The intercostal muscles are a group of muscles located between the ribs, playing a significant role in the mechanics of breathing. These muscles contribute to the continuous process of respiration, which is fundamental to sustaining life. They work in conjunction with other respiratory muscles to facilitate the intake of oxygen and the expulsion of carbon dioxide.
Location and Basic Function
The intercostal muscles are situated in the spaces between adjacent ribs, known as the intercostal spaces. Their primary function involves assisting in the movement of the rib cage during respiration. This movement is crucial for changing the volume of the thoracic cavity, which in turn drives air into and out of the lungs.
They are organized into several layers, each contributing uniquely to the breathing process. These muscles are instrumental in both quiet, everyday breathing and more strenuous, forced breathing.
Specific Roles of Each Type
The intercostal muscles are categorized into three main groups: the external intercostals, the internal intercostals, and the innermost intercostals. The external intercostals are positioned superficially, with their fibers running downwards and forwards from the rib above to the rib below. These muscles primarily contract during inhalation, pulling the ribs upwards and outwards, which increases the volume of the chest cavity.
Conversely, the internal intercostals lie beneath the external layer, with their fibers oriented downwards and backwards. These muscles become active during forced exhalation, contracting to pull the ribs downwards and inwards. This action helps to decrease the volume of the thoracic cavity, expelling air from the lungs. The innermost intercostals are the deepest layer, sharing a similar fiber orientation and function with the internal intercostals. They also contribute to forced exhalation and help stabilize the rib cage during respiratory movements.
Intercostal Muscles and the Mechanics of Breathing
The coordinated action of the intercostal muscles, along with the diaphragm, orchestrates the complete process of breathing. During inhalation, the diaphragm contracts and flattens, while the external intercostal muscles contract, elevating the ribs. This combined action expands the thoracic cavity, increasing its internal volume. As the volume of the thoracic cavity increases, the pressure within the lungs decreases below atmospheric pressure, causing air to flow into the lungs.
For quiet exhalation, the external intercostals and diaphragm simply relax. This relaxation allows the rib cage to move downwards and inwards due to gravity and elastic recoil, decreasing the volume of the thoracic cavity. The reduction in volume increases the pressure within the lungs above atmospheric pressure, passively pushing air out. During forced exhalation, such as during exercise or coughing, the internal and innermost intercostal muscles actively contract, forcefully pulling the ribs down and in. This rapid decrease in thoracic volume creates a greater pressure gradient, expelling a larger volume of air more quickly.