Respiratory Anatomy Part 1 – Introduction

Lesson Overview

In this lesson, we introduce the physiology of the respiratory system.


Gain an understanding of the foundational physiology of the respiratory system.


Define breathing and respiration. Explain what causes breathing and its purpose. Describe the contents of the thoracic and abdominal cavities and how they change during breathing. Understand why lungs cannot be moved directly at will and the principal muscle involved in breathing. Describe the diaphragm and the movement of the diaphragm during relaxed breathing. Explain problems that a person may have with their diaphragm.

Respiration & Breathing Defined

  • Breathing is the movement of air into and out of the lungs.
  • The primary purpose of breathing is to supply fresh oxygen to the blood and cells, and to discharge carbon dioxide.
  • Respiration is the act of breathing.
  • Breathing is defined as the “physical and chemical processes by which oxygen is conveyed to tissues and cells, and the oxidation products, carbon dioxide and water, are given off.” (

Taking Air Into & Out from Lungs

Breathing is the process of taking air into and expelling it from the lungs… The passage of air into and out of the lungs is movement, one of the fundamental activities of living things. Breathing… is caused by a three-dimensional changing of shape in the thoracic and abdominal cavities. – Leslie Kaminoff

Starts with Independent Life, Ends When Life Ceases

Breathing starts with independent life outside the mother and ends when life ceases… During most of one’s life, the depth and rate of breathing are self-regulated through the nervous system to meet the purposes of breathing: to supply in a regulated and controlled way the fresh oxygen which is constantly needed by the cells, and to discharge the carbon dioxide accumulated in them. – B.K.S. Iyengar

Oxygenation of the Blood

The unchanging and constant goal of respiration is the oxygenation of the blood. The cells in the tissues need oxygen to function properly, and it is brought to them via the arterial blood, which comes from the lungs and the heart. This mechanism produces a waste product, carbon dioxide, which is carried in the venous blood back to the heart and lungs… Since oxygen cannot be stored in the body, respiration is required without cease, day and night. However, the act of breathing does not always strictly depend on the oxygen needs of the body. [It] may also serve other purposes and be dependent on other circumstances. – Blandine Calais-Germain

How it Works

  • Breathing is caused by the thoracic and abdominal cavities changing shape.
  • Breathing occurs not only at the “level of the organs” but also at the “level of movement.” It involves both the nervous system and muscles.

Organs & Systems Work Together

Many different organs and systems in your body work together to enable us to do what we simply call “breathing.” – Baxter Bell, MD

The Exchange of Oxygen & Carbon Dioxide

Every cell in the body needs to breathe—taking up oxygen, burning fuel, generating energy, and giving off carbon dioxide. This process, known as cellular respiration, depends on an exchange—moving oxygen all the way from the atmosphere to lungs, to blood, and to cells,and at the same time moving carbon dioxide from cells to blood, to lungs, to atmosphere. – H. David Coulter

An Interface Between Organs & Movement

Breathing occurs on the level of the internal organs, just like the beating of the heart; but in contrast to the heart, it also involves muscles, certain parts of the skeleton and joint articulations. It cannot be separated from these. Breathing thus becomes an interface between two levels: the level of the organs and the level of movement. It can therefore be controlled, albeit with limitations, by the nervous systems’ management of either one of these two levels. – Blandine Calais-Germain

The Thoracic and Abdominal Cavities


The torso is divided into the thoracic and abdominal cavities, which change shape during breathing.

Thoracic and Abdominal Cavities Defined

Our torso is divided into two main sections: the thoracic cavity and the abdominal cavity. The thoracic cavity includes everything contained in the rib cage which extends from the base of the neck to a few inches above the navel. Essentially, it is a sealed-off container for the lungs, with the heart resting in between. The abdominal cavity consists of the part of the trunk that begins at the lower border of the rib cage and fills the space down into the pelvis. This sack contains the digestive organs. The two cavities are separated by a thin strong muscle called the diaphragm. – Irene Dowd

Thoracic Cavity

  • Heart
  • Lungs
  • When breathing, this cavity changes shape and volume.
  • Respiratory muscles increase the dimension of the thoracic cavity, lowering air pressure in the lungs (i.e. creating a vacuum); air is received into the lungs.
  • When the muscles relax and the diaphragm billows back up, the air in the chest is compressed, allowing it to move out.

Abdominal Cavity

  • Stomach
  • Liver
  • Gall Bladder
  • Spleen
  • Pancreas
  • Small and Large Intestines
  • Kidneys
  • Bladder
  • When breathing, this cavity changes shape, but not volume.
  • To be precise when speaking of abdominal changes during normal breathing, anatomy experts use terms such as “bulge,” “swell” or “puff out.” This is because the belly is not actually “expanding” as noted in the previous point about volume.

