•Fig. 18.5 Experimentaldatashowingthatincreasesincarotidsinuspressureresultindecreasedsympatheticefferentnerve(Symp)activityandincreasedcardiacvagalnerveactivity.(AdaptedfromKollaiM,KoizumiK.Pflügers Arch. 1989;413:365.)100 50 0 60 100 140 180 220 Sinus pressure (mm Hg) Vagal activity (% of max) 100 50 0 Symp. activity (% of max) •Fig. 18.6 Intravenous Infusions of Blood or Electrolyte Solutions Tend to Increase the Heart Rate Through the Bainbridge Reflex and to Decrease the Heart Rate Through the Baroreceptor Reflex. Theactualchangeinheartrateinducedbysuchinfusionsistheresultofthesetwoopposingeffects.Intravenous infusion Increases right atrial pressure Atrial receptors stimulated Bainbridge reflex – + Increases cardiac output Increases arterial pressure Baroreceptor reflex Heart rate
Stimulation of the atrial receptors increases not only the heart rate but also urine volume. Reduced activity in the renal sympathetic nerve fibers may partially account for this diuresis. However, the principal mechanism appears to be a neurally mediated reduction in vasopressin (antidiuretic hormone) secretion by the posterior pituitary gland (see ). Stretch of the atrial walls also releases atrial natriuretic peptide (ANP) from the atria.
ANP, a 28–amino acid peptide, exerts potent diuretic and natriuretic effects on the kidneys (see also ) and vasodilator effects on the resistance and capacitance vessels. Thus ANP is an important regulator of blood volume and blood pressure.
Incongestiveheartfailure,NaClandwaterareretained,mainlybecausestimulationbytherenin-angiotensinsystemincreasesthereleaseofaldosteronefromtheadrenalcortex.TheplasmalevelofANPisalsoincreasedincongestiveheartfailure.ByenhancingtherenalexcretionofNaClandwater,ANPgraduallyreducesfluidretentionandtheconsequentelevationsincentralvenouspressureandcardiacpreload.
Rhythmic variations in heart rate, occurring at the frequency of respiration, are detectable in most individuals and tend to be more pronounced in children. The heart rate typically accelerates during inspiration and decelerates during expiration (Fig. 18.7 *The myocytes of the ventricles secrete a related peptide in response to stretch. This peptide, termed brain natriuretic peptide (BNP) because of its initial discovery in the central nervous system, has actions similar to those of ANP (see •Fig. 18.7 Respiratory Sinus Arrhythmia. Notethattheheartrateincreasesduringinspirationanddecreasesduringexpiration.(ModifiedfromWarnerMR,etal.Am J Physiol. 1986;251:H1134.)50 80 110 140 Expiration Time (sec) Inspiration 3 6 9 12 150 Heart rate(beats/min)