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Respiratory System Anatomy

From Mediwikis
Anatomy of the Respiratory System

The thoracic cavity (chest) contains the bulk of the anatomy of the respiratory system. This section does not include the anatomy of the oral/nasal cavities, pharynx/larynx as these are covered elsewhere.

Muscles and movements

Quiet breathing is using some of the inspiratory and expiratory movements of the thoracic wall but mainly using the Diaphragm while forced respiration is involves deep inspiration and expiration and uses the accessory muscles and abdominal muscles to assist.

Intercostal muscles contract to increase the transverse and antero-posterior diameters of the thorax. The ribs conduct a "pump handle" movement which expand the thorax laterally. Pump handle movements change it anterio posteriorly. Elevation of ribs causes anterior movement and the raising of the sternum also increases the volume


Primary Cartilaginous is when the joints are joined by hyaline cartilage (synchondrosis). Examples are the xiphisternal joint and joint between rib one and manubrium

Secondary Cartilaginous joint is when the articulating surface is covered in fibrocartilage which is more resistant than hyaline cartilage. An example would be the manubirosternum joint.

Costochondral joints are between the ribs and costal cartilage, these have a primary cartilaginous joint but have a further membrane around the outside called the periosteum which binds them together.

All other joints are synovial. This involves a small potential space into which synovial fluid is secreted by the syonvial membrane. These are the costovertebral, costotransverse and interchondral joints.

Accessory Muscles

Abdominal wall muscles are rectus abdominus, transversus abdominus and obliques. Accessory respiratory muscles are neck muscles, back muscles and shoulder girdle. Accessory muscles increase volume of thorax longitudinally while diaphragm pushes downwards though contents of abdomen stop further contraction. Abdominal muscles relax in inspiration. During forced expiration abdominal muscles push contents of abdomen up to squeeze air out of lungs.

The accessory muscles are:

  • Right and left Quadratus Lumborum, at the back going from the pelvis to the 12th rib (iliac crest on the pelvis).
  • Sternoclcleidomastoid from the mastoid process in the skull to the sternum and clavicle
  • Pectoralis minor, under pectoralis major in the chest
  • Scalene: 3 of them in the neck, anterior, middle and posterior going from the cranial vertebrae to rib 1 and 2.

Muscle Anatomy

External intercostal muscles go from the rib tubercles to costochondral junctions i.e. diagonally across the intercostal spaces from the spine towards the sternum round the outside of the tibs. Internal and innermost intercostal muscles go from the sternum to the angle of the rib. Essentially opposite direction to external ones.


The main boney features of the repiratory system are based around the thoracic skeleton, this is made up of:

  • 12 Thoracic vertebrae
  • Sternum
  • 12 pairs of ribs

The thoracic skeleton is designed to protect the organs it encases, and aids in the inflation and deflation of the lungs when breathing to enable effective respiration. Ribs 1,2,10,11,12 are atypical. Ribs 11 and 12 are floating as they do not have any costal cartilage. 1-7 are true ribs while 8-12 are false.

Each ribs' head has two facets. There is a small bump called a tubercle on the neck with another articular face. The neck leads on to the body and the curve is called the angle. Each rib also has an intercostal groove for blood vessels and nerve on the underside.

Thoracic Vertebrae

There are 12 thoracic vertebrae to match the 12 pairs of ribs that form the thoracic skeletal framework, both ribs and vertebrae are labelled from the top down, (the most superior being 1 moving down to the most inferior being 12). In specimens the thoracic vertebrae are characterised by the presence of articulation points for these ribs, these is the key way to identify the thoracic vertebrae from any other vertebrae present in the body. Other distinguishing features of the thoracic vertebrae include:

  • Heart shaped almost square in transverse cross-section vertebral body
  • Long spinous process

Each thoracic vertebrae articulates with it's own rib and the one below it, for example T6 articulates with the 6th rib and the 7th rib, T7 with the 7th rib and 8th rib and so on. However there are exceptions to this rule in that T10, T11, T12 (and sometimes T9) only articulate with their own ribs and not the one below.


The number of ribs present in the body is subject to anatomical variation (with more or less pairs possible) but the standard number is seen to be 12 pairs. At each rib starts posteriorly with the head of the rib articulating with the vertebral column, the rib then sweeps forward and down in a slight curve to end in a section of costal cartilage.

The 12 ribs are then divided into "true" and "false" ribs:

  • True Ribs = rib pairs 1 to 7
There are seven pairs of true ribs identified by their costal cartilages articulating with the sternum directly and the sternum alone.
  • False Ribs = rib pairs 8 to 12
There are five pairs of false ribs identified by their costal caritage not articulating with the sternum directly or with the sternum alone.
Each of the costal cartilages of ribs 8 to 10 articulates with the costal cartilage of the rib above.
The costal cartilages of the ribs 11 to 12 do not articulate with anything and are therefore known as the "floating ribs".

