The Cardiopulmonary System

The cardiovascular system is a body system in which blood flows through the body, with the help of the heart, to transport nutrients, waste, and gases. The organs of the cardiovascular system include the heart, blood vessels, and blood. One function of the cardiovascular system is to transport materials to and from cells throughout the body. These materials include nutrients, oxygen, and carbon dioxide. A second function is to help defend against the invasion of a pathogen, a disease-causing organism or virus that causes harm to a host organism. This defense is mounted by white blood cells, found in the bloodstream, that travel to an area of injury or infection.

The heart receives blood from the body and pumps it to the lungs, where gas exchange occurs. The heart then receives blood from the lungs and pumps it to the rest of the body.

The blood vessels form a closed system that helps deliver blood to and from the heart. There are three major types of blood vessels: arteries, veins, and capillaries. Blood vessel type is distinguished by the direction of blood flow, not the contents (i.e., amount of oxygen) carried in the blood vessel. Each artery transports blood away from the heart to the lungs or to the body tissues under high pressure. In the organs, arteries branch into smaller blood vessels. An arteriole is a small blood vessel connecting an artery with capillaries.

A capillary is a tiny, thin-walled blood vessel that is the site of nutrient and gas exchange. Capillaries form a network of capillary beds that surround tissues and saturate them with nutrients and dissolved gases in blood. Capillaries converge to form venules, and these venules branch into veins. A vein is a blood vessel that returns blood to the heart. Heart structure varies in animal groups. Regardless of the specific structure, in the vertebrate heart, the atrium (plural, atria) is a chamber that receives blood from the veins. The ventricle in the vertebrate heart is a chamber that pumps blood away from the heart. Fish have a single atrium and ventricle and only a single circuit. Amphibians have two atria and one ventricle, but they have both pulmonary and systemic circuits. Pulmonary circuits carry blood away from the heart to the lungs to be oxygenated, and then back to the heart. Systemic circuits carry oxygenated blood away from the heart to the rest of the body. Most reptiles have two atria and one ventricle, but the ventricle has a partial division that increases the efficiency of the two circuits. In crocodilian reptiles, the ventricle division is complete. Both birds and mammals have four-chambered hearts (two atria and two ventricles) and both pulmonary and systemic circuits.

Mammalian blood circulation has two main circuits: the pulmonary circuit between the heart and the lungs and the systemic circuit between the heart and the rest of the body. In the pulmonary circuit, blood with low oxygen levels but higher carbon dioxide levels travels from the heart to the lungs, where gas exchange occurs. Carbon dioxide is removed, and oxygen dissolves into the blood. The newly oxygenated blood travels back to the heart. The systemic circuit pumps blood with high oxygen levels to the tissues of the body. Details of these steps may vary from one organism to the next, but the general process is the same in organisms that have a circulatory system.

In mammals, low-oxygen blood returning from the body collects in the venae cavae, the largest veins in the body that carry deoxygenated blood, and enters the right atrium of the heart. A contraction of the heart pushes the blood into the lower chamber, the right ventricle. The next contraction pushes the blood into the pulmonary arteries that connect to the two lobes of the lungs. The pulmonary artery branches into capillaries wherein oxygen and carbon dioxide are exchanged with the lung tissue. The newly oxygenated blood travels back to the heart through the pulmonary veins.

Blood from pulmonary veins re-enters the heart in the left atrium. A contraction of the heart pushes the blood into the left ventricle. The next contraction pushes the blood out of the heart through the aorta. The aorta branches into many arteries that transport the blood to the rest of the body. The arteries branch further and reduce in diameter to form arterioles, then further to form capillaries. Within the tissues and organs, capillaries are the site of gas exchange, with oxygen entering the cells and carbon dioxide entering the blood. Blood with low oxygen levels returns through veins to the venae cavae and back to the right atrium of the heart.

