The thymus and adrenal glands, along with other tissues secrete hormones, which travel through the blood to reach the target organs. The thymus gland helps in the development of the immune system. It secretes thymosin which aids in cell-mediated immunity by differentiating T-lymphocytes and humoral immunity by secreting antibodies.
The adrenal glands consist of adrenal cortex and medulla. The adrenal cortex secretes mineralocorticoids and glucocorticoids. The primary mineralocorticoid is aldosterone, which maintains the body’s fluid volume, electrolytes, osmotic pressure and blood pressure. The glucocorticoids secreted by the adrenal cortex stimulate gluconeogenesis, lipolysis and proteolysis. Glucocorticoids include cortisol, cortisone and corticosterone hormones. Cortisol helps in anti-inflammatory reactions and also stimulates RBC production and suppresses the immune response. The adrenal cortex also secretes small amounts of testosterone, which stimulates the development of secondary sexual characters.
The adrenal medulla secretes adrenaline or epinephrine and noradrenaline or norepinephrine, collectively called the catecholamines. These hormones increase the strength of heart contractions, heartbeat and the rate of respiration. They are also rapidly secreted in response to stress and emergency situations and are thus called the emergency hormones or hormones of fight or flight. They are also known as 3F glands and 4S glands, where the 3Fs stands for fright, fight and flight and 4Ss stands for sugar metabolism, salt metabolism, sex hormones and source of energy.
The tissues present in the heart, kidneys and gastrointestinal tract are not categorised as endocrine glands, but still secrete hormones. In the heart, the atrial wall secretes atrial natriuretic factor (ANF), which helps decrease blood pressure. When blood pressure increases, ANF is secreted. This dilates the blood vessels, thereby reducing blood pressure. The juxtaglomerular cells present in the kidneys produce erythropoietin that stimulates the formation of RBCs. The gastrointestinal tract secretes hormones, namely gastrin, secretin, cholecystokinin and gastric inhibitory peptide. Gastrin secreted by the gastrin cells of the stomach stimulates the secretion of pepsinogen and hydrochloric acid.
Secretin, secreted by the duodenal wall stimulates the exocrine region of the pancreas to secrete water and bicarbonate ions into the duodenum. Cholecystokinin secreted by the small intestine stimulates the pancreas and liver to secrete pancreatic enzymes and bile respectively. Gastric inhibitory peptides secreted by the small intestine inhibit gastric secretion and motility. Non-endocrine tissues secrete hormones such as angiogenin and vascular endothelial growth factors (VEGF) which are responsible for the repair and regeneration of tissues.