6. Thymus Gland
- It is located between lungs behind sternum on the ventral side of aorta.
- It secretes Thymosins (peptide hormones).
Functions of thymosins
- Differentiation of T-lymphocytes. It provides cell-mediated immunity.
- Promote antibody production for humoral immunity.
- Thymus is degenerated in old individuals. So, production of thymosins decreases. As a result, immune responses of old persons become weak.
7. Adrenal Gland
- It has 2 parts: Adrenal cortex & Adrenal medulla.
a. Adrenal cortex
- It has 3 layers: inner zona reticularis, middle zona fasciculata & outer zona glomerulosa.
- It produces the following corticoid hormones:
Glucocorticoids (mainly cortisol)
- Involved in carbohydrate metabolism.
- Stimulate gluconeogenesis, lipolysis and proteolysis.
- Inhibit cellular uptake and utilization of amino acids.
- Maintain cardiovascular system and kidney functions.
- Cortisol stimulates RBC production.
- Produces anti-inflammatory reactions and suppress immune response.
Mineralocorticoids (mainly aldosterone)
- Regulate the water (body fluid volume), electrolytic balance, osmotic pressure and blood pressure.
- Aldosterone stimulates reabsorption of Na+ & water from renal tubules and excretion of K+ and PO4^3- ions.
Androgenic corticoids
- For growth of axial hair, pubic hair and facial hair during puberty.
- Deficiency of corticoid hormones affects carbohydrate metabolism. It causes acute weakness and fatigue. This condition is called Addison’s disease.
b. Adrenal medulla
- Produces catecholamine hormones such as Adrenaline (epinephrine) & Noradrenaline (norepinephrine).
- They are rapidly secreted in response to stress emergency situations so called emergency hormones (hormones of Fight or Flight).
- These increase alertness, pupillary dilation, piloerection (rising of hairs), sweating, heartbeat, heart contraction and respiratory rate.
- Stimulate glycogenolysis to increase glucose in blood. Also stimulate lipolysis and proteolysis.
8. Pancreas (Islets of Langerhans)
- A composite (heterocrine) gland i.e. exocrine + endocrine.
- Islets of Langerhans are the endocrine part.
- There are about 1-2 million Islets (1-2% of pancreatic tissue).
- α cells and β cells in the islets secrete peptide hormones such as Glucagon and Insulin respectively. They maintain Glucose homeostasis in blood.
Glucagon
- Hyperglycemic factor. It:
- Acts on hepatocytes and stimulates glycogenolysis resulting in an increased blood sugar (hyperglycemia).
- Stimulates gluconeogenesis.
- Reduces the cellular glucose uptake and utilization.
Insulin
- Hypoglycemic factor. It:
- Acts on hepatocytes and adipocytes to enhance cellular glucose uptake and utilization. So, glucose from blood rapidly moves to hepatocytes and adipocytes. Thus, blood glucose level decreases (hypoglycemia).
- Stimulates glycogenesis (glucose converts to glycogen).
- Prolonged hyperglycemia leads to Diabetes mellitus (loss of glucose through urine and formation of harmful compounds like ketone bodies). Treatment is insulin therapy.