A pulmonary hematoma is a collection of blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 within the tissue

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 of the lung

Lung

The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

. It may result when a pulmonary laceration

Pulmonary laceration

A pulmonary laceration is a chest injury in which lung tissue is torn or cut. An injury that is potentially more serious than pulmonary contusion, pulmonary laceration involves disruption of the architecture of the lung, while pulmonary contusion does not...

 fills with blood. A lung laceration filled with air is called a pneumatocele

Pneumatocele

A pneumatocele, or pneumatocyst is a cavity in the lungs filled with air that may result from pulmonary trauma. A pneumatocele results when a lung laceration, a cut or tear in the lung tissue, fills with air. A rupture of a small airway creates the air-filled cavity. Pulmonary lacerations that...

. In some cases, both pneumatoceles and hematomas exist in the same injured lung. Pulmonary hematomas take longer to heal than simple pneumatoceles and commonly leave the lungs scarred. A pulmonary contusion

Pulmonary contusion

A pulmonary contusion is a contusion of the lung, caused by chest trauma. As a result of damage to capillaries, blood and other fluids accumulate in the lung tissue. The excess fluid interferes with gas exchange, potentially leading to inadequate oxygen levels...

 is another cause of bleeding within the lung tissue, but these result from microhemorrhages, multiple small bleeds, and the bleeding is not a discrete mass but rather occurs within the lung tissue. An indication of more severe damage to the lung than pulmonary contusion, a hematoma also takes longer to clear. Unlike contusions, hematomas do not usually interfere with gas exchange

Gas exchange

Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...

 in the lung, but they do increase the risk of infection

Infection

An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 and abscess

Abscess

An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...

formation.