Chemical Coordination and Integration: Functions, Glands

The chemical coordination and integration of all physiological functions in the animal body are jointly regulated by the neural and endocrine systems.

• The endocrine glands carry out the control and coordination of body functions.
• They are ductless glands that secrete various hormones to control and coordinate body functions.
• When compared to invertebrates, vertebrates have more complex and evolved endocrine systems.
• Plants do not have these glands, but they do secrete plant hormones that regulate various processes.

Key Terms: Endocrine system, glands, hormones, pituitary gland, hypothalamus, hormone-receptor complex, endocrine glands, hormones, sells turcica, glads

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Human Endocrine System

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The endocrine system in humans consists of many ductless glands that regulate the functions of the body or evoke certain responses in the body.

• The human body has various endocrine glands located in different parts.
• They include the pituitary gland, pineal gland, thymus, thyroid gland, parathyroid, pancreas (dual gland), adrenal gland, and gonads (testes and ovaries).
• The liver, kidney, heart, and gastrointestinal tract also produce hormones in small traces to coordinate the functioning of these organs.
• The Hypothalamus consists of groups of secretory cells called nuclei which secret various hormones.
• These hormones released by the hypothalamus are essential in regulating the synthesis and secretion of pituitary hormones.

The Endocrine System

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Endocrine Glands

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Endocrine glands are ductless glands that control and coordinate different body functions or evoke different responses in the body.

• These glands release chemicals called hormones, which regulate different body functions.
• These hormones are secreted directly into the bloodstream.
• Hormones are non-nutrient chemicals that act as intracellular messengers and are produced in trace amounts.
• Hormones directly don't take part in any chemical reactions; instead, they stimulate metabolic reactions.
• The endocrine system in invertebrates is simple with very few hormones, and that of vertebrates is complex with a large number of hormone secretions to coordinate and integrate body activities.

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Glands and their Hormonal Secretions

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Various Glands along with their Hormonal Secretions are tabulated below:

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Functions of Endocrine Glands

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The following are the functions of various endocrine glands:

Pituitary Gland

The pituitary glands are located in a bony cavity called sella turcica, attached to the hypothalamus via a stalk.

• Pituitary glands can be divided into anterior pituitary and posterior pituitary.
• The pituitary gland is considered the master gland of the body, as it is capable of controlling other glands.
• They play a vital role in growth and development.
• They also help in uterine contraction during childbirth and milk ejection from mammary glands.

Pituitary gland

Pineal Gland

The pineal gland is located on the dorsal side of the forebrain.

• The hormone secreted by the gland is called the melatonin hormone.
• Melatonin hormones help regulate the body's diurnal rhythm like the sleep-wake cycle, body temperature, etc.
• These glands develop serotonin derivatives of melatonin that can affect the sleep cycle.
• They also control and regulate other body processes like metabolism, menstrual cycle, pigmentation, and defense capability. 

Thyroid Gland

These glands are located on either side of and anterior to the trachea or windpipe.

• The isthmus connects these bilobed glands.
• Thyroid glands are composed of stromal tissues and follicles.
• Thyroid follicles consist of follicular cells, and they produce two types of hormones- triiodothyronine (T3) and tetraiodothyronine or thyroxine (T4).
• Iodine is the vital element required for the synthesis of thyroid hormone.
• Thyroid deficiency will lead to goitre or hypothyroidism.
• During pregnancy, hypothyroidism can impact a child's growth and cause mental retardation.
• Children may have a low intelligence quotient, deaf, or have abnormal skin. 

Thyroid Glands

Parathyroid Gland

These glands are located on the backside of the thyroid glands.

• These glands secrete the peptide hormone called parathyroid hormone (PTH).
• PTH regulates the concentration of calcium ions in the blood.
• They also help maintain the phosphate level in the blood.
• They also help in the reabsorption of calcium from renal tubules and digestive tracts.
• It also aids in the intestinal absorption of vitamin D and phosphate.

Thymus

The gland is located on the dorsal side of the heart and the aorta between the lungs and behind the sternum.

• They secrete the peptide hormone thymosin.
• The hormone helps the immune system function properly and helps in the production of T-lymphocytes.
• These hormones also produce antibodies to provide humoral immunity to the body.

Adrenal Gland

These pairs of glands are located on the anterior part of each kidney.

