Composed of glands that secrete hormones into the circulatory system . Hormones are secreted in minute amounts into

2. Characteristics of theEndocrine System • Composed of glands that secrete hormones into the circulatory system. • Hormones are secreted in minute amounts into the interstitial space. • Hormones eventually enter the circulatory system and arrive at specific target tissues.

3. Characteristics of theEndocrine System • Functions are similar to the nervous system. • Differences; • Amplitude-modulated vs. Frequency-modulated • Response of target tissue to hormones is usually slower and of longer duration than that to neurons.

4. Chemical Structureof Hormones • Peptides & Proteins: Most hormones are either peptides or proteins and are usually referred to as peptide hormones. • Amines: Amine hormones are derivatives of the amino acid tyrosine.  • Lipids & Steroids: Steroid hormones are produced by the adrenal cortex and the gonads. 

6. Control ofSecretion Rates • Hormones control the rates of many activities in the body. • The rate at which each hormone is secreted is controlled by a negative feedback mechanism. • Three major patterns of regulation: • Non-hormone substance (e.g. insulin)

8. Control ofSecretion Rates • Hormones control the rates of many activities in the body. • The rate at which each hormone is secreted is controlled by a negative feedback mechanism. • Three major patterns of regulation: • Non-hormone substance (e.g. insulin) • Stimulation by the nervous system (e.g. epinephrine)

10. Control ofSecretion Rates • Hormones control the rates of many activities in the body. • The rate at which each hormone is secreted is controlled by a negative feedback mechanism. • Three major patterns of regulation: • Non-hormone substance (e.g. insulin) • Stimulation by the nervous system (e.g. epinephrine) • Hormone from another endocrine tissue (e.g. TRH, TSH)

12. Transport and Distributionin the Body • Hormones are dissolved in the blood plasma and transported in free form or bound to a protein carrier. • As a result, hormones can be distributed throughout the body relatively quickly. • Hormones diffuse from the capillary to the interstitial space.

13. Transport and Distributionin the Body • Lipid-soluble hormones diffuse through the walls of all capillaries. • Water-soluble hormones must pass through pores.

14. Metabolism and Excretion • Hormones are only active in the body for a certain time because they are destroyed and eliminated shortly after they are secreted. • Half-life = length of time that it takes to eliminate half of the total amount of hormone that was secreted.

15. Metabolism and Excretion • Water-soluble hormones have relatively short half-lives because they are rapidly broken down by enzymes. • These hormones normally have concentrations that increase and decrease rapidly in the blood. • They generally regulate activities that have a quick onset and a short duration.

16. Metabolism and Excretion • Lipid-soluble hormones are usually bound to protein carriers. • The rate at which these hormones are broken down is greatly reduced. • Therefore, these hormones have longer half-lives.

17. Metabolism and Excretion • Hormones are removed from the blood in four ways. • Excretion (kidney, liver) • Metabolism (enzymes) • Active transport (actively transported into cells and secreted again) • Conjugation (attach water-soluble molecules to hormone and then excreted by kidney or liver).

18. Interaction of Hormoneswith Their Target Tissues. • Hormones only interact with cells that have binding sites that are specific for the particular hormone.

20. Classes ofHormone Receptors. • Hormones can be placed into one of two major categories. • Hormones that cannot pass through the plasma membrane. • Hormones that can pass through the plasma membrane. • As a result, hormone receptors need to be located in different locations.

22. Membrane-BoundHormone Receptors. • Some receptors are located in the membrane of the target tissue. • After a hormone binds to the receptor, the receptor initiates events that lead to a response. • Some receptors alter membrane permeability.

24. Membrane-BoundHormone Receptors. • Some receptors are located in the membrane of the target tissue. • After a hormone binds to the receptor, the receptor initiates events that lead to a response. • Some receptors alter membrane permeability. • Some receptors activate G proteins.

26. Membrane-BoundHormone Receptors. • Some receptors are located in the membrane of the target tissue. • After a hormone binds to the receptor, the receptor initiates events that lead to a response. • Some receptors alter membrane permeability. • Some receptors activate G proteins. • Some receptors alter intracellular enzyme activity.

28. IntracellularHormone Receptors. • Some receptors are located in the target cell. • They are located either in the cytoplasm or in the nucleus of the target cell. • Once the hormone binds to the receptor, the effects of the hormone take place.

30. Functions of theEndocrine System • Metabolism and tissue maturation. • Ion regulation. • Water balance. • Immune system regulation. • Heart rate and blood pressure regulation. • Control of blood glucose and other nutrients. • Control of reproductive functions. • Uterine contractions and milk release.

31. Pituitary Glandand Hypothalamus • Pituitary gland is responsible for secreting nine hormones. • Hypothalamus regulates the pituitary gland’s secretions.

32. Pituitary Gland • Divided into a posterior and anterior portion. • Posterior pituitary is also known as the neurohypophysis, because it is continuous with the brain. • Anterior pituitary is also known as the adenohypophysis, because it acts more as a gland.

35. Relationship of thePituitary Gland to the Brain • The hypothalamus and the anterior pituitary are connected to each other via blood vessels. • Hypothalamus produces neurohormones that travel to the anterior pituitary via the blood. • Neurohormones then leave the blood and act on the cells in the anterior pituitary.

37. Relationship of thePituitary Gland to the Brain • Some neurohormones acts as releasing hormones and others act as inhibiting hormones. • This is how the hypothalamus controls the anterior pituitary gland. • See table 18.1 for a complete description of the hormones of the hypothalamus.

39. Relationship of thePituitary Gland to the Brain • There are no blood vessels that connect the hypothalamus to the posterior pituitary. • Instead, neurohormones produced in the hypothalamus travel to the posterior pituitary via axons.

41. Hormones of thePosterior Pituitary • Posterior pituitary stores and secretes two polypeptide hormones: • Antidiuretic hormone (ADH) • Oxytocin

42. Antidiuretic Hormone (ADH) • ADH is synthesized in the hypothalamus and transported to the posterior pituitary. • ADH is then released into the circulatory system and carried to the primary target tissue in the kidney. • ADH promotes water retention and reduces urine volume.

44. Hormones of theAnterior Pituitary • Anterior pituitary hormones are called tropic hormones.

46. Hormones of theAnterior Pituitary • Anterior pituitary hormones are called tropic hormones. • We will only address one of the many tropic hormones: • Thyroid-stimulating hormone

48. Thyroid-stimulatingHormone (TSH) • TSH is also known as thyrotropin. • Stimulates the synthesis and secretion of thyroid hormones from the thyroid gland. • TSH secretion is controlled by TRH from the hypothalamus.

49. Thyroid Gland & Hormones • Thyroid hormones exist in two forms: • Triiodothyronine (T3) • Tetraiodothyronine (T4) (also called thyroxine) • Approx. 90% T4 and 10% T3 secreted.

50. Thyroid Gland & Hormones • Thyroid hormones are transported in the blood in combination with plasma proteins. • Approx. 70-75% of T3 & T4 are bound to thyroxin-binding globulin (TBG). • 20-30% are bound to other plasma proteins. • Therefore, high half-life.