The Bainbridge reflex (or Bainbridge effect or atrial reflex) is a cardiovascular reflex causing an increase in heart rate in response to increased stretching of the wall of the right atrium and/or the inferior vena cava as a result of increased venous filling (i.e., increased preload). It is detected by stretch receptors in the wall of the right atrium, the afferent limb is via the vagus nerve, it is regulated by a center in the medulla oblongata of the brain, [1] and the efferent limb involves reduced vagal activity and increased sympathetic nervous system outflow. [2]
Mechanistically, the increased heart rate evoked by the Bainbridge reflex acts to match heart rate (and hence cardiac output) to effective circulating blood volume on a beat-to-beat basis. This, in combination with other cardiovascular reflexes, helps maintain homeostatic equilibrium of the circulation. [3] The Bainbridge reflex may also contribute to respiratory sinus arrhythmia as intrathoracic pressure decreases during inspiration causing increased venous return. [3] [4]
The reflex is named after Francis Arthur Bainbridge, an English physiologist. The Bainbridge reflex was one of the first neural cardiovascular reflexes to be described and initiated a period of intense research into neural regulation of the heart. [5]
The reflex was originally demonstrated by Bainbridge in 1915 who observed an increase is heart rate following infusion of blood or saline into the jugular vein of anaesthetized dogs. [6] The response was reduced by cutting the cardiac sympathetic nerves and abolished by cutting the vagus nerve and he therefore concluded that it was a neural reflex. While the reflex may raise heart rate by as much as 40% to 60%, [7] initial attempts to replicate Bainbridge's observations were frequently unsuccessful [8] and this inconsistency was only explained in 1955 when Coleridge and Linden found that the type of heart rate response (increase or decrease) depended on the resting heart rate and the rate of increase in volume. [9] While Bainbridge only described an increase in heart rate in response to increased blood volume, a 'reverse Bainbridge reflex', namely a decrease in heart rate following reduced venous return has since been described. [2]
The Bainbridge reflex and other cardiovascular reflexes, such as the arterial baroreceptor reflex [10] and the Bezold-Jarisch reflex, [11] influence heart rate and circulatory homeostasis. The Bainbridge reflex responds to increased blood volume, whereas the baroreceptor reflex responds to changes in arterial blood pressure and the Bezold-Jarisch reflex responds to mechanical and chemical stimuli acting on the left ventricular wall. These reflexes are supplemented by the intrinsic sensitivity to stretch of pacemaker cells in the sinoatrial node. [12]
The Bainbridge reflex is most strong when heart rate is low; when heart rate is already high, additional venous return to the right atrium (i.e. additional increases in blood volume) will indirectly cause relatively greater stimulation of arterial baroreceptors which will reduce the heart rate. Thus, the effect of the Bainbridge reflex on heart rate may be counteracted by the baroreceptor reflex so that the net effect is determined by the balance of both reflexes, or, rather, the balance of factors determining their individual amplitude. [7] [13]
Increased blood volume in the right atrium leads to stretching of the atrial walls. This stretching is sensed by atrial stretch receptors [7] (which are located at the venoatrial junction [13] ), causing an increase in the firing rate of group B nerve fibers (low pressure receptors). [1] The information about the degree of atrial stretch is then conveyed through afferent fibres of the vagus nerve (cranial nerve X) to the medulla oblongata; efferents controlling heart rate (chronotropy) and contraction strength (inotropy) are then conveyed back to the heart through sympathetic nerves as well as the vagus nerve. [7] Unusually, tachycardia is mediated by increased sympathetic activity to the sinoatrial node with no fall in parasympathetic activity.[ citation needed ] Effects on cardiac contractility [13] [1] and stroke volume are insignificant. [1] Bainbridge reflex is attenuated by both anticholinergics and beta-adrenergic receptor antagonists in innervated hearts (as one or the other afferent part of the reflex arc mediating the Bainbridge reflex is blocked), [14] and can be entirely abolished by bilateral vagotomy (as the afferent portion of the reflex arc is entirely destroyed). [13]
The Bainbridge reflex is the predominant but not the only mechanism mediating increases in heart rate in response to increased atrial stretch: stretching of the pacemaker cells of the sinoatrial node has a direct positive chronotropic effect on the rate of the sinoatrial node, and may by itself increase heart rate by as much as 15%. This local response involves stretch-activated ion channels, as was demonstrated by stretching single isolated pacemaker cells while recording their cellular electrical activity. [15] This has led to the suggestion that the response discovered by Bainbridge should be referred to as an 'effect' rather than a 'reflex'. [16]
Bradycardia, also called bradyarrhythmia, is a resting heart rate under 60 beats per minute (BPM). While bradycardia can result from various pathologic processes, it is commonly a physiologic response to cardiovascular conditioning or due to asymptomatic type 1 atrioventricular block.