• The gland consists of 2 tissues- the central adrenal medulla and the outer adrenal cortex.
• The adrenal medulla secretes two hormones- adrenaline and noradrenaline hormone, commonly called catecholamines.
• The adrenal cortex secretes hormones like mineralocorticoids and glucocorticoids.
• These hormones are collectively called corticoids.
• Adrenaline hormones are called emergency hormones as they are secreted during emergencies or fight or flight situations.
• It increases alertness, causes sweating, increases heart rate and respiration, dilation of the pupil, etc.

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Pancreas

The Pancreas is a dual gland, i.e., it acts as both endocrine and exocrine glands. The endocrine part possesses "islets of Langerhans", which contain two types of cells- alpha cells (α-cells) and beta cells (β-cells). The alpha cells secrete hormones called glucagon, and the beta cells secrete insulin. Both are peptide hormones. Both these hormones regulate blood sugar levels. Glucagon promotes glycogenolysis, thereby increasing the glucose levels in blood or hyperglycemia. Insulin hormone regulates glucose levels. It promotes cellular glucose uptake, thereby decreasing the glucose levels in the blood.

Testes

Testes are located in the scrotal sacs outside the abdominal cavity.

• They perform dual functions- as a primary sex hormone and an endocrine gland.
• The testis consists of interstitial cells and seminiferous tubules.
• Interstitial cells, also called Leydig cells, produce the hormone testosterone.
• This male hormone regulates the maturation of primary sex organs, spermatogenesis (sperm production), development of beards, and moustaches.

Ovary

The ovaries are located in the lower pelvic region of the abdominal cavity.

• It is the primary sex organ in females.
• Ovaries focus on the release of eggs or ovum.
• Ovaries secrete two essential hormones called Estrogen and progesterone.
• Estrogen helps in the development of mammary glands, and sex organs, and it regulates the menstrual cycle and ovulation.
• They also help in maintaining water balance in the body.
• Progesterone causes changes to the female body during pregnancy.
• Hence the hormone is also called pregnancy hormones.

Hormones of Heart, Kidney and Gastrointestinal Tract

The heart secretes a hormone called Atrial natriuretic factor (ANF) which is known to lower blood pressure.

• The hormone gets released when the blood pressure increases.
• Erythropoietin is the hormone secreted by the kidney and helps in the formation of RBC.
• Secretin, cholecystokinin, gastrin, and gastric inhibitory peptides are the hormones secreted by the gastrointestinal tract.

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Mechanism of Hormone Action

The mechanism of Hormone Action is as follows:

• Hormones control and streamline the activity of the target tissues by binding to a specific protein called hormone receptors, which are located in the target tissues.
• They form a hormone-receptor complex.
• This binding allows the hormones to exert their effect on the target tissues.
• Once the hormone-receptor complex is formed, specific biochemical changes happen within the target tissue. 
• Hormones will interact with intracellular receptors and membrane receptors.

Mechanism of hormone action

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Hormones on the Basis of Chemical Nature

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Hormones can be classified based on their chemical nature: 

• Steroids- testosterone, progesterone, cortisol.
• Peptide, polypeptide, and protein hormones- glucagon, insulin, pituitary hormone, hypothalamic hormones.
• Iodothyronines- thyroid hormones.
• Amino acid derivatives- epinephrine.
• Steroid receptors are intracellular receptors, whereas peptide receptors are membrane receptors.
• The hormones that bind with membrane-bound receptors do not enter the target tissue; instead, they generate a second messenger, which controls and coordinates cellular metabolism.
• The steroid hormones that interact with intracellular receptors regulate chromosome function and gene expression by interacting with the genome and the hormone-receptor complex.

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Things to Remember

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• The endocrine glands primarily regulate the control and coordination of functions in the human body.
• Endocrine glands are ductless glands that release a chemical called hormones to coordinate and regulate the body's physiological activities.
• The primary endocrine glands of the human body are- the pituitary gland, pineal gland, thymus, thyroid gland, parathyroid, pancreas (dual gland), adrenal gland, and gonads (testes and ovaries).
• The kidney, heart, and gastrointestinal tract also produce some hormones.
• Hormones regulate the body’s functions by interacting with the target tissue and forming a hormone-receptor complex.