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Secretion – Definition, Types and Clinical Relevance

A secretion is a fluid produced and released by glands or specialized cells in the body, serving specific biological functions such as digestion or immune defense.

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Things worth knowing about "Secretion"

A secretion is a fluid produced and released by glands or specialized cells in the body, serving specific biological functions such as digestion or immune defense.

What Is a Secretion?

In medicine, a secretion refers to a fluid that is actively produced and released by glands or specialized cells in the body. Secretions serve essential biological roles, including aiding digestion, protecting mucosal surfaces, supporting reproduction, and contributing to immune defense. The term derives from the Latin secretum, meaning something set apart or separated.

Secretions are distinct from excretions (waste products such as urine or sweat), as they are purposefully created for specific bodily functions rather than being metabolic waste.

Types of Secretions

The human body produces a wide variety of secretions, classified by their origin and function:

Exocrine secretions: Released by exocrine glands onto body surfaces or into body cavities. Examples include saliva, gastric acid, bile, pancreatic juice, tears, and mucus.
Endocrine secretions (hormones): Released directly into the bloodstream by endocrine glands to regulate bodily functions. Examples include insulin, cortisol, and thyroid hormones.
Paracrine and autocrine secretions: Signaling molecules released by cells that act on neighboring cells or the secreting cell itself.

Composition and Properties

The composition of a secretion depends on its function. Common components include:

Water and electrolytes (e.g., sodium, potassium, chloride)
Proteins and enzymes (e.g., amylase in saliva, pepsin in gastric secretion)
Mucins, which protect mucosal surfaces
Immunoglobulins (e.g., secretory IgA in saliva and breast milk)
Hormones and signaling molecules
Lipids (e.g., in bile)

Key Secretions at a Glance

Saliva

Saliva is produced by the salivary glands and serves to moisten the oral mucosa, begin the digestion of carbohydrates via the enzyme amylase, and protect tooth enamel. An adult produces approximately 0.5 to 1.5 liters of saliva per day.

Gastric Secretion

Gastric secretion contains hydrochloric acid, the enzyme pepsin, and the so-called intrinsic factor, which is necessary for the absorption of vitamin B12. It facilitates the breakdown of food and helps destroy ingested pathogens.

Mucus

Mucus is a viscous secretion that moistens and protects mucosal surfaces throughout the respiratory tract, gastrointestinal tract, and other organs. It forms a physical barrier against bacteria, viruses, and foreign particles.

Bile

Bile is produced in the liver and stored in the gallbladder. It contains bile salts that are essential for the digestion and absorption of dietary fats in the small intestine.

Pancreatic Secretion

Pancreatic secretion contains a range of digestive enzymes (e.g., lipase, proteases, amylase) and bicarbonate, which neutralizes the acidic content entering the small intestine from the stomach.

Clinical Relevance

Changes in the quantity or composition of secretions can indicate or contribute to disease:

Increased mucus production: Commonly occurs in respiratory infections, asthma, and chronic obstructive pulmonary disease (COPD).
Reduced saliva production (xerostomia): May result from medications, radiation therapy, or autoimmune conditions such as Sjogren syndrome.
Pathological secretions: Purulent secretion (pus) indicates bacterial infection; blood-tinged secretions may suggest injury or malignancy.
Hormonal dysregulation: Overproduction or underproduction of endocrine secretions (hormones) leads to conditions such as diabetes mellitus or thyroid disorders.

Diagnosis and Analysis of Secretions

Secretions can be analyzed in the laboratory to help diagnose diseases. Common investigations include:

Sputum analysis in respiratory conditions
Stool examination for digestive enzymes or occult blood
Hormone measurements in blood or urine
Microbiological cultures for pathogen identification

References

Pschyrembel Clinical Dictionary. 268th Edition. De Gruyter, Berlin 2020.
Schmidt, R.F., Lang, F., Heckmann, M. (Eds.): Physiology of Humans. 31st Edition. Springer Medizin, Heidelberg 2010.
World Health Organization (WHO): Human Physiology and Disease Reference Materials. WHO Press, Geneva 2021.

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