Heart sounds are generated by the turbulance created by blood flowing through the heart, specifically when the heart valves close. Normal heart sounds can be characterized by a "lub" "dub" that occurs with a heart beat. The first heart sound (lub) occurs when the atrioventricular (AV) valves close and the second heart sound (dub) is heard when the semilunar valves close. Heart conditions can be identified by variation in these heart sounds, such as a murmur. Click below to hear the bear heart sounds.
|This heart sound was obtained from a bear cub in March.||This heart sound was obtained from an adult male black bear in March.|
Electrocardiography is an interpretation of the electrical activity in the heart. Contractions of the atria are initiated near the end of ventricular diastole, which is initiated by depolarization of the atrial myocardial cells (sinoatrial node). Atrial depolarization is elicited at the P wave of the electrocardiogram (ECG lead II trace). The excitation and subsequent development of tension and shortening of atrial cells cause atrial pressures to rise. Active atrial contraction forces additional volumes of blood into the ventricles (often referred to as "atrial kick"). The atrial kick contributes a significant volume of blood toward ventricular preload (approximately 20%). At normal heart rates, the atrial contractions are considered essential for adequate ventricular filling. As heart rates increase, atrial filling becomes increasingly important for ventricular filling because the time interval between contractions for passive filling becomes progressively shorter. Atrial fibrillation and/or asynchronized atrial-ventricular contractions can result in minimal contribution to preload, via atrial contraction. Throughout diastole, atrial and ventricular pressures are nearly identical due to the open atrioventricular values which offer little or no resistance to blood flow. It should also be noted that contraction and movement of blood out of the atrial appendage (auricle) can be an additional source for increased blood volume.
Ventricular systole begins when the excitation passes from the right atrium through the atrioventricular node, and through the remainder of the conduction system (His bundle and left and right bundle branches) to cause ventricular myocardial activation. This depolarization of ventricular cells underlies the QRS complex within the ECG. As the ventricular cells contract, intraventricular pressures increase above those in the atria, and the atrioventricular valves abruptly close. Closure of the atrioventricular valves results in the first heart sound, S1. As pressures in the ventricles continue to rise together in a normally functioning heart, they eventually reach a critical threshold pressure at which the semilunar valves (pulmonary valve and aortic valve) open.
In the field, researchers can obtain ECG recordings from black bears. Click below to view.