The appearance of bloodshot or red eyes, medically termed conjunctival injection, is a frequent symptom accompanying various systemic illnesses. This visible redness results from changes occurring on the transparent membrane covering the white part of the eye and the inner surface of the eyelids, known as the conjunctiva. While the symptom can be alarming, it most often represents a localized manifestation of the body’s generalized immune response attempting to combat an underlying condition.
The Biological Mechanism of Redness
The white part of the eye, the sclera, is covered by the conjunctiva, a thin layer containing numerous minute blood vessels. When the body encounters an infectious agent or reacts to an allergen, it initiates an inflammatory response involving the release of specific chemical mediators into the bloodstream and surrounding tissues.
One of the primary chemicals released is histamine, a compound stored in mast cells that acts as a local signaling molecule. Histamine’s function in this context is to increase blood flow to the affected area by causing the tiny blood vessels within the conjunctiva to widen, a process called vasodilation. As these capillaries engorge with a greater volume of blood, they become significantly more visible against the white background of the eye.
This increased blood flow is an intentional physiological action designed to facilitate the delivery of immune cells, like white blood cells, and other protective proteins to the site of irritation or infection. The resulting redness is essentially a temporary traffic jam of blood cells and fluid in the eye’s superficial vessels. Furthermore, the increased permeability of these dilated vessels allows immune components to pass more easily from the blood into the surrounding tissue to neutralize the pathogen.
Common Illnesses That Affect the Eyes
Bloodshot eyes frequently accompany viral upper respiratory infections, such as the common cold or the flu. In these cases, the virus can directly infect the conjunctiva, causing a highly contagious condition known as viral conjunctivitis, often characterized by watery discharge. The generalized inflammation and irritation from a cold, including coughing and sneezing, can also contribute to the eye’s redness.
Bacterial infections, while less common in adults than viral causes, also lead to conjunctivitis, but typically present with a thicker, pus-like discharge that may cause the eyelids to stick together. This type of infection requires a different course of treatment than a viral one. Additionally, pressure built up from severe sinus infections can sometimes cause vessels in the eyes to become congested, contributing to the bloodshot appearance.
Allergic reactions are another common cause of red eyes, although this is a non-infectious process involving the same histamine release mechanism. Allergens like pollen or pet dander trigger an exaggerated immune response that manifests as redness, intense itching, and tearing in both eyes. In all these sickness-related scenarios, the redness is a direct or indirect consequence of the body’s defensive inflammatory pathways.
When to Seek Medical Attention
While bloodshot eyes are frequently benign, certain accompanying symptoms signal a need for immediate professional medical evaluation. Any sudden or significant change in vision, such as blurriness, double vision, or the appearance of halos around lights, should be considered an urgent matter. Severe eye pain that goes beyond simple discomfort is a serious red flag, as is extreme sensitivity to light, known as photophobia.
If you experience any of the following symptoms, seek prompt medical care:
- Thick, yellow, green, or brown discharge that constantly reappears or glues the eyelids shut upon waking.
- Redness that is intensely focused in only one eye and rapidly worsens.
- Redness accompanied by a severe headache, nausea, or vomiting.
Home Relief and Comfort Measures
For cases of bloodshot eyes linked to mild illness or irritation, several non-pharmacological strategies can help soothe the discomfort and reduce the redness. Applying a cool compress or a washcloth soaked in cold water over closed eyes can help by causing the superficial blood vessels to constrict, which temporarily reduces the visible engorgement. Over-the-counter artificial tears, or lubricating eye drops, can wash away irritants and provide moisture to relieve dryness and a gritty feeling.
Ensuring adequate hydration supports the body’s overall function and tear production, which maintains eye comfort. It is helpful to avoid known eye irritants such as cigarette smoke, strong chemical fumes, or excessive screen time that can exacerbate dryness. Practicing meticulous hand hygiene is necessary, especially avoiding rubbing the eyes. This prevents mechanical irritation of sensitive tissue and avoids spreading infectious agents.