Heart and Lungs : The heart and lungs are the primary contents of the thorax. They are interconnected with very large blood vessels. The heart sends oxygen-poor blood through the pulmonary arteries to the lungs, which oxygenate it and return it to the heart through the pulmonary veins. The pulmonary arteries arise from one large pulmonary trunk, and then begin branching exponentially once they enter the lungs in order to reach the functional respiratory units and pick up oxygen. The smallest pulmonary veins then take the oxygenated blood backwards through the lungs and empty into the back of the heart through four larger pulmonary veins. The oxygen-rich blood is then pumped by the heart out into the body through the aorta. Deoxygenated blood from body tissues returns to the heart through the superior and inferior vena cava and the cardiac cycle repeats continuously. The pulmonary veins and arteries are the only case where arteries carry deoxygenated blood and veins carry blood that has been oxygenated.
The respiratory system (called also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for the process of respiration in an organism. The respiratory system is involved in the intake and exchange of oxygen and carbon dioxide between an organism and the environment.
In air-breathing vertebrates like human beings, respiration takes place in the respiratory organs called lungs. The passage of air into the lungs to supply the body with oxygen is known as inhalation, and the passage of air out of the lungs to expel carbon dioxide is known as exhalation; this process is collectively called breathing or ventilation. In humans and other mammals, the anatomical features of the respiratory system include trachea, bronchi, bronchioles, lungs, and diaphragm. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveoli air sacs in the lungs.
In fish and many invertebrates, respiration takes place through the gills. Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.
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