, and the difference could be accounted for by liquid which could be aspirated through the trachea of the foetal lamb. On ventilation of the lungs for 2 hr, without first allowing the escape of any lung liquid, lung weight measurements indicated that about 66% of the lung liquid had been taken up in mature lambs and about 50% in immature lambs.7. It was concluded that the rate at which lymph is formed in the lungs is greater per kilogram body weight in foetal than in new-born lambs and greater in them than in ewes. The increase in lymph flow at the start of ventilation could account for the removal of about 40% of the liquid present in the lungs of the mature foetus and about 25% of the liquid in the lungs of the immature foetus.

SUMMARY1. The permeability of lung capillaries to macromolecules was investigated in immature and mature foetal lambs, new-born lambs and young sheep. The placental circulation of the foetal animals was maintained intact after delivery by Caesarian section. New-born lambs and sheep were mechanically ventilated. Samples of plasma and lymph that had drained from the lung via the thoracic duct were collected over a period of 1-5 hr.2. The proteins in plasma and lymph samples were separated by fractionation on columns of Sephadex G-200. Plasma yielded three peaks of protein concentration. The Kav value of each peak was determined, and, by calibrating the columns with known proteins, the mean radius of equivalent sphere (a) of the proteins in peak I was shown to be similar to that of fibrinogen > 110 A, peak II to y-globulin -54 A and peak III to albumin -34 A. Lung lymph contained the same three constituent peaks as plasma but in lower concentration. In all four groups mean lymph/plasma concentration (L/P) ratio was significantly different for each of the three peaks, being lowest for the largest molecules (peak I) and highest for the smallest (peak III).3. In five mature foetal lambs polydisperse polyvinylpyrrolidone labelled with 1251 ([1251]PVP) was injected i.v. early in the experiment: count rates in fractionated samples showed for plasma a continuous decline with time after injection, and for lung lymph an increase to a maximum then a decline. Steady-state L/P ratios for eleven fractions of PVP of differing molecular size ranging from 110 to 17 A were derived by compartmental analysis. For a given molecular size PVP L/P ratios were similar to protein L/P values.4. The regression of PVP L/P ratio on Kav was linear (correlation R. D. H. BOYD AND OTHERS coefficient r = 0.99), and the slope of the regression of protein L/P ratio on Kay was significantly steeper for new-born lambs than for mature foetuses (P < 0 025) and sheep (P < 0.005), and steeper for immature foetuses than sheep (P < 0 01).5. PVP and protein L/P ratios (mature foetuses) plotted against a showed a sigmoid relation with agreement between the two sets of L/P ratios. The goodness of fit between our experimental results and Landis & Pappenheimer's (1963) capillary pore theory (eqn. (1)) was examined: L/P ratios for the larger molecules ( > 75 A) appeared to be too high. By recalculating ratios on the assumption that the largest molecules (110 A) escape unrestricted from the capillary via leaks, the discrepancy disappears.6. Values for pore radius (r), and pore area per unit path length (A/Ax) have been calculated for each of the four groups; r ranged from 90 to 150A, A/Ax from 3-3 to 0-2 cm x 103.kg-'. In new-born lambs the value of r was significantly smaller, and A/Ax larger than that of any other group. The inferences to be drawn from these results are discussed.

SUMMARY1. The reflex responses to baroreceptor stimulation have been compared in eight pregnant and eight non-pregnant anaesthetized female rabbits.2. The vascularly isolated, innervated carotid sinus was exposed for 30 sec to a series of non-pulsatile pressures ranging from 30 to 230 mmHg. The contralateral sinus nerve and both aortic nerves were cut. Systemic arterial pressure and heart rate were measured at each sinus pressure.3. The range of arterial pressure change which could be evoked from the isolated innervated sinus was less in the pregnant than in the non-pregnant rabbits. Mean changes were 98 and 61 mmHg respectively. On the other hand changes in heart rate were similar in the two groups (45 and 43 beats/ min respectively).4. The smaller blood pressure response in the pregnant animals resulted from a lesser rise in systemic arterial pressure at low levels of sinus pressure. At high sinus pressures the blood pressure fell to a similar level in both groups of animals.5. Pressure on the great vessels by the gravid uterus was not a factor since there was no consistent difference between the responses obtained with the rabbit lying on its back or on its side.6. Mechanisms which might be responsible for the difference found are discussed.

SUMMARY1. The reflex cardiovascular effects of changes in pressure within the isolated carotid sinus have been examined in sixteen anaesthetized pregnant rabbits.2. Comparison of the mean results at sinus pressures of 40 and 200 mmHg showed that heart rate fell by 32-1 beats min-' and arterial pressure was reduced by 48-2 mmHg. Cardiac output, measured by thermal dilution, fell by 25-7 ml. min-'.kg-1 and total peripheral resistance was reduced by 0-292 mmHg.ml.-' min.kg.3. The corresponding changes previously reported in a group of seventeen non-pregnant female rabbits were a similar fall in heart rate of 34.5 beats min-' but significantly greater reductions in arterial pressure of 69-8 mmHg and in total peripheral resistance of 0X432 mmHg. ml.-' min. kg. However, the fall in cardiac output of 12-6 ml. min-' . kg-1 was significantly smaller.4. These observations show that the smaller blood pressure response of the pregnant rabbit to alterations in sinus pressure is due to a reduced change in total peripheral resistance.