In the human body system or in many animals for that matter, there are certain collection of cells which are named as ‘glands’ which will produce a secretary substance that will either be secreted directly to the blood or to a cavity within the body or on to the skin surface. These glands can be located deep within the body as in the case of pancreas and liver or else can be located near the skin surface such as in sweat and sebaceous glands. The sizes of the glands and the way it function along with their product can vary and according to these changes the glands can be categorized in many different ways.
These glands will secrete hormones in to the circulation and would not be having a tube structure for the secretions to flow. Instead, the secretions will travel through its membranes and will enter the blood stream.
These glands will have its own tubular secretary passages and would make its secretions to go through these passages and appear on the surface of a cavity or on the skin. Depending on the way they secrete their productions, these glands can be further classified in to apocrine, holocrine and merocrine glands.
The apocrine glands will give away part of its cellular structure at the time of secretion and sweat glands can be taken in as one of the examples. Holocrine glands will give away the entire cell at the time of secretion and sebaceous glands are examples for such kind of activity. The merocrine glands on the other hand do not get self mutilated but instead makes use of a mechanisms known as ‘exocytosis’ in secreting their secretions and mucous glands are examples for such action.
When considering the secretions that these glands are making, some of it can be heavy in proteins such as in the case of serous glands which secretes a watery substance. Certain glands will produce a secretion which is thick and often is composed of carbohydrate rich substances. Mucous glands can be named as a possible example for such a gland. Sebaceous glands will secrete a more lipid rich secretion when compared to others.
Lastly, the structure of a gland can also be classified according to their shape and when the glands maintain a tubular structure it can be named as a ‘tubular gland’ whereas if it contains a sac like component for storage of secretions, it can be named as an alveolar gland.