Other animals have variations on the blood circulation seen in mammals. For instance, fish have a single circuit that pumps blood from the capillaries in the gills to those capillaries found within the body. Gases exchange in this systemic circuit, and then carbon dioxide is transported back to the gills, where it is expelled.

Amphibians, such as frogs, have a two-circuit circulatory system, but lack a distinction in the divisions of the heart. One circuit pumps blood from the heart to the lungs and back to the heart, while the other pumps blood from the heart to the capillaries of the body and back to the heart.

The purpose of the respiratory system is to help with gas exchange; oxygen is taken in and carbon dioxide is removed from the body. Respiratory structures in organisms vary, but all structures have similar functions. In order to maximize gas exchange, respiratory structures tend to have a large surface area, facilitated by blood flowing close to these surfaces.

The trachea (plural, tracheae) is a respiratory structure found in both invertebrates and vertebrates. In vertebrates, the trachea leads to the lungs, which are specialized organs for gas exchange. In invertebrates, the tracheae make up a system of tubes that delivers oxygen to cells.

Gills are filamentous structures on either side of the throat of a fish. Fish bring water into their mouths and draw the water over the feathery gills, which contain capillaries. Blood flows through the capillaries in the opposite direction to the flow of water, to maximize oxygen uptake. After gas exchange, low oxygen water is expelled from the fish.

Lungs are localized organs found in tetrapods that are not found all throughout the body like the tracheal system of arthropods (i.e., insects). They allow gas exchange to take place. Lungs are found in the chest cavity, enclosed by the ribs and diaphragm.

Breathing in mammals is the process of moving air between the environment and the lungs. Inhaling causes the contraction of the diaphragm, a muscle beneath the lungs. The movement of the diaphragm is downward toward the abdomen, increasing the volume of space in the chest and lungs and decreasing the pressure within the lungs. Air moves into the lungs to equalize the pressure. During exhalation, the diaphragm relaxes and moves upward toward the lungs. Relaxing the diaphragm decreases the volume in the chest cavity and increases the pressure in the lungs. Air moves out of the lungs to equalize the pressure.

Typically air enters through the nostrils, where it is moistened, warmed, and filtered by structures called cilia. Each cilium (plural, cilia) is a small, hair-like projection from cells that functions to filter air in the respiratory system. Air also encounters mucous membranes lining the nasal passage. These membranes aid in air filtration by trapping particles in a sticky layer of mucus. The movement of cilia on the cells lining the airway pushes the mucus, with its trapped particles, toward the throat. Once in the throat, mucus is expelled by mechanical means such as coughing or enters the digestive system for elimination.

Following entry through the nose, air travels through the pharynx (or throat) before reaching the larynx, or the voice box. Sounds are made in the larynx by air passing over vocal folds (or cords). After air reaches the larynx, it travels past the epiglottis. The epiglottis is a flap of connective tissue that covers the esophagus and helps ensure only air, not food particles, travels to the trachea. From the trachea, air flows into the bronchial system. The trachea branches into two bronchi, each leading to a separate lung. The bronchi branch further into smaller tubes called bronchioles. At the end of each bronchiole is a cluster of alveoli. An alveolus (plural, alveoli) is a small air sac in the lungs where gas exchange occurs in mammals.

Alveoli have a single layer of epithelial cells on their surface and are surrounded by a web of capillaries that also have a single layer of epithelial cells on their surface. The air in the alveoli has a higher oxygen concentration than the blood that arrived in the lungs from the heart. Oxygen diffuses from each alveolus into the blood through the thin layer of cells surrounding the alveolus. Similarly, the blood has a higher concentration of waste carbon dioxide transported from the cells of the body. Carbon dioxide diffuses from the blood into the alveoli. The oxygen then travels in the blood back to the heart, where it is pumped to the rest of the body. The carbon dioxide is expelled from the lungs into the environment during exhalation.

Not all organisms accomplish gas exchange in the same manner as mammals. For example, fish have specific structures, the gills, that remove oxygen from water. Other organisms have different structures or processes to exchange gases based on their body construction or environment.