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Pneumonia Pathophysiology

Updated: May 24, 2022

Pneumonia is an acute respiratory infection of the lung parenchyma caused by one or co-infecting pathogens. These pathogens cause the lung parenchyma (alveoli) to become inflamed and fill with pus and fluid, limiting oxygen intake and making gas exchange ineffective.

In these notes, we’ll be going over the Pathophysiology of Pneumonia and its manifestations, but before getting into that make sure that you have understood the Anatomy & Physiology of the Respiratory System.


Classification of Pneumonia

Pneumonia is an umbrella term for a group of respiratory infections caused by several different microorganisms. These range from bacteria, mycobacteria, fungi and viruses, and they can all be acquired in different environments. There are many ways to classify Pneumonia and like a lot of things in this world, the researchers can’t seem to agree.

Some want to classify it depending on the infective pathogen, others want to structure it with severity and some have even suggested that we classify pneumonia depending on which areas it affects. However, when it comes to nursing schools, most have agreed to classify Pneumonia according to the environment and that’s what we’ll be using here today.

There are four main types of Pneumonia:

  1. Community-Acquired Pneumonia: This occurs within the community setting, meaning areas outside a hospital or care home. It also includes patients that have developed pneumonia within the first 48 hours of being admitted in a hospital or care home.

  2. Health Care-Associated Pneumonia: The infection would occur in patients who are not admitted to hospitals but would still be in contact with health care professionals. This includes patients in nursing homes, patients receiving home-based care, out-patient clinics, family members with a multidrug-resistant bacterial infection and patients who would have been in an acute hospital for more than 2 days within the past 3 months.

  3. Hospital-Acquired Pneumonia: This occurs in patients who have been hospitalised for more than 48 hours.

  4. Ventilator-Associated Pneumonia: This infection develops at least 48 hours after endotracheal intubation.

There’s a fifth category of pneumonia that can technically happen in any of the above-mentioned environments and that is:

  1. Aspiration-Pneumonia: This type of infection occurs because food, saliva, liquid or vomited is inhaled into the airways instead of passing through the oesophagus and into the digestive system.

The Causes of Pneumonia

As we mentioned above, Pneumonia can be caused by several different microorganisms but bacterial infections seem to dominate most of the infections. Table 1 gives an overview of the most common bacterial pathogens found in pneumonia.

Bacterial Pathogen

Community-Acquired Pneumonia

S. Pneumoniae

S. Aureus


Gram Negative Bacilli

Health Care Associated Pneumonia

Methicillin-Resistant Staphylococcus Aureus (MRSA)

Vancomycin-Resistant Enterococcus (VRE)

Hospital-Acquired Pneumonia

Enterobacter Species

E. Coli

H. Influenza

Klebsiella Pneumoniae

Pseudomonal Aeruginosa


S. Pneumoniae

Ventilation Pneumonia

Methicillin-Resistant Staphylococcus Aureus (MRSA)

Vancomycin-Resistant Enterococcus (VRE)

Aspiration Pneumonia

S. Aureus

Streptococcus Species

Gram Negative Bacilli

Aside from bacterial infections, Community-Acquired Pneumonia is often found to be caused by viral pathogens including COVID-19, Cytomegalovirus, Herpes Simplex Virus, Adenovirus and Respiratory Syncytial Virus. And lastly, Aspiration-Pneumonia can be caused by exogenous pathogens such as gastric contents, food and toxic environmental chemicals.

The Pathophysiology of Pneumonia

In healthy individuals, the upper parts of the airways are designed to block off any harmful particles. This mechanism keeps the lower end of the respiratory tract sterile and safe from pathogens. In Pneumonia, however, this screening mechanism would have been damaged by previous or ongoing illnesses and harmful pathogens manage to find their way into the lower respiratory tract.

In addition, pathogens can also enter the lungs through the circulatory system. Bloodborne pathogens can travel through the body until they reach the pulmonary circulation. There they enter the pulmonary capillary bed and settle at the bottom of the lungs.

