Venous return and mean systemic filling pressure: physiology and clinical applications.

Venous return is the flow of blood from the systemic venous network towards the right heart. At steady state, venous return equals cardiac output, as the venous and arterial systems operate in series. However, unlike the arterial one, the venous network is a capacitive system with a high compliance. It includes a part of unstressed blood, which is a reservoir that can be recruited via sympathetic endogenous or exogenous stimulation. Guyton's model describes the three determinants of venous return: the mean systemic filling pressure, the right atrial pressure and the resistance to venous return. Recently, new methods have been developed to explore such determinants at the bedside. In this narrative review, after a reminder about Guyton's model and current methods used to investigate it, we emphasize how Guyton's physiology helps understand the effects on cardiac output of common treatments used in critically ill patients.

Benefits behind barriers in physiology education.

The COVID-19 pandemic forced academics to switch to online teaching whether they were prepared or not. The speed and enthusiasm with which educators embraced online teaching suggest that challenges change the perspective for the better. The teaching challenges with the current coronavirus situation mimic the poliovirus attack Dr. Arthur C. Guyton encountered. Dr. Guyton was forced to switch his career from becoming a cardiovascular surgeon to a physiology educator and a researcher. His immense contributions to the field of physiology is an example of how challenges can bring benefits. Flipped teaching has been gaining attention among educators because of its ability to engage students in learning. The COVID-19 pandemic pushed educators to adopt this instructional design based on its conduciveness to technology, as well as its blend of both asynchronous and synchronous components of online teaching. Just like Dr. Guyton's enormous impact on medical education and research in spite of the challenges he faced, we must be creative during this pandemic through innovative teaching methods, which will serve as a gift for the future of physiology education.

Role of the kidney in the pathogenesis of hypertension: time for a neo-Guytonian paradigm or a paradigm shift?

The "Guytonian paradigm" places the direct effect of arterial pressure, on renal excretion of salt and water, at the center of long-term control of blood pressure, and thus the pathogenesis of hypertension. It originated in the sixties and remains influential within the field of hypertension research. However, the concept of one central long-term feedback loop, through which arterial pressure is maintained by its influence on renal function, has been questioned. Furthermore, some concepts in the paradigm are undermined by experimental observations. For example, volume retention and increased cardiac output induced by high salt intake do not necessarily lead to increased arterial pressure. Indeed, in multiple models of salt-sensitive hypertension the major abnormality appears to be failure of the vasodilator response to increased cardiac output, seen in salt-resistant animals, rather than an increase in cardiac output itself. There is also evidence that renal control of extracellular fluid volume is driven chiefly by volume-dependent neurohumoral control mechanisms rather than through direct or indirect effects of changes in arterial pressure, compatible with the concept that renal sodium excretion is controlled by parallel actions of different feedback systems, including hormones, reflexes, and renal arterial pressure. Moreover, we still do not fully understand the sequence of events underlying the phenomenon of "whole body autoregulation." Thus the events by which volume retention may develop to hypertension characterized by increased peripheral resistance remain enigmatic. Finally, by definition, animal models of hypertension are not "essential hypertension;" progress in our understanding of essential hypertension depends on new results on system functions in patients.

Venous return and clinical hemodynamics: how the body works during acute hemorrhage.

Venous return is a major determinant of cardiac output. Adjustments within the venous system are critical for maintaining venous pressure during loss in circulating volume. This article reviews two factors that are thought to enable the venous system to compensate during acute hemorrhage: 1) changes in venous elastance and 2) mobilization of unstressed blood volume into stressed blood volume. We show that mobilization of unstressed blood volume is the predominant and more effective mechanism in preserving venous pressure. Preservation of mean circulatory filling pressure helps sustain venous return and thus cardiac output during significant hemorrhage.