Most people think of the diaphragm as one muscle, the dome-shaped sheet beneath your lungs that powers every breath you take. In reality, the human body contains several structures that anatomists and clinicians call diaphragms, from large muscular partitions to microscopic barriers inside blood vessels. The exact count depends on how broadly you define the term. A strict anatomical count recognizes at least four or five named macro-level diaphragms, while an osteopathic clinical framework formally identifies five, and if you include smaller sensory and cellular structures that carry the same name, the number climbs higher still.
The Thoracic Diaphragm, the One You Already Know
When someone says “the diaphragm,” they almost always mean the thoracic diaphragm. It is a double-domed, musculotendinous sheet that separates the chest cavity from the abdominal cavity, with a continuous ring of muscle fibers surrounding a flat central tendon.1PubMed. Anatomy, Abdomen and Pelvis: Diaphragm When those muscle fibers contract, the dome descends, pressure in the chest drops, and air rushes into the lungs. At the same time, pressure rises in the abdomen, pushing the belly wall outward.2PubMed. Mechanics of the respiratory muscles This pressure seesaw is the core engine of breathing.
But the thoracic diaphragm does more than ventilate the lungs. The cyclical pressure swings it creates in the abdomen act like a pump for blood flow, squeezing venous blood out of the abdominal organs and back toward the heart with every breath.3PubMed Central. The “Abdominal Circulatory Pump”: An Auxiliary Heart during Exercise? This circulatory role becomes especially important during exercise, when breathing rate and depth both increase. The diaphragm also contributes to core stabilization, to coughing, and to every expulsive effort the body makes, from vomiting to childbirth.4PubMed Central. Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals
Embryologically, this muscle has a surprisingly complex origin. It develops from three separate tissue sources: the septum transversum, which forms the initial barrier between chest and abdomen; the pleuroperitoneal folds; and somite-derived cells that supply the actual muscle fibers.5PubMed Central. Development of the diaphragm, a skeletal muscle essential for mammalian respiration This multi-source assembly helps explain why the thoracic diaphragm is both structurally unique and vulnerable to developmental defects, a point that matters clinically.
The Pelvic Diaphragm
At the bottom of the trunk sits the pelvic floor, a muscular sling that anatomists formally call the pelvic diaphragm. It mirrors the thoracic diaphragm in a fundamental way: where the thoracic diaphragm forms the roof of the abdominal cavity, the pelvic diaphragm forms its floor. Its two major jobs are supporting the weight of the abdominal and pelvic organs and controlling the openings for the urethra, rectum, and, in women, the vagina.6PubMed Central. Pelvic floor anatomy and applied physiology
The main muscle of the pelvic diaphragm is the levator ani, a funnel-shaped structure on each side of the lower pelvis composed primarily of striated muscle. It forms from three component muscles: the puborectalis, pubococcygeus, and iliococcygeus.7PubMed. Anatomy, Abdomen and Pelvis: Levator Ani Muscle Recent anatomical work in women has shown that some of these muscle bundles attach directly to the rectum in a twisted configuration, allowing the levator ani to lift the center of the perineum and narrow the opening through which organs could otherwise descend.8PubMed Central. Twisted orientation of the muscle bundles in the levator ani functional parts in women: Implications for pelvic floor support mechanism
Below the levator ani is the perineal membrane, a connective-tissue sheet spanning the urogenital triangle. In women, it has two distinct regions: a dorsal portion of pure fibrous tissue connecting the vaginal wall and perineal body to the bony pelvis, and a ventral portion embedded with sphincter muscles around the urethra.9PubMed Central. Structure of the perineal membrane in females: gross and microscopic anatomy Men have a comparable but architecturally different arrangement in the same region.10PubMed Central. Architecture of structures in the urogenital triangle of young adult males; comparison with females The perineal membrane is sometimes described as a secondary layer reinforcing the pelvic diaphragm above it.
