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16

The Endocrine System

Part A

Human Anatomy & Physiology, Sixth Edition

Elaine N. Marieb

PowerPoint^® Lecture Slides prepared by Vince Austin, University of Kentucky

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Endocrine System: Overview

Endocrine system – the body’s second great controlling system which influences metabolic activities of cells by means of hormones
Endocrine glands – pituitary, thyroid, parathyroid, adrenal, pineal, and thymus
The pancreas and gonads produce both hormones and exocrine products

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Endocrine System: Overview

The hypothalamus has both neural functions and releases hormones
Other tissues and organs that produce hormones – adipose cells, pockets of cells in the walls of the small intestine, stomach, kidneys, and heart

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Major Endocrine Organs

Figure 16.1

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Hormones

Hormones – chemical substances secreted by cells into the extracellular fluids

Regulate the metabolic function of other cells

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Target Cell Specificity

Hormones circulate to all tissues but only activate cells referred to as target cells
Target cells must have specific receptors to which the hormone binds

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Target Cell Activation

Target cell activation depends on three factors

Blood levels of the hormone
Relative number of receptors on the target cell
The affinity of those receptors for the hormone

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Interaction of Hormones at Target Cells

Three types of hormone interaction

Synergism – more than one hormone produces the same effects on a target cell
Antagonism – one or more hormones opposes the action of another hormone

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Control of Hormone Release

Blood levels of hormones:

Are controlled by negative feedback systems

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Major Endocrine Organs: Pituitary (Hypophysis)

Pituitary gland – two-lobed organ that secretes nine major hormones

Neurohypophysis – posterior lobe (neural tissue) and the infundibulum

Receives, stores, and releases hormones from the hypothalamus

Adenohypophysis – anterior lobe, made up of glandular tissue

Synthesizes and secretes a number of hormones

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Pituitary-Hypothalamic Relationships: �Posterior Lobe

The posterior lobe is a downgrowth of hypothalamic neural tissue
Has a neural connection with the hypothalamus
Synthesizes oxytocin and antidiuretic hormone (ADH)
These hormones are transported to the posterior pituitary

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The Posterior Pituitary and Hypothalamic Hormones

Posterior pituitary – stores and releases antidiuretic hormone (ADH) and oxytocin
ADH influences water balance
Oxytocin stimulates smooth muscle contraction in breast tissue, the uterus and blood vessels

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Oxytocin

Oxytocin is a strong stimulant of uterine contraction
Regulated by a positive feedback mechanism to oxytocin in the blood
This leads to increased intensity of uterine contractions, ending in birth
Oxytocin triggers milk ejection (“letdown” reflex) in lactating women
Pitocin, artificial oxytocin, is often injected to help stop bleeding after birth

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Oxytocin

Synthetic and natural oxytocic drugs are used to induce or hasten labor
Plays a role in sexual arousal and satisfaction in males and nonlactating females
Thought to participate in the bonding between mother and baby

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Antidiuretic Hormone (ADH) AKA:Vasopressin

ADH helps to avoid dehydration

Prevents urine formation

Osmoreceptors monitor the solute concentration of the blood
Alcohol inhibits ADH release and causes copious urine output
Hyposecretion of ADH leads to idiabetes nsipidus

Urine output can exceed 20 – 30 liters/day

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Pituitary-Hypothalamic Relationships: �Anterior Lobe

The anterior lobe of the pituitary is an outpocketing of the oral mucosa
There is no direct neural contact with the hypothalamus

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Adenophypophyseal Hormones

The six hormones of the adenohypophysis:

Are abbreviated as GH, TSH, ACTH, FSH, LH, and PRL

growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, follicle stimulating hormone, luteinizing hormone, prolactin

Regulate the activity of other endocrine glands

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Metabolic Action of Growth Hormone

GH stimulates liver, skeletal muscle, bone, and cartilage to produce growth factors and stimulate mitosis
Direct action promotes lipolysis and inhibits glucose uptake

