Glands are organs that have the ability to synthesize and secrete special biologically active substances. They are called secrets, and the function itself is called secretory.
The human body has a huge number of different glands, but most of them are microscopic and only a few are relatively large in size. Microscopic glands in the walls of the tubular organs of the digestive, respiratory, urinary and reproductive systems produce mucus, which moisturizes and protects the walls of the cavity, which is why the inner lining of the tubular organs is called mucosa. The single-celled glands (goblet cells), which are part of the epithelium of the gastrointestinal tract, and other small digestive glands produce enzymes needed to digest food. The numerous small glands in the skin are sweat and sebaceous glands. With sweat and sebum, harmful, unnecessary compounds are removed from the body.
In those cases when the body requires a large amount of a special substance (secret) to perform its functions, complex large glands, isolated from other organs, are engaged in its production. Such glands are, for example, the female mammary gland, pancreas , lacrimal glands, large salivary glands, etc.
The structure of the glands
All multicellular glands have a similar structure: they consist of an accumulation of “working” cells specializing in the secretion of various substances (the so-called parenchyma), and supporting cells that form the framework, or stroma, of the gland. The stroma gives the gland its shape, nerves and blood vessels pass through it, delivering “building material” to the working cells. Depending on the origin, the nature of the secretion produced, the presence of the excretory ducts, the glands are divided into groups.
Types of glands
Most of the glands have excretory ducts through which the secretion enters the surface of the body or mucous membranes. Such glands are called exocrine (exo – “outward”, crino – “release”), or glands of external secretion . These include all glands of the skin, lacrimal, salivary glands, liver, etc. Glands that do not have excretory ducts and secrete secretions (hormones) directly into the blood are called endocrine (endo – “inside”), or endocrine glands . Hormones are highly active substances that, in very small quantities, can affect various bodily functions. The endocrine glands include the pituitary gland, pineal gland, adrenal glands, thyroid, parathyroid and thymus (thymus) glands. The sex glands (ovary and testicle) and the pancreas belong to the glands of mixed secretion, because possess both exocrine and endocrine functions.
Inside the gland, the parenchymal cells are grouped into areas of a certain shape, depending on which the alveolar, tubular and alveolar-tubular glands are distinguished. They can be simple or branched. For example, alveolar glands can consist of a single vesicle, or alveoli (simple alveolar gland), several alveoli (branched alveolar), or a large number of alveoli forming clusters (complex alveolar). In the tubular glands, the main structural component is the tubule, in the alveolar-tubular – both the vesicle and the tubule. As a rule, large glands have a complex alveolar-tubular structure, which allows them to produce a large volume of secretion.
The structure of the salivary glands
The salivary glands, the ducts of which open into the oral cavity, belong to the digestive glands and produce substances necessary for the digestion of food. Small salivary glands are scattered throughout the mucous membrane of the tongue and mouth: they are found on the lips, cheeks, palate, gums. These glands continually produce small amounts of saliva to hydrate the oral mucosa. There are three pairs of large salivary glands: parotid, submandibular and sublingual (Fig. 1). They are located outside the mouth, but their ducts open into it.
The parotid gland , the largest, weighs about 30 g and is located on the lateral surface of the face in front and below the auricle. The excretory duct of the parotid gland goes under the skin across the cheek, then pierces the buccal muscle and opens on the inner surface of the cheek at the level of the second upper large molar. The submandibular gland weighs 15 g and is located under the skin in the area of the floor of the mouth (in the so-called submandibular triangle). Its excretory duct opens into the oral cavity on the hyoid papilla on the side of the frenum of the tongue. The sublingual gland (weight – about 5 g) is located in a fold of the mucous membrane at the bottom of the oral cavity. The main duct of the sublingual gland opens together with the duct of the submandibular gland on the sublingual papilla, and several small ducts have openings along the sublingual fold of the mucous membrane.
Large salivary glands have a lobular structure. Each lobule is alveolar-tubular. Connecting with each other, the tubes form a system of outflow ducts, which merge into a common excretory duct. In newborns, the salivary glands are poorly developed, their rapid growth occurs in the period from 4 months to 2 years. An increase in the size of the large salivary glands is observed up to 25-30 years, and after 55-60 years they decrease.
