Hormones

Definition and Classification of Hormones

Hormones are chemical messengers produced by endocrine glands that regulate physiological processes throughout the body. They can be classified based on their chemical structure and mechanism of action.

• Amino acid-based hormones: These hormones use surface cell receptors and typically act via second messenger systems. Examples include insulin and epinephrine.

• Steroid hormones: Derived from lipids, these hormones can enter cells and directly activate DNA, triggering protein synthesis. Examples include cortisol and testosterone.

Key Point: Hormones differ in their mechanisms of action, including neural, hormonal, and neural stimulation.

Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland are central to endocrine system regulation. The hypothalamus controls the pituitary gland, which in turn secretes hormones that affect various target organs.

• Posterior pituitary: Stores and secretes hormones made in the hypothalamus, such as ADH and oxytocin.

• Anterior pituitary: Produces and secretes its own hormones, including GH, FSH, TSH, ACTH, and prolactin.

Example: The hypothalamus secretes releasing hormones that stimulate the anterior pituitary to release specific hormones into the bloodstream.

Major Endocrine Glands and Their Hormones

Various endocrine glands secrete hormones that regulate growth, metabolism, and homeostasis.

• Thyroid gland: Thyroxine (T4) and triiodothyronine (T3) regulate metabolism.

• Parathyroid gland: Parathyroid hormone (PTH) controls calcium levels.

• Adrenal cortex: Mineralocorticoids (aldosterone), glucocorticoids (cortisol), and gonadocorticoids (androgens).

• Adrenal medulla: Epinephrine and norepinephrine for fight-or-flight response.

• Pancreas: Insulin (beta cells) and glucagon (alpha cells) regulate blood glucose.

• Pineal gland: Melatonin regulates circadian rhythms.

• Thymus: Thymosin promotes T lymphocyte maturation.

• Ovaries: Estrogen and progesterone.

• Testes: Testosterone.

Hormone Disorders

Disorders can arise from hormone imbalances or dysfunctions in endocrine glands.

• Diabetes mellitus: Caused by insufficient insulin production or response.

• Diabetes insipidus: Deficiency of ADH.

• Thyroid disorders: Hypothyroidism (low thyroid hormone), hyperthyroidism (excess thyroid hormone), and Graves' disease (autoimmune hyperthyroidism).

• Addison's disease: Adrenal gland dysfunction.

Example: In Graves' disease, autoantibodies stimulate the thyroid gland, causing excessive thyroid hormone production.

Blood

Characteristics and Functions of Blood

Blood is a connective tissue composed of plasma and formed elements. It performs essential functions such as transport, regulation, and protection.

• Characteristics: Blood has a specific pH, temperature, and viscosity.

• Formed elements: Include red blood cells (RBCs), white blood cells (WBCs), and platelets.

• Functions: Transport of oxygen, nutrients, hormones, and waste; regulation of pH and temperature; protection against pathogens.

Example: Hemoglobin in RBCs binds oxygen for transport throughout the body.

Hemostasis

Hemostasis is the process that stops bleeding after injury. It involves vascular spasm, platelet plug formation, and coagulation.

• Vascular spasm: Immediate constriction of blood vessels.

• Platelet plug formation: Platelets adhere to damaged endothelium.

• Coagulation: Cascade involving clotting factors such as prothrombin, thrombin, fibrinogen, and fibrin.

Equation:

Blood Disorders

Blood disorders affect the composition and function of blood.

• Anemia: Reduced RBC count or hemoglobin.

• Polycythemia: Excess RBCs.

• Leukemia: Cancer of WBCs.

• Leukocytosis: Increased WBC count.

• Thrombocytopenia: Low platelet count.

• Hemolytic disease of the newborn: RBC destruction due to Rh incompatibility.

Thrombus vs. Embolus

A thrombus is a stationary blood clot, while an embolus is a clot that travels through the bloodstream, potentially causing blockages.

Blood Types and Antigens

Blood types are determined by the presence of specific antigens and antibodies. Compatibility is crucial for safe transfusions.

Universal donor: Type O. Universal receiver: Type AB.

Additional info: Blood type compatibility prevents immune reactions during transfusions.

Rh Typing

The Rh factor is another antigen that determines blood compatibility. Rh-positive individuals have the antigen; Rh-negative do not.

• If mismatched blood is received: Immune reaction may occur, leading to hemolysis.

• Mother-fetus Rh mismatch: Can cause hemolytic disease of the newborn if the mother is Rh-negative and the fetus is Rh-positive.

Example: Rh incompatibility is prevented by administering Rh immunoglobulin to Rh-negative mothers.