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Metabolic Action of Growth Hormone

Figure 16.6

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Prolactin (PRL)

In females, stimulates milk production
Blood levels rise toward the end of pregnancy
Suckling stimulates PRH release and encourages continued milk production

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Thyroid Gland

The largest endocrine gland, located in the anterior neck
Thyroid hormone – the body’s major metabolic hormone
Consists of two closely related iodine-containing compounds

T₄ – thyroxine; has two tyrosine molecules plus four bound iodine atoms
T₃ – triiodothyronine; has two tyrosines with three bound iodine atoms

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Thyroid Hormone

TH is concerned with:

Glucose oxidation
Increasing metabolic rate
Heat production

Other endocrine cells, the parafollicular cells, produce the hormone calcitonin

Calcitonin lowers blood calcium levels

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Thyroid Gland

Figure 16.7

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Thyroid Hormone

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Adrenal (Suprarenal) Glands

Adrenal glands – paired, pyramid-shaped organs atop the kidneys
Structurally and functionally, they are two glands in one

Adrenal cortex
Adrenal medulla

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Adrenal Cortex

Synthesizes and releases steroid hormones called corticosteroids

Mineralocorticoids �(chiefly aldosterone)
Glucocorticoids �(chiefly cortisol)
Gonadocorticoids �(chiefly androgens)

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Adrenal Cortex

Figure 16.12a

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Mineralocorticoids

Regulate the electrolyte concentrations of extracellular fluids
Aldosterone – most important mineralocorticoid

Maintains Na⁺ balance by reducing excretion of sodium from the body
Stimulates reabsorption of Na⁺ by the kidneys

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Mineralocorticoids

Aldosterone secretion is stimulated by:

Rising blood levels of K⁺
Low blood Na⁺
Decreasing blood volume or pressure

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Glucocorticoids (Cortisol)

Help the body resist stress by:

Keeping blood sugar levels relatively constant
Maintaining blood volume

Cortisol provokes:

Gluconeogenesis (formation of glucose from noncarbohydrates)
Rises in blood glucose, fatty acids, and amino acids

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Excessive Levels of Glucocorticoids

Excessive levels of glucocorticoids:

Depress cartilage and bone formation
Inhibit inflammation
Depress the immune system

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Gonadocorticoids (Sex Hormones)

Most gonadocorticoids secreted are androgens (male sex hormones), and the most important one is testosterone
Androgens contribute to:

The onset of puberty
The appearance of secondary sex characteristics
Sex drive in females

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Adrenal Medulla

Made up of cells that secrete epinephrine and norepinephrine (catecholamines)
Secretion of these hormones causes:

Blood glucose levels to rise
Blood vessels to constrict
The heart to beat faster
Blood to be diverted to the brain, heart, and skeletal muscle

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Pancreas

A triangular gland, which has both exocrine and endocrine cells, located behind the stomach
Pancreatic islets (islets of Langerhans) produce hormones (endocrine products)
The islets contain two major cell types:

Alpha (α) cells that produce glucagon
Beta (β) cells that produce insulin

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Glucagon

A hormone that is a potent hyperglycemic agent
Its major target is the liver, where it promotes:

Release of glucose to the blood from liver cells
Glycogenolysis – the breakdown of glycogen to glucose
Gluconeogenesis – synthesis of glucose from lactic acid and noncarbohydrates

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Insulin

A hormone that is a potent hypoglycemic agent
Insulin:

Lowers blood glucose levels
Enhances transport of glucose into body cells

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Regulation of Blood Glucose Levels

The hyperglycemic effects of glucagon and the hypoglycemic effects of insulin

Figure 16.17

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Diabetes Mellitus (DM)

Results from hyposecretion or hypoactivity of insulin
The three cardinal signs of DM are:

Polyuria – huge urine output
Polydipsia – excessive thirst
Polyphagia – excessive hunger and food consumption

Hyperinsulinism – excessive insulin secretion, resulting in hypoglycemia