How the Thoracic and Pelvic Diaphragms Work Together
These two large diaphragms do not operate in isolation. Dynamic MRI studies in healthy women have shown that the thoracic diaphragm and the pelvic floor move in parallel: both descend during inhalation and both rise during exhalation.11PubMed. Phase-locked parallel movement of diaphragm and pelvic floor during breathing and coughing-a dynamic MRI investigation in healthy females The same synchronized pattern holds during coughing, when both structures must manage sudden spikes in abdominal pressure.
The pelvic floor muscles physiologically act as expiratory muscles, contracting during exhalation in synergy with the abdominal wall muscles and relaxing during inhalation.12PubMed Central. Breathing, (S)Training and the Pelvic Floor-A Basic Concept During heavy physical exertion, this coordination intensifies. The pelvic floor and abdominal muscles co-contract to brace against high intra-abdominal pressure, protecting the pelvic organs from being forced downward. Physical medicine literature recognizes the pelvic floor as a primary expiratory muscle and a key player in generating the intra-abdominal pressure needed for everything from lifting weights to singing loudly.13PubMed. The Role of the Pelvic Floor in Respiration: A Multidisciplinary Literature Review
This coordinated system matters clinically. When the thoracic diaphragm and pelvic floor fall out of sync, people can develop problems ranging from incontinence to chronic low back pain. Rehabilitation approaches for pelvic floor dysfunction often include breathing retraining precisely because these two diaphragms are meant to work as a unit.
Diaphragms Inside the Skull
The brain has its own diaphragm-like partitions. These are folds of dura mater, the tough outer membrane that wraps the brain, and they serve as internal dividers within the cranial cavity.
The tentorium cerebelli is the largest of these. It is a semi-circular sheet of dura stretching across the posterior cranial fossa, separating the cerebellum below from the cerebral hemispheres above.14PubMed Central. Tentorium Cerebelli: Muscles, Ligaments, and Dura Mater, Part 1 Its job is partly structural: it prevents the weight of the brain’s upper portions from bearing down on the brainstem and cerebellum underneath.15PubMed Central. The Tentorium Cerebelli: A Comprehensive Review Including Its Anatomy, Embryology, and Surgical Techniques The tentorium also creates a clinically important boundary. When brain swelling or a mass lesion pushes tissue past the edge of the tentorium (a process called herniation), the consequences can be life-threatening because vital brainstem structures sit right below.
A smaller cranial diaphragm is the diaphragma sellae, a fold of dura forming a partial roof over the pituitary gland where it sits in its bony cradle at the base of the skull.16PubMed Central. A Morphometric Study of the Foramen of Diaphragma Sellae and Delineation of Its Relation to Optic Neural Pathways through Computer Aided Superimposition This little membrane has a central opening through which the pituitary stalk passes, connecting the gland to the brain above. Surgeons care about the diaphragma sellae because its position relative to the optic nerves affects surgical approaches to pituitary tumors and because its anatomy varies considerably from person to person.
The Thoracic Outlet and the Mouth Floor
An osteopathic clinical model formally counts five diaphragms in the body: the tentorium cerebelli, the tongue and floor of the mouth, the thoracic outlet, the thoracic diaphragm, and the pelvic floor.17PubMed Central. The Five Diaphragms in Osteopathic Manipulative Medicine: Myofascial Relationships, Part 1 The first, fourth, and fifth on that list have already been covered. The remaining two are worth understanding on their own terms.
The floor of the mouth is formed primarily by the mylohyoid muscle, which stretches between the two halves of the jawbone to create a muscular hammock supporting the base of the oral cavity.18PubMed. Anatomy, Head and Neck, Mylohyoid Muscle It acts as a true horizontal partition, separating the space under the tongue from the soft tissues of the neck below. In osteopathic thinking, tension or restriction in this “diaphragm” can influence swallowing, tongue movement, and the mechanics of structures in the neck. Even in conventional anatomy, the mylohyoid is recognized as defining a boundary: infections or fluid collections above it behave differently than those below it, and surgeons treat these as distinct compartments.
