The capillaries are very small; in fact, capillaries are the smallest of all blood vessels. They form the connection between veins and arterioles in the circulatory system. However, capillaries tend to be found everywhere. Unlike veins and arteries, the main function of capillaries is not to transport blood. They allow the movement of substances, mainly gases, oxygen and carbon dioxide, in and out of the capillary. Capillaries have very thin walls, only one cell thick, which allow substances (such as oxygen) to diffuse through the wall effortlessly. They're also incredibly narrow; so narrow that blood cells pass through it one at a time. When arteries divide into arterioles and continue to decrease in size as they approach the muscles, they become capillaries. Here, the capillaries form a network structure (capillary bed), forming a network throughout the muscle. This allows for rapid and efficient transfer of oxygen-carrying red blood cells to the site where they are needed. Thanks to the combined structure of thin walls and a large surface area, capillaries allow the diffusion of oxygen and carbon dioxide with ease. This is ideal for the respiratory system which is responsible for oxidizing the blood. Gas exchange occurs in the lungs to reoxidize the blood. Air travels through the airways to reach the lungs and back up to be exhaled into the environment. The alveoli are located at the terminal point of the respiratory tract. The alveoli have a sac-like structure. In biological systems, the structure and functions of components are interrelated. Alveoli have a specialized structure for efficient gas exchange. In the structure of the alveoli (alveolus), it seems to have the shape of a hollow cavity which is coupled with elastic fibers...... in the center of the paper ...... absorption. Microvilli contain various protein channels and plasma membranes that contain pumps that use ATP. They enable the effective movement of nutrient subunits through diffusion and active transport. These subunits include monosaccharides, nucleic acids, glycerol, amino acids, and fatty acids. To reach the circulatory system, nutrient subunits pass through the wall of the small intestine to enter a dense network of capillaries found within the villi. The capillary will allow the transport of nutritional subunits to reach the body tissues; this movement of nutrients into the blood is known as absorption. The villi increase the surface area of ​​the small intestine over which food can be absorbed. Waste products from this process, such as fiber, are then pushed into the colon, where they wait to be expelled via a bowel movement..