Table of Contents
Introduction
Lungs are the primary respiratory organs found in humans and most animals. The respiratory system consists of lung parenchyma and airway. The airway consists of the bronchus, which bifurcates into trachea and further divides into bronchioles and alveoli.
Humans have two lungs – a right lung and a left lung. It is found in the thoracic cavity of the chest. It is found near the backbone on either side of the heart. The lungs function to draw oxygen from the air and transport into the bloodstream and to remove carbon dioxide from the blood. This whole process is known as gas exchange. Lungs also supply the airflow with the help of which human speech is possible.
The diaphragm is the muscle which is responsible for driving gaseous exchange in the lungs. Both the lungs together weigh around 1.3 kg, the right one being heavier than the left. The lungs are enclosed in a pleural cavity called pleurae that is filled with pleural fluid. The pleural fluid between the outer and inner membranes smoothes the process of breathing and prevents friction.
Lung – Anatomy
The lungs are conical in shape with a rounded narrow apex at the top. At the bottom is a broad concave base that lies on the convex surface of the diaphragm. Anatomically, lungs have an apex, three surfaces and three borders.
Borders of Lungs
The three borders are anterior, posterior and inferior borders. The anterior border coincides with the pleural reflection and is known to form a cardiac notch in the left lung. The cardiac notch is a concavity that accommodates the heart. The posterior border extends from C7 to T10 and is a thicker structure. The inferior border is thin in nature and separates the base of the lung from the costal surface.
Surfaces of Lungs
The three surfaces are medial, costal and diaphragmatic surfaces. The costal surface is layered by the costal pleura and is found along the lines of sternum and ribs. It connects the medial surface at the anterior and posterior borders and also joins the diaphragmatic surface at the inferior border. The medial surface is related to the sternum anteriorly and to the vertebrae posteriorly. The diaphragmatic surface is convex on the upper side forming the floor of the thoracic cavity and concave on the lower side, forming the roof of the abdominal cavity.
The lungs have a central recession called hilum from where the blood vessels and airway arrive into the lungs and make up the root of the lung. The pleural membranes that surround the lung are double serous membranes in between which the pleural fluid is present. Each of the lungs is divided into lobes and lobules.
Right and Left Lung
The right lung has three lobes – an upper, middle and lower one. The right lung usually weighs between 155g – 720g in males and 100g to 590g in females.
The left lung has two lobes – an upper lobe and a lower lobe. It does not have a middle lobe like the right lung. It weighs between 110g – 675g in men and 100g – 590g in women.
Microanatomy
The lungs are a part of the lower respiratory tract, and they house the bronchial tubes that branch from the trachea. The main bronchus tube divides multiple times after entering the lungs and forms bronchioles that eventually form the minute air sacs called alveoli. The alveoli is the location of gaseous exchange between the respiratory system and blood capillaries. From the hilum, the lymphatic vessels, nerves, bronchus, and pulmonary arteries and veins enter the lungs.
All portions of the respiratory tract such as the trachea, bronchi and bronchioles are lined with respiratory epithelium. The respiratory epithelium is ciliated and scattered with goblet cells that secrete mucus.
Alveoli
Alveoli, also known as air space or air sacs, are millions of hollow cup-shaped cavities found in the lungs where exchange of oxygen and carbon dioxide takes place. The alveoli makes up the functional tissue of the lungs called lung parenchyma that occupy around 90% of the total lung space.
The alveoli consists of two types of alveolar cells and an alveolar macrophage. The two types of cells, also known as pneumocytes, are denoted as type I and type II cells.
The type I cells are made up of squamous epithelium — they are thin and enable gaseous exchange. They make up the alveolar septa that separates the alveoli from each other. The type II alveolar cells are smaller and line the alveoli. They secrete lung surfactant and epithelial lining fluid that decreases the alveolar surface tension.
The alveolar macrophage is important for immunity as they remove depositions from the alveolar surface such as loose RBCs.