The ribs themselves have few notable features, they are made up of a somewhat bulbous head, running through to a flattened neck, on this neck is a projection known as the tubercle. As you move round the angle of the rib there is an indent known as the costal groove that runs around the inferior edge of the rib. Each of these contains a nerve, artery and vein. The first rib is flatter and wider than the others and is described as atypical. The floating ribs are also classed as atypical.


The sternum acts as an anchor point for the thoracic skeleton and with the thoracic vertebrae holds the ribs in place to effectively form a cage around the thoracic viscera.

The sternum is made up of three segments, these are the manubrium, body and xiphisternum. The manubrium is the most superior segment of the sternum and is placed just between and slightly inferior to the two clavicles. Moving down the sternum the body is next, this is the largest segment of the sternum. The body of the sternum articulates with the manubrium above at the sternal angle (level with the 2nd thoracic rib), thoracic ribs and the xiphisternum below. The xiphisternum is the most inferior segment of the sternum and is also the smallest of the three segments, it articulates with the body of the sternum above. Note that the xiphisternum is also known as the xiphoid process, they are one and the same.

The different segments of the sternum articulate with each other through links known as symphyses (symphysis = singular e.g. symphysis pubis, see osteology of the pelvis), these are very tight almost immovable joints.

Blood Vessels

The Aorta arches on the left side of the Carina. This can be seen on a chest X-ray as the Aortic Knuckle and is important as if it is enlarged it can be a sign of swelling in the aorta and therefore heart failure. Three arteries branch off from the aorta. The one furthest left becomes the left subclavian artery, supplying blood to the left arm. The middle one becomes the left common carotid artery. The one on the right side is the brachiophalic artery/trunk and splits into two smaller arteries: The right subclavian and the right common carotid. The superior Vena Cava branches into the left and right brachiophalic veins which each divide into the left and right internal jugular veins and the left and right subclavian veins all of which run roughly parallel to their artery counterparts.

Inferior to the Carina and on the right side is the point where another vein drains into the Vena Cava. This is the Azygos vein and connects the vena cava to the intercostal veins amongst other vessels (which vary depending on the person). It also connects the Hemiazygos vein and Accessory Hemiazygos vein, which is superior to the hemizygous vein. They each connect half of the left intercostal veins.

The arteries that supply the thoracic wall come from either the descending Aorta or the left and right thoracic arteries. The intercostal arteries branch from these and go round the rib cage in the intercostal grooves. The thoracic arteries are connected to the subclavian arteries on each side.

Venous drainage is provided by the intercostal veins which drain into either the internal thoracic veins or the azygous system as described above. The internal thoracic veins drain into the brachicephalic veins.

The lungs are supplied mainly by the pulmonary arteries. There are also smaller bronchial arteries arising from the superior thoracic aorta or one of the right intercostal arteries.

The Pleura

The concept of the pleura is one that can be hard to get your head around. One way to think of it is to imagine a two thin sheets of material placed one on top of the other to form two layers with a narrow space inbetween. An object is then placed on top of these two layers and the two layers of material are wrapped around it. So, from the outside working in, there is a layer of material, then a space, then another layer of material and then the object. This concept then works when applied to the pleura surrounding the lungs.

The two layers of the pleura are made up of connective tissue and mesothelial cells. From the outside working in, as before, there is the parietal pleura layer, a space (known as the pleual space) and then the visceral pleura layer, then the lung. Within the pleural space is a small amount of serous fluid that acts to reduce friction between the two pleural layers.

There are two of these pleural packets, one for each of the lungs, the parietal pleura makes up the borders of the pleural cavity and the visceral pleura is associated with the viscera (the organ, i.e. the lung in this case) and is attached to the surface of the lungs.

The parietal pleura is divided into the cervical pleura (posterior, attached to vertebrae of spine), the costal pleura (ribs), Diaphragmatic pleura and mediastinal pleura.

Boundaries of the Pleural Cavity

The boundaries of the pleural cavity can be thought of as the extent to which the parietal pleura reaches and covers the inside of the thoracic cavity. The boundaries of the pleural cavity are as follows, remember there is two pleural cavities one on each side of the thoracic cavity separated by the mediastinum containing the heart in the middle. :

  1. Running down the lateral side of the thoracic wall on the internal side of the ribs from above rib I in the root of the neck, down to just above rib X
  2. Running across the diaphragm to the edge of the mediastinum
  3. Running up the mediastinum to meet at the apex of the lungs with the pleura running down beneath the ribs

In this way the pleura forms a continuous casing over the whole of each lung.

