This is an excerpt from Kinetic Anatomy 4th Edition With HKPropel Access by Robert S. Behnke,Jennifer L. Plant & Jennifer L. Plant.
The two major organs housed within the thorax are the heart and lungs. As with all other anatomical structures, the heart and lungs need nerves and blood vessels to accomplish their functions.
The nerves of the heart are cardiac branches of the vagus nerve fibers arising from trunks of the sympathetic nervous system. When stimulated, the sinoatrial node (SA node) of the heart, found in the area of the right atrium near the superior vena cava, sends the impulse to the right and left atria myocardium. Specialized tissue (myocardial cells) of the atrioventricular bundle (AV node; located in the lower aspect of the interatrial septum) receives the impulse after it has passed through the atrium (figure 10.16). The impulse continues through the bundle of His (atrioventricular bundle), which divides into left and right branches (becoming known as myofibers of conduction, or Purkinje fibers) that enter the muscular walls (myocardium) of the ventricles and the papillary muscles. The result is atrial contraction rapidly followed by ventricular contraction.
The respiratory center is a group of cell bodies located on each side of the medulla oblongata. Arising from these centers are two sets of nerves (figure 10.17): (1) the phrenic nerves, arising from the cervical plexus and leading to the diaphragm, and (2) the intercostal nerves, which innervate the intercostal muscles. When the respiratory center is stimulated by carbon dioxide, it sends an impulse over the phrenic nerves, causing the diaphragm to contract (pull downward) and increasing space in the thoracic cavity. At the same time, the intercostal nerves cause the intercostal muscles to contract, lifting the ribs and increasing the space within the thoracic cavity. This change in capacity of the thoracic cavity creates a change in atmospheric pressure, causing air to rush in and distend the lungs (inspiration). When the lungs have been distended to a certain point, sensory nerves running from the air sacs, via the vagus nerve, send an impulse to the respiratory center to inhibit it. This stops the center’s impulses to the phrenic and intercostal nerves, causing the diaphragm and intercostal muscles to relax and resulting in a reduction in the size of the thoracic cavity, forcing air out of the lungs (expiration).
Two of the largest arteries of the heart are the pulmonary artery, coming from the right ventricle, and the aorta, coming from the left ventricle (figure 10.18). These arteries have valves at their ventricular ends to prevent any backflow of blood. The pulmonary artery has a three-flap valve known as the pulmonary semilunar valve, and the aorta has a similar valve known as the aortic semilunar valve.
The right and left coronary arteries come from the aorta and are located on the outer surface of the heart, supplying blood flow to the muscular walls of the heart (myocardium). The branches of the right coronary artery include the posterior (dorsal) interventricular and marginal arteries, which supply the anterior surface of the right ventricle; the aortic and pulmonary branches, which supply the aorta and pulmonary arteries; the interventricular, which supplies both ventricles; the right atrial, which supplies the right atrium surface; and the right marginal, which supplies the inferior surfaces of both ventricles. The branches of the left coronary artery include the aortic and pulmonary branches, which supply the aorta and pulmonary arteries; the circumflex, which supplies the left atrium and ventricles; the anterior (ventral) interventricular, which supplies both ventricles; and the left atrial, which supplies the left atrium. While not the only area of concern regarding the heart, it should be noted that the right and left coronary arteries (and their branches) often are the sites of blockage that diminish or cease blood flow to areas of the heart tissue (muscle) and thus cause a “heart attack.” Frequently, one of the major causes for this condition is the accumulation of low-density lipoprotein (LDL) cholesterol within these vessels, which reduces or stops blood flow to the heart muscle. Poor diet, lack of exercise, overweight, and smoking are some of the habits that can contribute to problems of the heart.
Blood from the body drains to the heart via two major veins: The superior vena cava (and its tributaries) drains the upper extremities, head, neck, shoulders, thorax, and a portion of the abdominal wall into the heart’s right atrium. The inferior vena cava (and its tributaries) drains the lower extremities, pelvis, abdominal viscera, and a portion of the abdominal wall into the right atrium. The left atrium contains the opening for the pulmonary veins, which bring the blood from the lungs to the heart.
The right and left coronary veins drain into the coronary sinus, which empties into the right atrium. Tributaries of the coronary sinus include the great cardiac vein, which drains the left atrium and both ventricles into the coronary sinus. The great cardiac vein also has a tributary: the left margin vein, which drains the left margin of the heart. Other coronary sinus tributaries include the inferior cardiac vein of the left ventricle, which drains the inferior surface of the left ventricle; the middle cardiac vein, which drains both ventricles and empties into the coronary sinus; the oblique vein of the left atrium, which drains the left atrium into the coronary sinus; and the small cardiac vein, which drains the right atrium and right ventricle into the coronary sinus (figure 10.18).
Respiratory Arteries and Veins
Blood flow to and from the lung tissues is accomplished through branches of the bronchial arteries and bronchial veins. More in-depth discussions of oxygen and carbon dioxide levels and changes in atmospheric pressures are found in coursework and texts in human physiology.
Arterial blood supplies the body cells with oxygen and is therefore well oxygenated. Venous blood is less oxygenated as it returns to the lungs. The blood in the pulmonary veins returning from the lungs to the heart is highly oxygenated, however (figure 10.19). In other words, the pulmonary veins are the only veins in the body that carry oxygen-rich blood.
A few veins of the heart do not drain into the coronary sinus. These veins include the anterior cardiac veins, which arise from the wall of the right ventricle and empty into the right atrium, and the venae cordis minimae, which are small veins in the heart walls that drain into the atria.
In the capillaries of the pulmonary vessels in the walls of the air sacs in the lungs is where the respiratory exchange of oxygen and carbon dioxide takes place (figure 10.20).