The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels. Heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrients to the tissue, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called the mediastinum.
The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system.
Baroreceptors are stretch receptors that sense blood pressure. Thus, increases in the pressure of blood vessel triggers increased action potential generation rates and provides information to the central nervous system. This sensory information is used primarily in autonomic reflexes that in turn influence the heart cardiac output and vascular smooth muscle to influence vascular resistance. Baroreceptors act immediately as part of a negative feedback system called the baroreflex, as soon as there is a change from the usual mean arterial blood pressure, returning the pressure toward a normal level. These reflexes help regulate short-term blood pressure. The solitary nucleus in the medulla oblongata of the brain recognizes changes in the firing rate of action potentials from the baroreceptors, and influences cardiac output and systemic vascular resistance.
Heart rate is the frequency of the heartbeat measured by the number of contractions of the heart per minute. The heart rate varies according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide. It is also modulated by numerous factors, including genetics, physical fitness, stress or psychological status, diet, drugs, hormonal status, environment, and disease/illness, as well as the interaction between these factors. It is usually equal or close to the pulse rate measured at any peripheral point.
A cardiac function curve is a graph showing the relationship between right atrial pressure (x-axis) and cardiac output (y-axis). Superimposition of the cardiac function curve and venous return curve is used in one hemodynamic model.
The sinoatrial node is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximately 15 mm long, 3 mm wide, and 1 mm thick, located directly below and to the side of the superior vena cava.
The cardiovascular centre is a part of the human brain which regulates heart rate through the nervous and endocrine systems. It is considered one of the vital centres of the medulla oblongata.
The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure causes the heart rate to decrease. Decreased blood pressure decreases baroreflex activation and causes heart rate to increase and to restore blood pressure levels. Their function is to sense pressure changes by responding to change in the tension of the arterial wall. The baroreflex can begin to act in less than the duration of a cardiac cycle and thus baroreflex adjustments are key factors in dealing with postural hypotension, the tendency for blood pressure to decrease on standing due to gravity.
The atrium is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves.
The Hering–Breuer inflation reflex, named for Josef Breuer and Ewald Hering, is a reflex triggered to prevent the over-inflation of the lung. Pulmonary stretch receptors present on the wall of bronchi and bronchioles of the airways respond to excessive stretching of the lung during large inspirations.
Vagal tone is activity of the vagus nerve and a fundamental component of the parasympathetic branch of the autonomic nervous system. This branch of the nervous system is not under conscious control and is largely responsible for the regulation of several body compartments at rest. Vagal activity results in various effects, including: heart rate reduction, vasodilation/constriction of vessels, glandular activity in the heart, lungs, and digestive tract, liver, immune system regulation as well as control of gastrointestinal sensitivity, motility and inflammation.
Venous return is the rate of blood flow back to the heart. It normally limits cardiac output.
Reflex bradycardia is a bradycardia in response to the baroreceptor reflex, one of the body's homeostatic mechanisms for preventing abnormal increases in blood pressure. In the presence of high mean arterial pressure, the baroreceptor reflex produces a reflex bradycardia as a method of decreasing blood pressure by decreasing cardiac output.
The Bezold–Jarisch reflex involves a variety of cardiovascular and neurological processes which cause hypopnea, hypotension and bradycardia in response to noxious stimuli detected in the cardiac ventricles. The reflex is named after Albert von Bezold and Adolf Jarisch Junior. The significance of the discovery is that it was the first recognition of a chemical (non-mechanical) reflex.
The Nicoladoni–Branham sign is named after Carl Nicoladoni, who first noticed the phenomenon of the pulse slowing in a patient with right arm phlebarteriectasia when the brachialis artery proximal to it was compressed. In modern medicine, the sign is elicited when pressure is applied to an artery proximal to an arteriovenous fistula and said to be positive if the following occurs:
Low pressure baroreceptors or low pressure receptors are baroreceptors that relay information derived from blood pressure within the autonomic nervous system. They are stimulated by stretching of the vessel wall. They are located in large systemic veins and in the walls of the atria of the heart, and pulmonary vasculature. Low pressure baroreceptors are also referred to as volume receptors,cardiopulmonary baroreceptors, and veno-atrial stretch receptors
Cardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; myocardium structure; the electrical conduction system of the heart; the cardiac cycle and cardiac output and how these interact and depend on one another.
Heart development, also known as cardiogenesis, refers to the prenatal development of the heart. This begins with the formation of two endocardial tubes which merge to form the tubular heart, also called the primitive heart tube. The heart is the first functional organ in vertebrate embryos.