Breathing Caused by Cavities Changing Shape

Although both the abdominal and thoracic cavities change shape, there is an important structural difference in how they do so. The abdominal cavity changes shape like a… water balloon. When you squeeze one end of a water balloon, the other end bulges… Your hand’s action only moves the fixed volume of water from one end of the flexible container to the other… In the context of breathing the abdominal cavity changes shape, but not volume. (In life processes other than breathing… the abdominal cavity does change volume.) In contrast to the abdominal cavity, the thoracic cavity changes both shape and volume; it behaves… similar to an accordion bellows.  – Leslie Kaminoff

The Diaphragm


  • The diaphragm is a dome-shaped muscle located at the base of the lungs. It separates the thoracic and abdominal cavities.
  • The diaphragm is a muscle. As with other muscles, the diaphragm can become tight or weak.


  • Lungs do not have muscular tissue and therefore, cannot be moved directly at will; the diaphragm is the principal muscle involved in breathing.
  • The diaphragm acts as a pump causing the lungs to inflate.
  • During relaxed breathing, on the in-breath, the diaphragm contracts (moves down) to give the lungs more room to fill up. The abdominal organs are compressed, and the abdomen typically puffs out.
  • On the out-breath, the diaphragm relaxes, and moves back up; the belly typically moves back in.
  • Like the heart, the diaphragm works without break.
  • The pericardium which surrounds the heart is connected via fascia to the diaphragm. Therefore, the heart moves along with the diaphragm during breathing.
  • When the diaphragm moves fully with the breath, Donna Farhi notes that nearby organs are “massaged, rolled, churned, and bathed in new blood, fluids, and oxygen.” These organs that move with the diaphragm include the heart, lungs, stomach, pancreas, gallbladder, small intestine, liver and spleen. (The Breathing Book)


  • When the abdomen is chronically tightened, the diaphragm is unable to move downward, and secondary muscles are recruited. 
  • Using the secondary muscles (designed for short-term use) for regular, deep breathing is tiring and ineffective.
  • If the diaphragm is tight, weak, or unable to move freely, it can affect stress level, circulation, emotions, digestion and spinal stability. (Yoga Tune Up)
  • Leslie Kaminoff tells a story in Your Diaphragm & The Cheetah (8 minute video) to explain a potentially unhelpful approach for a weak diaphragm. A weak diaphragm may make one think of doing resistance training, such as placing a sandbag on the belly and then doing belly breaths. Kaminoff suggests, instead, to consider that the nature of the diaphragm is to create a three-dimensional shape change in the cavities as long as it has the space to do so. So instead of thinking about resistance training for the diaphragm, he suggests that “breath training is about accessory muscle retraining.”

The Principal Muscle

The diaphragm is the principal muscle that causes three-dimensional shape change in the thoracic and abdominal cavities. – Leslie Kaminoff

Acts Like a Pump

Most regular breathing occurs because of the diaphragm. It acts like a pump at the base of the lungs. – Blandine Calais-Germain

Irregular Dome Shape

The diaphragm is a large muscular and fibrous wall which simultaneously separates and connects the thorax and the abdomen…. It is shaped like a large, irregular dome, which is very thin and more developed at the back than at the front… Its edges are attached to the internal outline of the rib cage. The top of the dome is situated… slightly higher than the xiphoid process… The base… corresponds to the level of the waist. As you can see, the levels at which the diaphragm attach to the trunk are very different. – Blandine Calais-Germain

Shaped by Organs it Encloses

Its structure extends through a wide section of the body—the uppermost part reaches the space between the third and fourth ribs, and its lowest fibers attach to the front of the third lumbar vertebra; “nipple to navel” is one way to describe it. The deeply domed shape of the diaphragm has evoked many images. Some of the most common are a mushroom, a jellyfish, a parachute, and a helmet. It’s important to note that the shape of the diaphragm is created by the organs it encloses and supports. Deprived of its relationship with those organs, its dome would collapse, much like a stocking cap without a head in it. It also has an asymmetrical double-dome shape, with the right dome rising higher than the left because the liver pushes up from below the right dome, and the heart pushes down from the left dome. – Leslie Kaminoff

Is it Necessary to Exercise the Diaphragm?

If you are relatively active and healthy, the diaphragm does not need any special “exercising.” However, if you have been sedentary, suddenly gained weight, suffered from periods of inactivity or developed poor posture, your diaphragm may not be working optimally. In these cases, you can use your asana practice to stretch areas of restriction and re-establish healthy posture. And you can use breathing practices, such as gradually lengthening your inhalation and exhalation, to improve your lung capacity over time. – Baxter Bell MD