Gland is the organ that secretes liquid chemicals having various purposes.
Exocrine gland secretes its product onto an epithelial surface by way of a duct.
Exocrine gland includes the salivary gland (Fig. 4-10) (Fig. 4-12) of oral cavity, the sweat gland (Fig. 15-13) and sebaceous gland (Fig. 15-17) of skin, and the female breast (Fig. 15-21).
The other type, endocrine gland lacking the duct secretes its product (hormone) into the bloodstream. In medicine, the inside of blood vessel is internal; that is how endocrine gland is named.
Unlike the exocrine gland’s product, the hormone can go throughout the whole body to affect the distant and multiple organs. Instead, effect of the hormone slowly appears.
In addition to the endocrine organs in the cartoon above, several other organs that are part of other body systems, such as bone, kidney, liver, and heart, have secondary endocrine functions.
< Endocrine organs >
Fig. 9-7. Pituitary gland.
The pituitary gland is placed on the sphenoid bone (Fig. 1-8) and is most multi-functioned among the endocrine glands.
Even if the pituitary gland (hypophysis) has size of a pea and weighs 0.5 gram (Fig. 13-24) (Fig. 13-35), it is divided into the adenohypophysis and neurohypophysis.
Hormones secreted by the adenohypophysis act on other endocrine glands to release hormones, causing numerous physiological responses.
As the adenohypophysis hormone, prolactin enables female to produce milk (Fig. 15-23) while growth hormone stimulates growth by cell division (Fig. 8-3).
The neurohypophysis secretes the following hormones. Oxytocin induces the uterus contraction during labor (Fig. 8-42). Antidiuretic hormone retains water in the body, by increasing reabsorption of water in the kidney (Fig. 6-11).
Fig. 9-12. Pituitary gland influenced by hypothalamus.
Hormones are released from the pituitary gland under the influence of the hypothalamus. Hormones of the hypothalamus are secreted to the adenohypophysis via a special blood vessel and to the neurohypophysis by way of the neuron. So, the pituitary gland is located close to the hypothalamus (Fig. 13-24).
Suppose the endocrine system is an athletic team. The pituitary gland (captain of athletes) influences other glands (other athletes) (Fig. 9-9) and is influenced by the hypothalamus (coach of team).
The pineal gland produces melatonin, which controls the sleep and wake cycle.
Fig. 9-14. Thyroid gland.
The bilateral lobes of the thyroid gland are connected by isthmus. It is a prevalent misunderstanding that the thyroid gland is located anterior to the thyroid cartilage. In fact, the thyroid gland is situated anterior to the cricoid cartilage (Fig. 5-10) and the trachea (Fig. 5-22).
Thyroid hormone increases the metabolism and has effects on almost all body tissues. For example, the appetite, absorption of food, and intestine motility are all influenced by thyroid hormone.
The parathyroid glands on the posterior surface of the thyroid gland may not be clearly seen during dissection (Fig. 9-14).
Parathyroid hormone stimulates a macrophage to break down bone and release calcium. It increases calcium absorption in the gastrointestinal tract (Fig. 4-45). It promotes calcium reabsorption in the kidney (Fig. 6-11).
The pancreas is an exocrine gland because it secretes pancreatic juice into the duodenum through the pancreatic duct (Fig. 4-53). The pancreas is also an endocrine gland because it secretes hormone directly into the blood vessel.
High blood glucose level stimulates the pancreas to release insulin. Insulin causes the liver to convert glucose in blood into glycogen to be stored; it also causes the skeletal muscle and fat to uptake glucose from blood. Result is the decrease of the blood glucose level. Reversely, low blood glucose level stimulates the pancreas to release glucagon. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the blood. Thus, insulin and glucagon keep the blood glucose level stable.
Diabetes is a metabolic disease in which the patient has high blood glucose level over a prolonged period. Diabetes is due to either the impaired insulin secretion from pancreas or the decreased ability of insulin to act on target tissues (liver, muscle, and fat).
The adrenal gland is composed of the adrenal cortex and adrenal medulla. It reminds us of the renal cortex and renal medulla (renal pyramid) (Fig. 6-14).
The adrenal cortex produces aldosterone, which increases the reabsorption of sodium in the kidney (Fig. 6-11), which results in high blood pressure. On the other hand, antidiuretic hormone from the neurohypophysis increases the reabsorption of water, causing the same effect (Fig. 9-11).
Cortisol, another hormone of the adrenal cortex, suppresses immunity (Fig. 12-3) to cause anti-inflammatory effects.
A hormone, ADRENALine is secreted by the ADRENAL medulla. Adrenaline induces the rapid beat of heart as well as the constriction of blood vessels. The action of adrenaline is a part of that of sympathetic nerve (Fig. 13-61).
Androgen and estrogen are made by the testis and ovary; they are made by the adrenal cortex (Fig. 9-22) as well. They induce the development of secondary sex characteristics. Although androgen is described as a male sex hormone, both male and female have varying degrees of androgen, as is also true of estrogen.
The endocrine system and autonomic nerve make appropriate reaction to the environment, both keeping homeostasis of the body. The endocrine system and autonomic nerve are influenced by the hypothalamus (Fig. 9-12) (Fig. 13-15). Their difference is that the endocrine system generates slower effects (Fig. 9-5) than the autonomic nerve does.