The Trachea

The trachea is basically just a tube that carries the air from the mouth and nose when we breath to the lungs and visa versa. It begins at spinal level C VI and ends at spinal level T4 where it divides (i.e. bifurcates) into the two main bronchi. The trachea is both flexible and stable in its nature as it is held open with several C-shaped rings of cartilage.

The only complete ring of cartilage is the circoid cartilage. This marks the start of the trachea and end of the larynx. Above the larynx is the pharynx and then the nasal and buccal openings.

Each of the two main bronchi supply one of the lungs, the left main bronchus to the left lung and the right main bronchus to the right lung. These bronchi then divide further into the smaller airways delving deeper into the lung tissue. Working stepwise from widest diameter to narrowest there is the:

Right/Left main bronchi → lobar bronchi (one to each lobe) → segmental bronchi → bronchioles

With each division there are more and narrower airways. The cartilage that holds the airways open is lost by the time you reach the bronchioles.

The Bronchioles have three generations: Lobular, terminal and respiratory. The respiratory bronchioles lead to alveolar ducts and alveolar sacks.

Clinical Note It is both interesting and important to note the differences between the left and right main bronchi. The right main bronchus has a wider diameter and also is more vertical in its nature than the left main bronchus. This means that in cases of chocking or aspiration of foreign material into the airways/lungs it is more likely to be lodged in the right side than the left.

The Lungs

The two lungs lie one either side of the midline separated by the mediastinum. The right lung is the larger of the two lungs as the left lung is restricted for space by the bulk of the heart within the mediastinum. The base of each lung lies level with the diaphragm and the pleura associated with the bases of the lungs is attached to the diaphragm.

The lungs are divided into lobes, the right lung into 3 lobes (superior, middle and inferior) and the left into 2 (superior and inferior). But both of them form the same basic shape, a blunted cone, flat at the bottom and pointed at the apex.

Each lung has an area known as the root and hilum, the hilum acts as a boundary around the root. The root is a collection of bronchi and vessels that enter and leave from each lung on their mediastinal surface (the surface nearest to the mediastinum in the midline). The hilum is made up of a fold in the pleura so that the structures that make up the root can leave the pleural cavity. There are subtle differences in the root and hilum between the two lungs.

The Right Lung

The root of the right lung acts as a conduit for:
  1. The right main bronchus
  2. 2 pulmonary arteries (that carry deoxygenated blood to the lungs)
  3. 2 pulmonary veins (that carry oxygenated blood away from the lungs)
  4. Smaller features include the vessels supplying blood to the bronchi and nerves and lymphatics that the lung needs

The Left Lung

The root of the left lung acts as a conduit for:
  1. The left main bronchus
  2. 1 large pulmonary artery (this splits into two in the body of the lung) (that carry deoxygenated blood to the lungs)
  3. 2 pulmonary veins (that carry oxygenated blood away from the lungs)
  4. Smaller features including vessels supplying blood to the bronchi and nerves and lymphatics that the lung needs

The lungs themselves have many indentations and grooves on their surface caused by nearby viscera. The viscera of note that cause obvious identifiable features on the lung surface are:

  • Oesophagus
  • Superior Vena Cava (SVC) (on the right lung only)
  • Thoracic Aorta continuing to the Aortic arch (on the left lung only)
  • Heart
  • Rib 1

These grooves are found on the mediastinal surface of the lungs, along with the root and hilum. Anatomy of the lungs, looking at the hilum of each.

Lungs rk.jpeg

There are two major sets of 2 recesses near the lungs. The costomediastinal recess is along the anterior edge of each lung where the mediastinal and costal pleura meet. The costodiaphragmatic recesses occur in each cavity and are between the most inferior part of the lungs and the most inferior part of the pleural cavity. The lungs expand into these recesses during inspiration and shrink away from them during expiration.

The costophrenic angles are where the costodiaphragmatic recesses are.

Bronchopulmonary segments are self contained anatomical units within the lung. They are each supplied by a segmental bronchus with arteries running through the middle of them and veins around the outside of them. They are separated by connective tissue and so are distinct enough to be removed if a disease is confined to one lobe. Each segment is supplied by a segmental bronchus and a pulmonary artery. The arteries all supply a single segment whereas the veins are intersegmental and therefore drain multiple segments.

There are 10 segments in each lung. The right lung the superior lobe has 3, the middle lobe has 2 and the inferior lobe has 5. In the left lung there are 4-5 segments in each lobe giving 8,9 or 10 segments. The segments themselves are cone-shaped. The apex of the cone is where the segmental bronchus and pulmonary artery enter. The base of the cone is lined up with one of the surfaces of the lungs while the sides of the cone touch veins and other segments.

The Diaphragm

The Diaphragm is attached to the xiphoid process of the sternum, the costal margin of the thoracic wall, the ends of the floating ribs and the vertebra or the lumbar region. Note that there are further ligaments which attach the diaphragm to other structures.

