How does a pulmonary embolism form in the body? 

A pulmonary embolism is a complex physiological event that occurs when a blood clot, or occasionally another substance, travels through the circulatory system and lodges in a pulmonary artery. This process typically begins far from the lungs, most commonly in the deep veins of the legs or pelvis. Understanding the journey of a blood clot from its origin to the lungs is essential for recognizing the risks associated with cardiovascular health and venous circulation. 

What We’ll Discuss in This Article 

• The initial development of a blood clot within the deep veins. 

• The mechanism by which a clot detaches and enters the bloodstream. 

• The pathway of the clot through the heart and into the pulmonary system. 

• How the narrowing of lung vessels leads to a sudden blockage. 

• The immediate effects of reduced blood flow on lung tissue and oxygen levels. 

• Critical safety information regarding symptoms and emergency response. 

The origin of a venous blood clot 

The formation of a pulmonary embolism almost always starts with a condition known as deep vein thrombosis, where a blood clot develops in a deep vein within the body. These clots typically form when blood flow is restricted or when the blood’s chemical composition changes, making it more prone to sticking together. The body usually manages clotting to heal wounds, but in the deep veins, these clots can grow without an external injury, often due to prolonged inactivity or damage to the vein wall. 

Most of these initial clots develop in the large veins of the calf, thigh, or pelvis. When blood flow slows down, which is common during long periods of sitting or bed rest, red blood cells and platelets can clump together. Over time, these clumps are reinforced by a protein called fibrin, creating a solid mass. While many clots remain attached to the vein wall, they pose a significant risk if they become unstable or if the surrounding blood flow is suddenly altered. 

Detachment and transit through the heart 

A pulmonary embolism occurs when a blood clot (thrombus) breaks free from a vein and travels through the bloodstream to the lungs. Once a piece of the clot detaches, it is referred to as an embolus. The embolus is carried by the venous blood, which is naturally flowing back toward the heart for re-oxygenation. Because veins get progressively larger as they move closer to the heart, the clot typically moves freely through the lower body’s venous system without getting stuck. 

The clot enters the right atrium of the heart and passes through the right ventricle, which is the chamber responsible for pumping blood to the lungs. During this phase; the heart’s contractions can sometimes break a large clot into several smaller fragments. These fragments are then propelled into the pulmonary artery, the main vessel that carries deoxygenated blood from the heart into the lung tissue. This transit is usually rapid and occurs without symptoms until the clot reaches the narrower vessels of the pulmonary system. 

Blockage within the pulmonary arteries 

As the pulmonary artery branches out into the lungs, the blood vessels become progressively narrower to allow for gas exchange at the microscopic level. The travelling clot eventually reaches a vessel that is too small for it to pass through, causing it to become lodged. This obstruction creates a physical barrier that prevents deoxygenated blood from reaching the air sacs where it would normally pick up oxygen. 

Depending on the size of the clot, the blockage can occur in the main pulmonary artery or in smaller branches deep within the lung. A large clot that straddles the branching point of the main pulmonary arteries is particularly serious as it can block blood flow to both lungs simultaneously. This mechanical obstruction immediately increases the pressure in the pulmonary system, forcing the right side of the heart to work much harder to push blood against the barrier, which can lead to rapid heart strain. 

Impact on oxygenation and lung tissue 

The primary consequence of a pulmonary embolism is a sudden mismatch between the air entering the lungs and the blood available to carry oxygen away. When a section of the lung is blocked off from the blood supply, it can no longer contribute to the body’s overall oxygen levels, leading to a state called hypoxia. This lack of oxygen triggers the sensation of sudden breathlessness and causes the heart rate to increase as the body attempts to compensate for the deficiency. 

In some cases, the lack of blood flow can lead to the death of lung tissue, a condition known as pulmonary infarction. This typically causes sharp chest pain when breathing in, as the inflamed lining of the lung rubs against the chest wall. The severity of the condition is determined by the number and size of the clots, as multiple blockages can significantly impair the heart’s ability to function. UK medical guidelines focus on using anticoagulant medication to stop this process and prevent new clots from following the same dangerous path. 

Conclusion 

A pulmonary embolism forms through a step by step process starting with a clot in the deep veins that travels through the heart and into the lungs. This blockage disrupts the vital exchange of oxygen and puts significant pressure on the cardiovascular system. Recognizing the signs of this formation process is essential for ensuring that medical treatment is administered before the blockage leads to severe complications. 

If you experience severe, sudden, or worsening symptoms, call 999 immediately. 

Authority Snapshot 

This article explains the physiological formation of a pulmonary embolism, based on established UK medical principles. It was written by Dr. Stefan Petrov, a UK-trained physician, and reviewed by the MyPatientAdvice medical team to ensure accuracy and safety. All content is strictly aligned with the current clinical standards provided by the NHS and NICE.