In human anatomy, the term “diaphragm” broadly refers to a thin, musculofibrous partition that separates different body cavities or structures. The word originates from the Greek “diaphragma,” meaning “partition” or “barrier,” aptly describing its role in the body. Anatomists use this term for several distinct structures that fulfill this separating function. While many anatomical divisions exist, only a few are formally designated as diaphragms due to their specific structural and functional characteristics.
The Respiratory Diaphragm
The respiratory diaphragm, often simply called “the diaphragm,” is a sheet of internal skeletal muscle positioned at the base of the chest, forming a separation between the thoracic cavity and the abdominal cavity. This dome-shaped muscle is the primary muscle involved in breathing. The diaphragm is slightly asymmetrical, with its right side typically sitting higher due to the presence of the liver beneath it.
During inhalation, the diaphragm contracts and flattens, moving downwards towards the abdomen. This action increases the vertical volume of the thoracic cavity, which reduces the internal pressure within the chest and draws air into the lungs. Conversely, during exhalation, the diaphragm relaxes and returns to its dome shape, decreasing the thoracic cavity volume and expelling air from the lungs. This continuous contraction and relaxation cycle is fundamental to respiration. The diaphragm receives its motor and much of its sensory innervation from the phrenic nerves, which originate from the cervical spinal nerves C3, C4, and C5.
The Pelvic Diaphragm
The pelvic diaphragm, also known as the pelvic floor, forms the muscular base of the pelvic cavity. This structure separates the pelvic cavity above from the perineal region below, acting as a crucial support system for pelvic organs. It is primarily composed of the levator ani muscle group, which includes the puborectalis, pubococcygeus, and iliococcygeus muscles, along with the coccygeus muscle and associated connective tissues.
The pelvic diaphragm plays a significant role in maintaining continence for both urinary and fecal functions. It supports the bladder, intestines, and uterus in females, preventing their descent. The muscles of the pelvic floor work in conjunction with other core muscles to maintain intra-abdominal pressure and provide stability. During activities like coughing or sneezing, the pelvic diaphragm helps manage pressure changes within the abdomen.
The Urogenital Diaphragm
The urogenital diaphragm is a distinct musculofascial structure located in the anterior part of the pelvic outlet, inferior to the broader pelvic diaphragm. It occupies the area between the pubic symphysis and the ischial tuberosities. This structure is primarily composed of the deep transverse perineal muscle and the external urethral sphincter. Some sources also include other muscles like the bulbospongiosus and ischiocavernosus as part of its composition.
The urogenital diaphragm’s primary function involves supporting the urethra and, in females, the vagina, as these structures pass through it. It is particularly important for the voluntary control of urination, as the external urethral sphincter, a key component, surrounds the urethra. Contraction of these muscles assists in the propulsion of urine at the end of urination and semen during ejaculation in males.
Beyond the Major Three: Other Diaphragmatic Structures
While the respiratory, pelvic, and urogenital diaphragms are the most commonly recognized, other anatomical structures also bear the “diaphragm” designation due to their partition-like functions. One such structure is the diaphragma sellae, a small, circular fold of dura mater. This structure forms an incomplete roof over the sella turcica, a saddle-shaped depression in the sphenoid bone at the base of the skull.
The diaphragma sellae covers and protects the pituitary gland, which is located within the sella turcica. It features a central opening that allows the pituitary stalk, or infundibulum, to pass through, connecting the pituitary gland to the hypothalamus. The size of the opening in the diaphragma sellae can vary among individuals.