The word “stoma” refers to two distinct structures in biology and medicine, defined by their unique location and function. In botany, it is a microscopic pore acting as a gateway for gas exchange in plants. In human anatomy and surgery, a stoma is a surgically created opening on the body’s surface designed to divert waste products. Understanding their locations clarifies their fundamental roles in sustaining life.
The Botanical Stoma: Location in Plants
The botanical stoma, pluralized as stomata, is a microscopic pore complex located primarily on the epidermis of leaves, though they can also be found on stems and other green parts of the plant. This structural placement on the outer layer allows for direct interaction with the atmosphere. Each pore is bordered by a pair of specialized cells known as guard cells, which swell and shrink to regulate the opening and closing of the pore.
In most terrestrial plant species, the majority of stomata are concentrated on the underside of the leaves. This positioning is termed hypostomatous, and it is a common adaptation in plants that grow in direct sunlight. Some plants, such as those with floating leaves like water lilies, have stomata exclusively on the upper surface to access the air above the water.
The density of these pores can vary widely. In dicot plants (broadleaf trees and shrubs), a higher number of stomata are typically found on the lower surface compared to the upper surface. Monocot plants, including grasses, often have a more equal distribution on both the upper and lower surfaces of their leaves.
The Medical Stoma: Placement on the Body
The medical stoma is a surgically established opening created on the abdominal wall to allow the passage of bodily waste. This opening is formed by bringing a segment of the intestine or urinary tract through the abdominal muscles and stitching it to the skin. The exact placement depends entirely on the type of organ being diverted.
Surgeons map out the location pre-operatively to ensure the stoma is situated on a flat, manageable surface, away from bony prominences, scars, and the patient’s waistline. A colostomy, which diverts the large intestine (colon), is commonly placed on the left side of the lower abdomen.
Types of Stomas
An ileostomy involves diverting the small intestine (ileum) and is most often positioned on the right side of the abdomen. For a urostomy, which diverts urine, a small section of the intestine is used to create a conduit. The urostomy stoma is usually placed in the lower right quadrant of the abdomen.
The stoma generally appears as a round or oval protrusion of moist, pink or red tissue, similar to the lining inside the mouth. The precise spot is individualized to promote ease of care and proper appliance adherence. The size of the opening can measure approximately one to two inches in width, and it may protrude slightly or lie flatter against the skin.
Function and Positional Necessity
For the plant, the location of stomata is a trade-off between maximizing \(\text{CO}_2\) intake for photosynthesis and minimizing water loss through transpiration. By concentrating the pores on the underside of the leaf, the plant reduces direct exposure to sunlight, wind, and higher temperatures. This decreases the rate of water evaporation.
This strategic placement ensures that the plant can efficiently exchange gases while maintaining a stable internal water balance. The guard cells then regulate the pore size in response to light, water availability, and internal \(\text{CO}_2\) levels.
In the human body, the placement of the surgical stoma on the abdominal wall is necessary for surgical access and patient management. The abdomen provides the most direct and safest route for a surgeon to bring the internal digestive or urinary tract to the body’s exterior. The chosen location must also be easily accessible for the patient to manage the external collection pouch.
The specific quadrant selection ensures that the correct section of the bowel or urinary tract is diverted. An accessible, flat location allows the ostomy appliance to form a secure seal, preventing leakage.