Once the pathogens are stuck,  some of the alveoli become inflamed and they fill up with a thick exudate that interferes with the exchange of oxygen and carbon dioxide. Naturally, the body has to fight this off so it sends Neutrophils into the alveoli, but in doing it fills the last few empty spaces of air in the alveoli. Following that, the mucosa starts to swell up causing mucosal oedema and partially occluding the bronchi, and in some cases, if the patient has other respiratory conditions bronchospasm would occur.

All of these factors limit the oxygen intake into the affected lung and create an imbalance in the Ventilation-Perfusion Ratio. Since the lungs do not have enough oxygen in them, they cannot fully oxygenate the venous blood entering the pulmonary circulation. The poorly oxygenated blood will then enter the left side of the heart and be pumped around the body. But because it has very little oxygen to distribute Arterial Hypoxemia develops.

This problem can occur in different areas of the lungs, depending on where the pathogens settle. If it attacks a large portion of one or more lobes it is referred to as Lobular Pneumonia. But if it attacks many different patches along the bronchi and stretches out to the nearest parenchyma it is called Bronchopneumonia.

The Symptoms of Pneumonia

As we’ve said earlier, Pneumonia can result from many different pathogens or environments and each type will have slightly different symptoms. A patient with streptococcal pneumonia would typically present with:

  1. Rapid onset of chills

  2. Fever spike (starting from 38.5 to 40.5°C)

  3. Pleuritic chest pain that becomes worse with deep breathing and coughing

  4. Tachypnoea

  5. Respiratory Distress

  6. Blood-tinged sputum (also present in Staphylococcal and Klebsiella pneumonia)

On the other hand, patients with a viral infection, mycoplasma infection or those infected with Legionella are likely to present with

  1. Pulse Temperature Deficit (ie. The pulse rate usually increases if a patient has a fever, but in this case, the patient will have a fever with a low pulse).

Some patients might have a more gradual onset of symptoms, starting with manifestations of an upper respiratory tract infection:

  1. Blocked nose

  2. Sore throat

  3. Headache

  4. Myalgia

  5. Low-grade fever

  6. Orthopnea (Dyspnoea when lying flat)

After a couple of days, the patient would develop mucopurulent sputum and if the infection is left untreated it would progress and develop

  1. Flushed cheeks

  2. Discoloured nail beds (central cyanosis)

Patients with cancer or patients on immunosuppressant medications would normally present with:

  1. Fever

  2. Crackles

  3. Lung consolidation (evident by Tactile Fremitus, Dull percussions, Bronchial breath sounds, Egophony and Whispered Pectoriloquy)

COPD Patients would already have several respiratory issues and in some cases, purulent sputum or slight worsening of their symptoms would be the only signs of Pneumonia.

The Risk Factors of Pneumonia

Patients that have chronic conditions, particularly ones that increase the production of mucus, interfere with lung function or suppress their immune system have a much higher chance of acquiring pneumonia. This is because their defence mechanism would already be compromised and so it becomes much easier for pathogens to access the lower respiratory tract.

The chance of acquiring pneumonia is higher if a patient has:

  1. COPD

  2. Asthma

  3. Cystic Fibrosis

  4. AIDS

  5. Diabetes

  6. Renal Impairment

  7. Heart Failure

The frequency of pneumonia has also been higher in patients that are:

  1. Bedbound

  2. Intubated

  3. Hospitalised

  4. Tobacco smokers

  5. Drug Addicts

  6. Alcoholics

  7. On Antibiotic Therapy

  8. Older in age

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  1. Mackenzie G. (2016). The definition and classification of pneumonia. Pneumonia (Nathan Qld.)8, 14.


  3. Hinkle, Janice. Brunner & Suddarth’s Textbook of Medical-Surgical Nursing . Wolters Kluwer Health. Kindle Edition.

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