The thoracic outlet (also called the thoracic inlet, depending on whether you describe it from the perspective of the chest or the neck) is the ring-shaped opening at the top of the rib cage. It is formed by the first ribs, the top of the breastbone, and the first thoracic vertebra. The lung apex, which protrudes slightly above this ring, is covered by a reinforcing fascial sheet called the suprapleural membrane, or Sibson fascia, which stabilizes the top of the lung against being displaced upward into the neck.19PubMed Central. Lung hernia: an updated narrative review This fascial layer, along with the scalene muscles that attach around the same area, functions as a horizontal barrier regulating what passes between neck and chest. Nerves, blood vessels, and the esophagus all thread through this region, and compression of any of them produces the well-known clinical syndrome called thoracic outlet syndrome.
Sensory Diaphragms
The word “diaphragm” also applies to structures in the sense organs, where its meaning is closer to the original Greek: a partition or barrier.
The iris of the eye is an adjustable muscular diaphragm that controls how much light reaches the retina. Its central opening, the pupil, widens in dim light and narrows in bright light. When the iris is damaged or absent, people experience severe glare disability. Prosthetic iris diaphragm implants have been developed for patients with traumatic iris loss, congenital absence of the iris, and other conditions; in one series, about 96 percent of patients reported subjective improvement in glare after implantation.20PubMed Central. Numerical model characterization of the sound transmission mechanism in the tympanic membrane from a high-speed digital holographic experiment in transient regime
The tympanic membrane, better known as the eardrum, is another diaphragm in the literal sense. It stretches across the ear canal and converts airborne sound waves into mechanical vibrations that pass into the middle ear.21PubMed. Biomechanics of the tympanic membrane Without it, sound energy would largely dissipate before reaching the tiny bones of the middle ear. Understanding the eardrum’s dynamic behavior remains an active area of research, because better models of how it transmits vibration lead to improved treatments for hearing loss and better designs for prosthetic eardrums.20PubMed Central. Numerical model characterization of the sound transmission mechanism in the tympanic membrane from a high-speed digital holographic experiment in transient regime
Microscopic Diaphragms Inside Blood Vessels
At the cellular level, the term “diaphragm” shows up in a completely different context. Certain specialized blood vessels, particularly the capillaries in your kidneys, intestinal lining, and endocrine glands, have tiny windows called fenestrae punched through their walls to allow rapid exchange of substances between blood and tissue. Many of these fenestrae are bridged by thin protein structures called fenestral diaphragms, formed by a protein known as PV1.
These molecular-scale diaphragms act as selective barriers. When researchers deleted the gene responsible for PV1 in mice, the fenestrae themselves still formed, but without diaphragms covering them, the capillaries leaked plasma proteins uncontrollably. The mice developed low blood protein levels, abnormal fat levels, and reduced survival.22PubMed Central. The diaphragms of fenestrated endothelia: gatekeepers of vascular permeability and blood composition In other words, these subcellular diaphragms are gatekeepers that keep the blood’s composition stable, filtering what gets out and what stays in. Loss of diaphragm function in these capillary beds led to sieving of proteins out of the blood, demonstrating that even at this tiny scale, a diaphragm’s partitioning role is essential.23Developmental Cell. PV1 is a key structural component for the formation of endothelial diaphragms and is necessary for endothelial barrier function
These microscopic diaphragms are not something most people ever hear about, but they underscore a point that runs through the entire list: the body uses partitions at every scale, from whole-cavity dividers down to molecular gates, and calls many of them diaphragms.