The Diaphragm has two domes, at rest the right sits higher than the left due to the liver. It contracts as a single unit and is mainly innervated by the phrenic nerves but recieves some input from some thoracic nerves. The central tendon is the most important bit and is continuous with the fibrous pericardium. There are two crus (pl crura) extending from each side of the diaphragm to the spinal chord, the right is longer than the left. There are 3 other arcuate ligaments: Median, medial and lateral

  • Median attaches to the aorta
  • Medial are psoas major (muscles in abdomen)
  • Lateral attaches to the quadratus lumborum

The diaphragm also has 3 hiatuses:

  • The Caval for the IVC and Right phrenic nerve at TVIII
  • Oesophageal for Oesophagus and associated blood vessels and vagal trunks at TX
  • Aortic for Descending Aorta , thoracic duct and azygous vein at TXII

It is supplied with blood by the Superior Phrenic arteries off the lower thoracic aorta. Branches of the internal thoracic arteries, pericardiacphrenic arteries also help. The largest vessels are the inferior phrenic arteries coming off the abdominal aorta.

The Diaphragm receives venous drainage from veins which parallel the arteries and drain into the azygous system, brachiocephalic (internal thoracic) and abdominal veins.

Diaphragm rk.jpeg


  • Dorsal column: Vertical trunk of whit matter within spinal cord.
  • A dermatome/myotome is the area of skin/muscle supplied by a single spinal nerve.

Each segment of the spine has two nerves coming off it. The spinal cord is the most inferior part of the CNS and involved in the integration of reflex responses with sensory information. The spine is mainly involved in the motor and reflex movements of limbs and trunk. The nerves connecting to the spine all have sensory, motor and autonomic neurones with them.

  • Sensory neurones connect to the dorsal grey matter and sensors in the body and moves into the spinal cord via the dorsal root.
  • Motor neurones connect to the ventral grey matter and effectors and moves via the ventral root.
  • The autonomic pathway in and out of the spine involves more than two neurones and travels via the dorsal root to synapse with the mediolateral grey matter.

The sympathetic trunk is a vertical trunk of ganglia which run up the spine on each side. ANS neurones either synapse with the ganglia at the same height of their neurone, travel up or down the trunk and then synapse or synapse with ganglions in-between the spinal cord and their target organ.

The rib cage is innervated by 12 spinal nerves. They split into an anterior and posterior ramus. The posterior rami supply the muscles and the skin of the back. The anterior rami curve round to form the intercostal nerves apart from the one around rib XII (nerve XII) which is the post costal nerve as it is not in an intercostal space.

The skin outside the rib cage is innverated by the lateral cutaneous branches of the intercostal nerves for the side of the chest and the anterior cutaneous branches for the anterior skin on the chest.

The parietal pleura are innervated by the pleural sensory branches of the intercostal nerves and the phrenic nerves for the medistinal and diaphragmatic pleura. The visceral pleura are innervated by nerves attached to bronchioles but pain is generally not felt from the vsiceral pleura.

The diaphragm is supplied by two (left and right) phrenic nerves. They originate from the rami of cervical nerves C3, C4 and C5 (3,4,5 keep the diaphragm alive). They pass vertically through the neck via the superior thoracic aperture. They supply the whole diaphragm including the cura, the muscular extensions which attach the diaphragm to the upper lumbar vertebrae.

The lungs themselves are innervated from nerves coming off the pulmonary plexus which is connected to the pulmonary branches of the vagus nerves (left and right 10th cranial nerves) and parts of the sympathetic trunk, it is located near the carena.

Referred pain

Referred pain is feeling pain in an area which is not the source of pain. This occurs, for example, during a heart attack when one feels pain in their left arm. It is mainly due to the point at which nerves enter the spinal cord. Intercostal nerves which contain sensory neurones with pleural branches link back up with the spine in places whose dermatome territories are not near the thorax. This means that pain felt by nerves in the parietal pleura would be felt in the shoulder, neck and trunk.

Surface Markings of Relevant Anatomy

The sternal angle is an important surface maring when thinking about the anatomy of the respiratory and cardiovascular systems. It can be felt by moving down from the jugular notch (just above the manubrium) where it forms a bump on the sternum. Not only does the sternal angle demarkate the level of the 2nd rib (felt immediately lateral to the sternal angle) which is useful in counting down ribs in chest examinations to localise features, but also indicates the level of other anatomical features including:

  • The intervertebral disc between T4 and T5
This is key in locating the upper border of the pericardium and therefore the change from superior mediastinum to inferior mediastinum
  • The bifurcation of the trachea
  • The pulmonary trunk lies just inferior
  • The changing point of the arch of the aorta into the ascending aorta and on the other side of the arch: the thoracic aorta