The right atrium and left atrium are separated by a wall of tissue called the atrial septum. The two lower chambers are called the ventricles, and are separated into the right and left ventricle by the ventricular septum.
The ventricles have thicker walls and pump blood to the lungs and body. The bundle branches are a part of the electrical system of the heart. The electrical system controls the heartbeat and…. The bundle of His is a part of the electrical system of the heart.
The atria, which have relatively thin walls, fill first, before squeezing the blood into the much stronger ventricles, which then contract to send blood coursing through our arteries.
Most reptiles have two atria and one ventricle. The only exceptions are the 23 living species of crocodilians alligators, caimans, crocodiles and gharials who, like birds and mammals, have four-chambered hearts with two atria and two ventricles Jones, ; Jensen et al.
In vertebrates, each heartbeat is initiated when a pacemaker region in one of the atria generates an electrical signal. The structure and exact location of the pacemaker region differ amongst species Jensen et al. This allows the body to increase or decrease the heart rate in response to metabolic demands Wang, The electrical signal from the pacemaker region spreads rapidly across the cardiac muscle cells of the atria via structures called gap junctions, and this ensures that the entire wall of each atrium contracts almost simultaneously.
Neurons called Purkinje fibers are also involved in this process in birds but, in general, the mechanisms responsible for the contraction of the atria are similar in most vertebrates.
However, the way in which the electrical signal travels from the atrium to the ventricle differs amongst vertebrates, and the evolution of this pathway has been the focus of considerable attention for many decades Davies, ; Jensen et al.
Now, in eLife, Vincent Christoffels of the University of Amsterdam and colleagues — including Bjarke Jensen and Bastiaan Boukens as joint first authors — report new and surprising findings about this phenomenon in alligators Jensen et al. Back in the 17th century, William Harvey had already noticed that the atria contract before the ventricles in a number of different animals.
This meant that the electrical signal generated in the pacemaker region must somehow be slowed down at the 'border' between the atria and the ventricles. In both mammals and birds, a layer of fibrous fatty tissue — which does not conduct electricity — insulates the ventricles from the atria. The only way that the electrical signal can pass from the atria to the ventricles is via a small structure called the atrioventricular node, which is positioned immediately above the septum that separates the left and right ventricles.
When the electrical signal reaches this node, it activates two bundles of neurons containing His fibers and Purkinje fibers that swiftly relay the impulse and cause the ventricles to contract simultaneously. However, in extant reptiles, the common ancestor of both birds and mammals, there does not seem to be an insulating layer or an anatomically defined node Davies, Instead, the electrical signal is slowed down by an intricate arrangement of myocardial fibers at the junction between the two atria and the ventricle.
In addition, recent studies have been unable to provide any anatomical evidence for a conduction system in the ventricle of reptiles. The electrical signal appears to be conveyed by the internal lining of the heart, which shares molecular signatures with the conduction system found in birds and mammals Jensen et al. While reptiles rely on their environment to maintain their temperature that is, they are ectothermic , mammals produce their own heat so they are endothermic.
The high levels of metabolism needed to produce enough warmth means that the resting and maximal metabolic rates of mammals and birds are about 10 times higher than those of ectothermic animals Bennett and Ruben, The cardiovascular system must keep up with these greater needs by delivering more oxygen to the body. The four-chambered heart provides an efficient solution by keeping oxygenated and non-oxygenated blood separate.
Humans, and indeed all mammals not to mention birds! Warm bloodedness requires a great deal of oxygen, for the oxygen is used to generate both ATP and heat. A four-chambered heart is an enormous evolutionary advantage over a three-chambered heart.
To understand this, you need to look at the chambers and the circuits together. Some babies are born with a ventricular septal defect , which means an opening between the left and right ventricles, which means that their hearts are acting like three-chambered hearts.
Surgery to correct the defect is necessary in order for the child to live a normal life. Remember the fish, with an atrium to receive blood from the body, and a ventricle to pump it out again? Well, with a three-chambered heart there are two ventricles and one atrium. The two atria emphasize a higher degree of separation between two of the circuits: the pulmonary circuit and the systemic circuit.
At this point you need to start thinking of the heart in terms of left and right. The right atrium receives deoxygenated blood low in O 2 , and high in CO 2 from the systemic circuit, and the left atrium receives oxygenated blood high in O 2 , and low in CO 2 from the pulmonary circuit. Don't forget that left and right in all these discussions always means the patient's left and right, which means you need to pay attention to whether any diagrams are in anterior or posterior view! This advance was only so good, however, because both atria pump the blood to the single ventricle.
In a three-chambered heart the blood pumped out of the ventricle is a mixture of both oxygenated and deoxygenated blood. This blood is pumped out to both the pulmonary and the systemic circuit in truth, because it is pumped right back to the tissues of the heart, it really goes to all three circuits.
For ectothermic cold blooded animals that is plenty of oxygen, but it's just not enough for you. Birds and mammals evolved a ventricular septum, turning one ventricle into two. The result is the evolution of entirely separate pulmonary and systemic circuits see Figure The blood sent to the lungs is completely deoxygenated, and the blood pumped out to the rest of the body is fully oxygenated.
The evolution of two ventricles, making a four-chambered heart, doubled the amount of O 2 being sent to the tissues. The amount of food and waste in the blood going to the systemic circuit is not so cut and dried see Cardiovascular and Lymphatic Circulation. In the human heart the right atrium sends deoxygenated blood from the body to the right ventricle, which then pumps it to the lungs pulmonary circuit. The left atrium sends oxygenated blood from the lungs to the left ventricle, which then pumps it to the body systemic circuit.
Figure When you look at the orientation of the heart at the bottom of the thoracic cavity see The Respitory System to learn about the pericardium you will see that, rather than being straight up and down, the heart is at an angle, and a bit twisted kind of like me! This is due in part to making room for the liver, and in part to the location of the many blood vessels that attach to the heart. Don't forget that the blood flow in the pulmonary and systemic circuits is continuous, meaning that blood from one circuit moves on immediately to the other circuit.
Next, the central location of the heart means that blood going to the lungs needs to be pumped both left and right, and blood going to the body needs to be pumped both up and down.
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