Acute inflammation is the immediate and early response to injury, characterized mainly by vascular changes with delivery of leucocytes to the site of injury to clear the invaded bacteria and damaged tissue.

Visit: Inflammation ; Inflammatory cells in acute and chronic inflammation ; Types of Acute Inflammation ; Chemical Mediators ; Chronic Inflammation ; Wound Healing .

Cardinal signs: 1. Rubor (redness) due to dilatation of arterioles

2. Calor (heat)

3. Dolor (pain) due to pressure on nerve endings by edema fluid and chemical mediator bradykinine

4. Tumor (swelling) due to edema.

5. Functio laesa (loss of function) due to inhibition of movement by pain and tissue necrosis.

Sequence of vascular changes:

1. Transient vasoconstriction of arterioles may be due to:

i) direct mechanical stimulation of capillaries

ii) local axon reflex (vasoconstrictor fibres).

2. Persistent vasodilation- first arterioles, followed by opening of capillary beds, caused by chemical mediators.

3. Increased blood flow mainly due to arteriolar dilatation and opening of capillary beds.

4. Slowing of the circulation (Stasis) due to

i) Increased viscosity of blood due to fluid loss

ii) Swollen capillary endothelium due to toxin.

5. Flow of protein-rich fluid into extra-vascular space called inflammatory exudates.

Inflammatory Exudate: due to:

i) Increased intra-vascular hydrostatic pressure.

ii) Increased permeability due to endothelial contraction with increased intercellular gaps, of the venules.

iii) Direct endothelial injury and detachment, involving venules, capillaries and arterioles.

iv) Increased tissue osmotic pressure.

6. Margination and immigration of leucocytes:

Normally red and white cells flow intermingled in the center of the vessel forming axial stream, separated from vessel wall by a clear cell-free plasmatic zone.

Due to slowing of the circulation, leucocytes fall out of the axial stream and come to plasmatic zone known as margination of leucocytes.

Leucocytes gradually adhere to the vessel wall known as pavementing of the leucocytes.

Leucocytes squeeze between the endothelial cells by the process of diapedesis and migrate through the vessel wall into the interstitial tissue, a process known as emigration of leucocytes (first neutrophils followed by monocytes and lymphocytes).

Emigrated leucocytes move towards the site of injury under the influence of chemotactic agents & the process is called chemotaxis.

Termination of Acute Inflammation:

1. Complete resolution- when minimal or no tissue damage and the involved tissues regenerate, there is restitution to normal structure. Example: Lobar pneumonia, acute infective hepatitis.

2. Healing by fibrosis - when there is substantial tissue destruction and the involved tissues do not regenerate. Example: Acute rheumatic carditis

3. Dissemination - cellulites, septicemia.

4.Progression to chronic inflammation-Example: Acute osteomyelitis