What is sweat?
Sweat is the fluid secreted on the surface of skin by the sweat glands present in deeper parts of the skin. Sweat is an essential part of normal bodily function. This liquid made up of water, sodium and a mix of minerals acts as the body's radiator fluid. When internal temperature rises, glands in human skin release sweat onto its surface, where it evaporates, cooling the skin and lowering the body temperature.
What are Sweat glands?
Sweat glands are the chief cutaneous glands which secrete a fluid called sweat. These are very fine, unbranched, lubular structures consisting of two following parts:
i) Secretory part
ii) Excretory part
Secretory part of sweat glands
These are the a lower coiled portion lying in the deeper parts of dermis.
Excretory part of sweat glands
These are elongated duct portion passing outward through dermis and epidermis to open on the surface of the skin through a minute pore. There are numerous capillaries surrounding the glands.
Description of sweat glands
Each of the sweat glands are made up of a single layer of epithelial cells lining a central lumen. In the secretory part, the cells are cubical or columnar which secrete the sweat into the lumen whereas in the duct portion, the cells are of simple epithelial type. The lining cells are externally enclosed by a basement membrane. In between the basement membrane and the lining epithelial cells, there are some contractile smooth muscle cells known as myoepithelial cells. Contraction of these cells helps in expulsion of sweat from the glands to the skin surface. Sweat glands are of two types, those are
Eccrine Sweat Glands
The eccrine sweat glands are the chief sweat glands as they constitute the majority and are found all over the body surface. However, they are more numerous on the palm, sole and forehead. The eccrine glands are smaller in size and they secrete a dilute and watery sweat.
Apocrine Sweat Glands
The apocrine glands are relatively fewer in number but larger in size. They are found only in some special regions such as axilla, areola of the nipples, around the genitalia etc. The apocrine sweat glands are embedded more deeply in the skin, mostly in the hypodermis.They secrete a viscid and oily sweat. Bacterial action on the apocrine sweat produces the characteristic foul body odour. The apocrine sweat glands become active from puberty, hence they are supposed to be concerned with sexual functions. The mammary glands and the ceruminal glands of the external ear are modified apocrine glands.
Composition of Sweat
The sweat mainly consists of the secretion of eccrine sweat glands. It is a watery fluid containing water (99.5%) and solids dissolved in it. The solids include various inorganic and organic materials. The chief inorganic constituent of sweat is sodium chloride, others comprise potassium, calcium, sulphur, iodine etc. The organic constituents present in sweat are urea, uric acid, lactic acid, creatinine, glucose etc. Thus, it is apparent that the sweat also contains various excretory materials e.g., inorganic salts, non-protein nitrogenous (NPN) compounds and many other metabolites that are found in urine.
Functions of sweat and Mechanism of sweating
Sweating is elicited mainly by a rise in the ambient temperature. The chief purpose of this is to maintain body temperature in hot climate. When sweat is evaporated, it takes away some heat (the latent heat) from the body and exerts a cooling effect on the body. This prevents rise of body temperature in hot climate in spite of rise in the environmental temperature. Sweat is easily evaporated in dry weather or low atmospheric humidity. But in hot humid days, the sweat is not easily evaporated as a result we feel hotter.
Sweating is regulated by neural reflexes. Sweat glands are innervated by sympathetic efferent fibers. These fibers are characteristic in that they are cholinergic in nature different to other sympathetic fibers which are noradrenergic. The anterior part of hypothalamus contains a heat sensitive centre which is excited by increase in ambient temperature or blood temperature as in muscular exercise and in turn stimulates sweat secretion via the cholinergic sympathetic nerves. Emotional and gustatory stimuli may also stimulate this hypothalamic centre causing sweat secretion.
In addition to thermo-regulation, the sweating mechanism also serves an excretory function because sweat contains water, salts, urea and other materials that are usually excreted through urine. The excretory function of sweat is very important, particularly in summer, when due to profuse sweating the urine formation is significantly reduced.