Key points• During exercise, cardiac output is regulated to match oxygen delivery to the metabolic demand.• This study evaluated the role of heart rate and peripheral vasodilation in the regulation of cardiac output during exercise.• We increased heart rate by atrial pacing in 10 healthy male individuals during three different conditions: at rest, during exercise and during femoral arterial ATP infusion at rest.• Increasing the heart rate by atrial pacing by up to 54 beats min −1 did not increase cardiac output in any of the three given conditions as there was a proportional decrease in stroke volume.• These results indicate that cardiac output is regulated by changes in peripheral vasodilatation, whereas an increase in heart rate is less important.Abstract In dogs, manipulation of heart rate has no effect on the exercise-induced increase in cardiac output. Whether these findings apply to humans remain uncertain, because of the large differences in cardiovascular anatomy and regulation. To investigate the role of heart rate and peripheral vasodilatation in the regulation of cardiac output during steady-state exercise, we measured central and peripheral haemodynamics in 10 healthy male subjects, with and without atrial pacing (100-150 beats min −1 ) during: (i) resting conditions, (ii) one-legged knee extensor exercise (24 W) and (iii) femoral arterial ATP infusion at rest. Exercise and ATP infusion increased cardiac output, leg blood flow and vascular conductance (P < 0.05), whereas cerebral perfusion remained unchanged. During atrial pacing increasing heart rate by up to 54 beats min −1 , cardiac output did not change in any of the three conditions, because of a parallel decrease in stroke volume (P < 0.01). Atrial pacing increased mean arterial pressure (MAP) at rest and during ATP infusion (P < 0.05), whereas MAP remained unchanged during exercise. Atrial pacing lowered central venous pressure (P < 0.05) and pulmonary capillary wedge pressure (P < 0.05) in all conditions, whereas it did not affect pulmonary mean arterial pressure. Atrial pacing lowered the left ventricular contractility index (dP/dt) (P < 0.05) in all conditions and plasma noradrenaline levels at rest (P < 0.05), but not during exercise and ATP infusion. These results demonstrate that the elevated cardiac output during steady-state exercise is regulated by the increase in skeletal muscle blood flow and venous return to the heart, whereas the increase in heart rate appears to be secondary to the regulation of cardiac output. Abbreviations BVC, brain vascular conductance; CVP, central venous pressure; dP/dt max , left ventricular contractility index; HR, heart rate; LBF, leg blood flow; LVC, leg vascular conductance; MAP, mean arterial pressure; MCA, middle cerebral artery; MCA V mean , brain vascular conductance index; PAMP, pulmonary artery mean pressure; PCWP, pulmonary capillary wedge pressure;Q , cardiac output; SV, stroke volume; SVC, systemic vascular conductance; V mean , mean blood velocity;V O 2 , oxygen uptake.

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The aim of this study was to determine the relationship between ultrasonographic kidney volume and modified body mass index through evaluating the kidney architectural appearance, variations between right and left kidney dimensions and correlation between ultrasonographic kidney volumes and modified body mass index in clinically healthy Nigerian indigenous dogs. The ultrasonography was performed to obtain the kidney dimensions in centimeter for the length (bipolar length), width and height of the kidney for both right and left kidneys to compute for the kidney volume in centimeter cube using the formula for the volume of an ellipsoid (L X W X H X 0.523), while the modified body mass index for dogs was obtained by taking the body weight in kilogram of each dog against the squared length of the trunk. The serum creatinine and urea parameters are within normal limits in dogs used for this study. This research revealed normal kidney architecture with hyperechoic renal capsules, hypoechoic renal cortex, anechoic medullary pyramids and hyperechoic renal pelvis. Left kidneys are significantly larger than the right kidneys and there was a weak positive correlation between right (r 2 = 0.21) and left (r 2 = 0.18) kidney volumes with modified body mass index in clinically healthy in Nigerian indigenous dogs. In conclusion, the left kidney appeared larger than the right kidney and there is a positive but weak relationship between ultrasonographic kidney volume and modified body mass index in healthy Nigerian indigenous dogs.