Cardiac Notch and Lingula of the Left Lung

What Is the Cardiac Notch of the Lung?

The cardiac notch of the lung is a deep, curved indentation in the anterior border of the left lung, where the lung edge pulls back to the left to leave space for the heart. It is one of the main features that make the left lung look different from the right lung.

Cardiac notch definition in anatomy

In anatomy, the cardiac notch (Latin incisura cardiaca pulmonis sinistri) is defined as the concave gap in the anterior border of the superior lobe of the left lung, lying in front of the heart and pericardium. Above the notch, the anterior border runs straight down behind the sternum; at the notch, it swings sharply away to the left; below it, the lingula curves back toward the midline.

Why only the left lung has one

The heart does not sit exactly in the middle of the chest. About two-thirds of it lies to the left of the midline, with its apex pointing down and to the left. The left lung grows around this bulge, so its front edge is cut away, while the right lung, with less of the heart beside it, has a straight anterior border. This is also one reason the left lung is slightly smaller, as covered on our which lung is bigger page.

Explained simply

Imagine two pillows pressed against either side of a ball that sits a little to the left. The left pillow needs a scooped-out curve to fit around the ball. That curve is the cardiac notch, and the part of the pillow that wraps underneath the ball is the lingula.

Where Is the Cardiac Notch on the Left Lung?

The cardiac notch is on the anterior border of the left lung, behind the left side of the sternum and the 4th to 6th costal cartilages, directly in front of the heart. It belongs to the superior lobe, above the lingula and in front of the oblique fissure.

Surface anatomy

On the front of the chest, the anterior border of the left lung runs down close to the midline behind the sternum until about the level of the 4th costal cartilage. There it turns outward to the left, curves around the heart, and descends to the 6th costal cartilage a few centimeters to the left of the sternum, where it meets the inferior border. The exact depth of the notch varies from person to person and changes slightly with each breath.

The cardiac notch of the pleura

The parietal pleura lining the chest wall has its own, shallower cardiac notch in the same place. Because the lung's notch is deeper than the pleura's, a narrow strip of pleural space in front of the heart holds no lung at rest. This pocket is part of the costomediastinal recess, which the lingula slides into during a deep breath in.

A small window to the heart

Where the lung is notched, the pericardium lies against the chest wall with no lung tissue in front of it. Doctors call this region the bare area of the pericardium, and when they tap on the chest it gives a dull note instead of the hollow sound of air-filled lung.

What Is the Lingula of the Lung?

The lingula is the thin, tongue-shaped lower front part of the superior lobe of the left lung, lying just below the cardiac notch and curving over the front of the heart. Its name is Latin for "little tongue."

Where is the left lung lingula?

The lingula sits at the front of the left side of the chest, below the cardiac notch and above the diaphragm, roughly behind the 5th and 6th costal cartilages. It lies in front of the oblique fissure and rests against the pericardium over the left ventricle. On a frontal chest X-ray it lies right next to the left border of the heart.

Segments, airway and vessels

The lingula is made of two bronchopulmonary segments: the superior lingular segment and the inferior lingular segment. They are aired by the lingular bronchus, a branch of the left upper lobe bronchus that divides into one segmental bronchus for each. A lingular branch of the left pulmonary artery runs with the airway, and lingular veins drain into the left superior pulmonary vein, all entering or leaving through the hilum.

Lingula vs right middle lobe

The lingula is the left lung's counterpart of the right middle lobe. Both lie at the front of the chest beside the heart, both have two segments, and both receive one bronchus from the main airway of their side. The difference is that the right middle lobe is a separate lobe, cut off by the horizontal fissure, while the lingula usually has no fissure of its own and stays part of the superior lobe.

Left Lung Anatomy: How Do the Cardiac Notch and Lingula Fit Together?

The cardiac notch and the lingula are two halves of the same shape: the notch is the gap cut into the superior lobe by the heart, and the lingula is the tongue of that lobe that wraps around the gap from below. Together they form the lower part of the anterior border of the left lung.

The left lung has two lobes, superior and inferior, separated by the oblique fissure. The superior lobe holds the apex, most of the anterior border, the cardiac notch and the lingula. The inferior lobe forms most of the base and the back of the lung. The table below lists the structures around the notch and what each one does.

Cardiac notch vs cardiac impression

The cardiac notch is easy to confuse with the cardiac impression. The notch is a gap in the lung's front edge, seen from the front. The cardiac impression is a hollow on the inner (mediastinal) surface of the lung, seen from the side, where the heart presses against it. Both lungs have a cardiac impression, but the left one is deeper, and only the left lung has a notch. The other parts of that inner surface are described on our mediastinum page.

What Is the Function of the Cardiac Notch?

The function of the cardiac notch is to give the heart and pericardium room on the left side of the chest without squeezing the lung or limiting the heart's movement. It is a shape that results from where the heart sits, rather than a structure that does active work.

• Room for the heart: the heart changes shape with every beat, and the notch lets it move freely against the chest wall.
• Spare space for breathing: during a deep breath in, the lingula and the edge of the notch slide forward into the pleural recess in front of the heart, so the lung can still expand fully.
• Gas exchange in the lingula: the lingula is ordinary lung tissue, so its two segments take part in gas exchange like the rest of the lung.
• A landmark: the area of dull sound over the notch helps doctors outline the heart on examination, and it is the region where the pericardium can be reached without passing through lung.

In short, the notch is the lung's way of sharing the left side of the thoracic cavity with the heart, and the lingula makes sure that space is still used for breathing.

How Do the Cardiac Notch and Lingula Develop?

The cardiac notch and lingula form before birth, as the left lung grows around the developing heart and its airway buds divide into lobes and segments. The shape is set by the heart's leftward position.

Early in development, the airway tube divides into right and left lung buds. The right bud branches into three lobar bronchi and the left into two, which is why the right lung ends up with three lobes and the left with two. The airway that would form a middle lobe on the left instead becomes the lingular bronchus inside the superior lobe. Meanwhile, the growing heart turns toward the left, and the expanding left lung molds itself around it, leaving the notch. Our respiratory system development page traces these stages in order.

Variations are common. The depth of the notch differs between people, and a small extra fissure sometimes separates the lingula from the rest of the superior lobe, making it look like a small middle lobe on the left side.

Common Conditions That Affect the Lingula

The cardiac notch itself is rarely affected by disease, but the lingula can be involved in the same conditions as other parts of the lung, and its position next to the heart makes these easy to spot on imaging. Our respiratory diseases section explains each condition in depth.

• Lingular pneumonia: infection that fills the lingula with fluid blurs the left border of the heart on a frontal chest X-ray, a clue known as the silhouette sign.
• Bronchiectasis: long-lasting widening of the airways often involves the lingula and the right middle lobe, partly because their long, narrow bronchi drain poorly.
• Collapse (atelectasis): a blocked lingular bronchus can make the lingula collapse, again changing the outline of the heart on X-ray.
• Nontuberculous mycobacterial infection: one form of this infection tends to affect the lingula and the right middle lobe, especially in older women.

Anyone with ongoing cough, fever or breathing problems should see a doctor, since these conditions need medical assessment.