During the day, small and large salivary glands secrete from 0.5 to 2 liters of saliva, consisting mainly of water (up to 99.5%), salts, amylase enzymes and some others, mucus, bactericidal substance lysozyme and immunoglobulins. The main function of saliva is to wet food and start digesting it. Under the action of saliva enzymes in the mouth, the breakdown of carbohydrates begins. The mucus in the saliva makes it easier to swallow. Lysozyme * disinfects the oral cavity. Saliva provides the dissolution of nutrients and the flow of their molecules for analysis into the taste buds of the tongue. The composition of saliva differs depending on the glands producing it. The parotid gland and small glands of the tongue secrete liquid saliva, which is rich in enzymes. The glands located at the root of the tongue and palate secrete a mucous secretion rich in mucin. The submandibular and sublingual glands, small glands of the lips and cheeks produce mixed saliva. The enzymatic composition and properties of saliva change with the age of a person, depending on the diet and type of food.
Salivation is a reflex act and increases already at the sight of food, in response to its smell, and even when thinking about food. The quality of food affects the amount and properties of saliva: the harder and drier the food, the more saliva is produced. The study of the reflex function of salivation in dogs by the great Russian scientist I.P. Pavlov formed the basis of the scientific direction he created – the physiology of higher nervous activity. I.P. Pavlov developed a conditioned reflex in dogs by first combining the delivery of food with an audio or visual signal, and then observing the production of saliva in response to the signal without presenting food. The conditioned reflex is based on the formation of neural connections between the centers of the brain .
The center of salivation is located in the medulla oblongata. It is to this center that a signal from receptors in the oral cavity comes when food enters the tongue. Here, even before the food enters the mouth, signals from the olfactory, visual and even auditory centers, carrying information about the smell, type and just the name of the food, arrive through the nerve connections. Therefore, salivation begins in advance, naturally, if the previous experience has already developed a corresponding conditioned reflex in a person. From the salivary center, the command for the production of saliva is transmitted to the glands along the autonomic nerves, while the parasympathetic nerves stimulate the secretion of a large amount of saliva, and the sympathetic ones reduce salivation and thicken saliva. Inhibition of salivation, leading to dry mouth, can be caused by pain, negative emotions, mental stress. On the contrary, profuse salivation causes poisonous substances, suffocation.
The type of food and the beginning of its processing in the oral cavity reflexively stimulate the secretion of gastric juice. Therefore, it is so important to properly organize food, observing all the “rituals” prior to eating, and paying attention to the quality and attractiveness of the foods consumed.
Lacrimal gland
Refers to the organs protecting the eyes and is part of the lacrimal apparatus. In structure, it is an alveolar-tubular gland. The lacrimal gland is located at the upper outer edge of the orbit (Fig. 2). The short excretory ducts of the lacrimal gland (10–12) open into the so-called conjunctival sac, formed by a thin transparent membrane ( conjunctiva ) that covers the outer surface of the eyeball and passes to the inner surface of the eyelids. Flowing down from above to the inner corner of the eye (to the nose), tears moisten the conjunctiva, wash away dust particles and neutralize microorganisms. Without tears, the conjunctiva and the cornea can dry out – the refractive power of the cornea is impaired. From the inner corner of the eye (lacrimal lake), tears flow through two lacrimal canals into the lacrimal sac , the lower end of which passes into the nasolacrimal duct , which opens into the nasal cavity. Therefore, tears eventually enter the nasal cavity, moisturizing its mucous membrane, and with profuse lacrimation, a person begins to blow his nose.
Every day, the lacrimal glands produce up to 10 ml of tears. This liquid has a slightly alkaline reaction, consists mainly of water and contains about 1.5% sodium chloride, 0.5% albumin protein, lysozyme and mucus. Due to the presence of lysozyme, tears have bactericidal properties. With tears, substances are released from the body that are formed during nervous tension or stress.
Lachrymation occurs continuously, stopping during sleep. This is a reflex process. Blinking movements of the eyelids promote the outflow of tear fluid. The secretion of the lacrimal glands increases with mechanical irritation of the cornea, with emotional arousal (anger, pain, joy). The pituitary hormone prolactin, which is important for the female body, promotes the production and secretion of tears, so women cry more often than men.
Using the example of the salivary and lacrimal glands, you got acquainted with the structure and work of a large group of organs – the glands of external secretion. They produce and secrete through the ducts substances that are important for the normal functioning of the human body.