When the Thoracic Diaphragm Fails
Because the thoracic diaphragm develops from multiple embryonic tissue sources, gaps can be left behind if that development goes wrong. The result is a diaphragmatic hernia, a hole through which abdominal organs can slide up into the chest. There are two classic types, classified by location. Bochdalek hernias occur in the left posterolateral portion of the diaphragm and are the more common type; Morgagni hernias appear at the front of the diaphragm and are rarer.24PubMed Central. Morgagni hernia: an uncommon pathology in adults
In newborns, a large Bochdalek hernia is a surgical emergency. Intestinal loops or even the stomach and spleen can migrate into the chest and compress the developing lung, sometimes lethally. In adults, smaller diaphragmatic hernias may go undetected for years and turn up incidentally on imaging. The separate embryonic origins of the diaphragm help explain why these defects tend to occur at predictable seam lines: the places where the septum transversum, pleuroperitoneal folds, and somite-derived muscle were supposed to fuse during fetal life.5PubMed Central. Development of the diaphragm, a skeletal muscle essential for mammalian respiration
Acquired diaphragmatic problems also exist. Phrenic nerve injury from surgery, trauma, or neurological disease can paralyze one or both sides of the diaphragm, reducing breathing capacity. And hiatal hernias, where the stomach pushes up through the normal opening for the esophagus, are extremely common in older adults.
Why So Many Structures Share One Name
The Greek word diaphragma simply means “partition” or “barrier across,” and that functional description applies at every level of the body. A structure counts as a diaphragm whenever it forms a roughly horizontal boundary separating one compartment from another, whether the compartments are the chest and abdomen, the brain and the cerebellum, or the inside and outside of a capillary.
Evolutionary anatomy offers one way to see why the thoracic diaphragm is the most developed of the lot. As mammals evolved from reptilian ancestors, the single large body cavity of reptiles was divided into separate thoracic and abdominal compartments. The muscular diaphragm that accomplished this division gave mammals a huge advantage: it allowed aspiration breathing powerful enough to sustain warm-blooded metabolic rates, while simultaneously generating the abdominal pressure needed for venous blood return and expulsive behaviors.4PubMed Central. Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals The thoracic diaphragm, in short, is the diaphragm that made mammalian life possible. But the other partitions the body builds, from the pelvic floor to the fenestral diaphragms in capillary walls, serve the same basic logic: separate compartments, control what crosses the boundary, and generate useful pressure gradients where needed.
The physical medicine literature connects at least the five macro-level diaphragms through continuous fascial chains. In the osteopathic model, restrictions in one diaphragm can transmit tension to another, contributing to symptoms far from the original problem site.17PubMed Central. The Five Diaphragms in Osteopathic Manipulative Medicine: Myofascial Relationships, Part 1 Whether or not you accept that clinical framework in full, the anatomical connections between these horizontal partitions are real. The thoracic diaphragm’s fascia blends with the psoas muscles and deep spinal ligaments, which in turn connect to the pelvic floor. Fascial layers in the neck link the thoracic outlet to the floor of the mouth. The body’s diaphragms are not isolated barriers stacked like floors in a building; they are connected elements in a system that manages pressure, motion, and compartmentalization from head to pelvis.
Counting Them Up
If you want a single number, the answer depends on what you include. The major named diaphragms that most anatomy and clinical sources recognize are:
- Thoracic diaphragm: the primary breathing muscle separating chest from abdomen.
- Pelvic diaphragm: the levator ani and coccygeus muscles forming the floor of the pelvis.
- Tentorium cerebelli: the dural fold separating the cerebrum from the cerebellum.
- Diaphragma sellae: the dural fold roofing over the pituitary gland.
- Urogenital diaphragm: an older term for the perineal membrane region below the pelvic floor, still widely used in clinical shorthand.
Add the thoracic outlet’s fascial barrier and the mylohyoid floor of the mouth, and you reach the osteopathic model’s count of five major horizontal partitions. Fold in the iris and eardrum as sensory diaphragms and the fenestral diaphragms of capillaries as cellular-scale barriers, and the total keeps climbing. No single textbook pins down one authoritative count, because the answer genuinely depends on scale and context. What stays consistent is the principle: at every level of organization, the body builds partitions, labels many of them diaphragms, and relies on all of them to keep its compartments separate, its pressures regulated, and its contents where they belong.