Lung Microbiota
The lung houses a complex variety of microbial communities. They include bacteria, bacteriophages, fungi and viruses. The core 9 bacteria are – Streptococcus, Staphylococcus, Veillonela, Megasphaera, Fusobacterium, Acinetobacter, Pseudomonas, Sphingomonas and Prevotella. These microbes interact with the epithelial cells of the airway and play a major role in maintaining homeostasis. The common fungal species found in the lung include Aspergillus, Saccharomyces, Candida and Malassezia.
Changes in the composition of the microbiota can cause diseases such as asthma, chronic obstructive pulmonary disease and cystic fibrosis.
Blood Supply
The lungs are supplied with blood by both pulmonary and bronchial circulation. The pulmonary circulation carries deoxygenated blood from the heart to the lungs and transports back oxygenated blood to the heart that can be supplied to the body. The bronchial circulation, on the other hand, transports oxygenated blood to the airway of the lungs with the help of bronchial arteries that come from the aorta.
There are three bronchial arteries; two in the left lung and one in the right lung that branch along with the bronchi and bronchioles. The average blood volume of the lungs is about 450ml that accounts for 9% of the total blood supply.
Nerve Supply
The lung is innervated from two sources – the pulmonary plexus that is a combination of sympathetic and parasympathetic innervation, and the phrenic nerve. The pulmonary plexus has afferent and efferent autonomic nerve fibres and it sits at the root of the lung. The vagus nerve (parasympathetic) helps in the dilation of blood vessels, constriction of the bronchi and also increases gland secretion. The sympathetic nerve, on the other hand, helps in the constriction of pulmonary vessels and dilation of the bronchi.
Refer: Difference between Type 1 and Type 2 Pneumocytes
Function of the Lungs
- Gaseous Exchange: The primary function of the lungs is to facilitate gaseous exchange. Alveoli, the functional unit of the lungs, have thin membranes that allow easy exchange of gases. But the lungs cannot expand or relax on their own during the breathing process. The diaphragm and intercostal muscles are two main respiratory muscles that help in inhalation and exhalation.
These muscles contract, pull up the rib cage and increase the volume of the lungs for inhalation. During breathing out, the diaphragm and intercostal muscles relax, bringing the rib cage back to its original position.
- Protection: The respiratory tract is lined with respiratory epithelium that is interspersed with goblet cells. The goblet cells secrete mucus which acts as an important barrier against air borne infections. The mucus also contains many antimicrobial compounds such as antioxidants, antiproteases, and defensins.
They have macrophages that destroy or engulf waste debris and microbes by the process of phagocytosis. They also have antigen presenting cells called dendritic cells that activate the T cells and B cells for adaptive immune response.
- Other Functions: The lungs help in regulating the blood pressure by becoming a part of the renin-angiotensin complex. It also maintains an acid-base homeostasis in the blood by expelling carbon dioxide during breathing.
Serotonin, bradykinin, leukotrienes and prostaglandins are excreted by the lungs. Drugs can be absorbed, modified and excreted via the lungs. They also filter out clots from veins that could otherwise cause a stroke.
Related Diseases
- Asthma: It is a long-term inflammatory disease that affects the airways of the lung. It obstructs the airflow and triggers spasms in the bronchioles. It is caused either due to genetic reasons or environmental factors such as air pollution and allergens. Most common symptoms include wheezing, coughing, shortness of breath and chest tightness.
- COPD: Chronic obstructive pulmonary disease is a long term progressive respiratory condition that worsens with everyday activities. It is caused by smoking, pollution or even genetic factors. The common symptoms include coughing and shortness of breath.
- Bronchitis: It is the inflammation of the bronchioles that leads to coughing of sputum, shortness of breath, wheezing and chest pain. Bronchitis can be both acute and chronic.
- Cystic Fibrosis: It is a recessive autosomal genetic disorder that affects the lungs, pancreas, kidney, liver and intestine. It leads to difficulty in breathing frequent lung infections.
- Pulmonary hypoplasia: It is a congenital malformation which is a condition where the lungs are underdeveloped. It results in a low number of bronchioles and alveolar segments.
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