Summary
Highlights
From the right atrium, deoxygenated blood flows to the right ventricle and then to the lungs via the pulmonary artery. After re-oxygenation in the alveoli, it returns to the heart through the pulmonary vein, entering the left atrium and then the left ventricle, ready to be pumped to the body again.
All members of the animal kingdom require oxygen for energy through cellular respiration, which produces carbon dioxide as a byproduct. The respiratory and circulatory systems work together to bring in oxygen and expel CO2, ensuring the body's proper functioning.
Not all animals use lungs for oxygen intake. Some arthropods use simple diffusion through pores, while amphibians can breathe through their skin (though they also have lungs or gills). Mammals and birds, being warm-blooded and larger, require more efficient respiration with lungs to meet their high metabolic demands and maintain body temperature. Fish use gills to extract dissolved oxygen from water.
Lungs are crucial for more complex animals, providing a large surface area for oxygen absorption through moist membranes. Air enters through the nose/mouth, passes the larynx, trachea, bronchi, and bronchioles, eventually reaching tiny sacs called alveoli. These alveoli, covered in capillaries, facilitate the exchange of oxygen into the blood and carbon dioxide out of the blood through simple diffusion. The CO2 is then exhaled.
Lungs themselves do not have muscles for contraction and expansion. Instead, the thoracic diaphragm, a large flat muscle beneath the lungs, drives the breathing process. When the diaphragm contracts and flattens, lung volume increases, causing pressure to drop and air to rush in. When it relaxes, pressure increases, and air is expelled.
The circulatory system, powered by the heart, moves blood and works in conjunction with the respiratory system. It follows a figure-8 pattern: oxygenated blood is pumped from the heart to the body, then deoxygenated blood returns to the heart, which then pumps it to the lungs for re-oxygenation before it returns to the heart to restart the cycle.
The four-chambered heart, particularly the powerful left ventricle, maintains high pressure to pump oxygenated blood through the aorta and into muscular arteries, which branch into smaller arterioles and capillaries. In capillary beds, oxygen is delivered to cells, and CO2 is picked up. Deoxygenated blood returns to the heart through veins, which have thinner walls and valves to prevent backflow. The superior and inferior vena cava deliver blood to the right atrium, the point of lowest pressure.
Humans are endotherms (warm-blooded), maintaining a constant internal temperature, which requires a high metabolism and thus a constant supply of oxygen via our efficient four-chambered hearts and lungs. Ectotherms (cold-blooded animals) have slower metabolisms and less demanding circulatory systems. Fish have two-chambered hearts, while reptiles and amphibians have three-chambered hearts, leading to less efficient blood oxygenation compared to endotherms.