In general, arterial smooth muscle reacts to carbon dioxide by creating vasodilation and responds to oxygen by creating vasoconstriction. The pulmonary blood vessels in the lungs are unique in that they have too high an oxygen voltage and are vasoconstrictionally narrowed as they fall. Bronchioles, smooth muscles that line the airways of the lungs, respond to high levels of carbon dioxide, which produces vasodilation, and vasocons constriction when carbon dioxide is low. These reactions to carbon dioxide and oxygen through the pulmonary blood vessels and smooth muscles of the bronchiolenatem tract help coordinate perfusion and ventilation in the lungs. Other different smooth muscle tissues exhibit extremes of abundant to weak sarcoplasmic reticulum, so the irritation-contraction coupling varies with its dependence on intracellular or extracellular calcium. [Citation needed] Smooth muscles with one unit are located in the walls of hollow organs; Smooth muscles with several units are located in the airways of the lungs and large arteries. Smooth muscle cells in a single piece contract synchronously, are coupled by gap junctions and have a potential for spontaneous action. Smooth multiunit cells lack lacunar junctions and their contractions are not synchronized. Multi-unit smooth muscles, the second type of smooth muscle observed, are made up of cells that rarely have gap junctions and are therefore not electrically coupled.
As a result, the contraction does not spread from one cell to another, but is limited to the cell that was originally stimulated. This type of smooth muscle is observed in the large airways to the lungs, in the large arteries, the muscles of the arrector pili connected to the hair follicles and the internal eye muscles that regulate the entry of light and the shape of the lens. Smooth muscles (so called because cells have no streaks) are present in the walls of hollow organs such as the bladder, uterus, stomach, intestines and in the walls of passages, such as arteries and veins of the circulatory system, and the airways of the respiratory, urinary and reproductive systems ((figure)). Smooth muscles are also present in the eyes, where it works to change the size of the iris and change the shape of the lens; and in the skin, where it causes the hair to stand in response to cold or anxiety. At the cellular level, smooth muscle acts as an involuntary, untrired muscle. Smooth muscles contain thick, thin filaments that do not compete with sarcomeres, resulting in an unspoken pattern. On microscopic examination, it appears homogeneous. Smooth muscle cytoplasm contains large amounts of actin and myosin. Actin and myosin act as the main proteins involved in muscle contraction. Actin filaments attach to dense bodies distributed throughout the cell. Dense bodies can be observed under an electron microscope and appear dark. Another important structure is the calcium-containing sarcoplasmic reticulum, which helps maintain contraction.
The shape of smooth muscles is fusiform, which is round in the middle and narrows at each end. Smooth muscles can tense and relax, but have greater elastic properties than striped muscles. This quality is important in organ systems such as the bladder, where the preservation of contractile tone is a necessity. The main function of smooth muscles is contraction. Smooth muscle consists of two types: one part and several parts. Smooth muscle with one unit consists of several cells connected by connexins that can be stimulated in a synchronous pattern of a single synaptic input. Connexins allow cell-to-cell communication between groups of smooth muscle cells with a single unit. This intercellular communication allows ions and molecules to diffuse between cells, creating calcium waves. This unique property of smooth muscle with one unit allows synchronous contraction.  Multi-unit smooth muscle differs from smooth muscle cell in that each smooth muscle cell receives its own synaptic input, so multi-unit smooth muscle has much finer control.
Since most smooth muscles have to function for a long time without a break, their power output is relatively low, but contractions can continue without consuming large amounts of energy. Some smooth muscles can also maintain contractions even when Ca++ is removed and myosinkinase is inactivated/dephosphorylated. This can be done as a subset of cross bridges between myosin heads and actin, called locking bridges, which keep the thick and thin filaments connected over a longer period of time without the need for ATP. This allows the maintenance of muscle tone in the smooth muscle, which lines the arterioles and other visceral organs with very low energy consumption. Recent research suggests that sphingosine-1-phosphate (S1P) signaling is an important regulator of smooth vascular muscle contraction. As transmural pressure increases, sphingosine kinase 1 phosphorylates sphingosine to S1P, which binds to the S1P2 receptor in the plasma membrane of cells. This leads to a temporary increase in intracellular calcium and activates the Rac and Rhoa signaling pathways. Together, they serve to increase MLCK activity and decrease MLCP activity, thus promoting muscle contraction. This allows the arterioles to increase resistance in response to increased blood pressure, thus maintaining constant blood flow. The Rhoa and Rac part of the signaling pathway provides a calcium-independent way to regulate the tone of resistance arteries.  Smooth muscle is found throughout the body around various organs and tracts.
Smooth muscle cells have a single nucleus and are spindle-shaped. Smooth muscle cells can suffer from hyperplasia, which mitotically divides to produce new cells. Smooth cells are not scratched, but their sarcoplasm is filled with actin and myosin, as well as dense bodies in the sarcolemma to anchor the thin filaments, and a network of intermediate filaments involved in firing the sarcolemma to the center of the fiber, shortening it in the process. Ca++ ions trigger contraction when released by SR and penetrate through open, voltage-controlled calcium channels. Smooth muscle contraction is initiated when Ca++ binds to intracellular calmodulin, which then activates an enzyme called myosin kinase, which phosphorylates the myosin heads so that they can form the transverse bridges with actin and then pull on the thin filaments. Smooth muscles can be stimulated by pacemaker cells, by the autonomic nervous system, by hormones, spontaneously or by stretching. The fibers of some smooth muscles have locking bridges, transverse bridges that move slowly without ATP; These muscles can maintain weak contractions for a long time. Monobloc smooth muscle tissue contains lacunar junctions to synchronize depolarization and contractions of the membrane so that the muscle contracts as a single unit. The one-piece smooth muscles in the walls of the intestines, called visceral muscles, have a stress-relaxation response that allows the muscle to stretch, contract, and relax as the organ expands. Smooth muscle cells in several parts do not have gaps, and the contraction does not spread from one cell to another. .