Endocrine System for the DAT
/Learn key DAT concepts related to hormones, endocrine glands, and hormonal axes plus practice questions and answers
Table of Contents
Part 1: Introduction to the endocrine system
Part 2: Classifying hormones
a) Peptide hormones
b) Steroid hormones
c) Tyrosine derivatives
Part 3: Endocrine glands and secreted hormones
a) Pancreas
b) Anterior and posterior pituitary glands
c) Thyroid and parathyroid glands
d) Adrenal cortex and adrenal medulla
Part 4: Hormonal axes
a) Hypothalamic-pituitary-adrenal (HPA) axis
b) Hypothalamic-pituitary-gonadal (HPG) axis
Part 5: High-yield terms
Part 6: Questions and answers
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Part 1: Introduction to the endocrine system
The endocrine system collaborates with the nervous system to influence metabolism, guide reproduction, and regulate growth. This comprehensive guide will give you an overview of endocrine-focused content essential for the DAT. Toward the end of this guide, you'll find DAT-style practice questions to assess your understanding.
For the DAT, a thorough understanding involves memorizing various hormones, glands, and their target cells, often referred to as effectors. To facilitate learning, we'll present this information in diverse formats. Grouping hormones by the glands that secrete them can be a helpful study strategy.
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Part 2: Classifying hormones
The endocrine system plays a critical role in maintaining bodily equilibrium, known as homeostasis. It operates by releasing hormones in response to existing conditions rather than instigating them. For instance, insulin is released when blood sugar levels are high, aiming to counteract, not provoke, low blood sugar levels. This operating principle is known as negative feedback. Unlike positive feedback, which amplifies a condition, negative feedback aims to diminish it through action.
The endocrine system achieves this by releasing hormones into the body's general circulation, primarily through the bloodstream. These hormones affect distant cells or tissues. This differs from autocrine signaling, where a molecule impacts the same cell, and paracrine signaling, where it affects neighboring cells or tissues.
Hormones, classified by their structure, fall into three primary types: peptide hormones, steroid hormones, and tyrosine derivatives.
a) Peptide hormones
Peptide hormones are derived from short amino acid chains linked by peptide bonds (O=C-NH). These hormones are essentially small proteins. Because of this, peptide hormones are typically polar and water-soluble. These properties facilitate their circulation through the bloodstream. However, due to their inability to easily traverse nonpolar cell membranes, peptide hormones generally rely on cell surface receptors to enact their effects.
Upon binding with the receptor, diverse responses may occur. The receptor might act as an ion channel or interact with proteins to generate an ion-channel effect.
The DAT often assesses knowledge concerning G-protein coupled receptors (GPCRs), a specific receptor system. When a hormone binds to GPCRs, they undergo a structural change, leading to the release of a G-protein by swapping a low-energy GDP for a high-energy GTP. This event initiates a "cascade" of secondary messengers like cAMP or IP3, triggering a series of reactions within the cell. These secondary messengers significantly amplify the hormone's impact, as a single hormone molecule binding to the cell's surface can trigger a considerable concentration of activated secondary messengers.
Figure 1: OVERVIEW OF GPCR FUNCTION
Here’s a roster of specific peptide hormones essential for DAT study. We’ll delve into their functions later in this guide.
| Anterior pituitary | Posterior pituitary | Parathyroid | Pancreas | Thyroid |
|---|---|---|---|---|
LH ACTH TSH Prolactin Endorphins Growth Hormones |
Oxytocin |
Insulin |
b) Steroid hormones
Steroid hormones are derived from cholesterol and fall into the lipid category. Remember, lipids are nonpolar, making them insoluble in water or blood. When in the bloodstream, steroid hormones rely on specialized protein transport molecules for conveyance.
Unlike peptide hormones, steroid hormones possess the unique ability to directly permeate cell membranes. This characteristic enables them to bypass the need for receptor systems on the cell membrane to elicit their effects. Steroid hormones directly traverse the membrane and can bind to receptors in the cytosol, be transported to the nucleus, and modulate transcription by binding directly to DNA.
Arranged based on their glandular source, here are the steroid hormones crucial for DAT study:
| Adrenal Cortex | Gonads (Testes and Ovaries) |
|---|---|
Aldosterone *More detailed coverage in our guide on the digestive and excretory systems |
Testosterone Progesterone *More detailed coverage in our guide on reproduction and development |
c) Tyrosine derivatives
Tyrosine derivative hormones, akin to steroid hormones, are lipid soluble and require a specific protein carrier for transportation within the bloodstream. Essentially modified versions of the amino acid tyrosine, these hormones tend to be small in size, distinguishing them from peptide and steroid hormones.
Categorized by their glandular origin, here's a rundown of the tyrosine derivative hormones vital for DAT study:
| Thyroid | Adrenal Medulla |
|---|---|
T4 (Contains 4 iodine atoms) |
Norepinephrine (Both categorized as catecholamines) |
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Part 3: Endocrine glands and secreted hormones
Distinguishing between the endocrine and exocrine systems is crucial. Exocrine systems discharge enzymes through ducts, expelling substances like sweat, mucous, and oils into external environments. On the other hand, endocrine systems release hormones directly into the bloodstream. Given this mode of transportation via the bloodstream, hormones require specific receptors for binding. This results in a slower yet enduring impact.
The interconnection between the endocrine and nervous systems is profound. Remember that the endocrine system originates from neural crest cells, which contribute not only to the peripheral nervous system but also to numerous associated structures.
While studying this section, it's essential to recognize the intricate ties between the nervous and endocrine systems. Some hormones can double as neurotransmitters, and secretions from endocrine glands might instigate negative or positive feedback effects on the central nervous system.
a) Pancreas
The pancreas plays a dual role as both an exocrine and endocrine gland. Its exocrine function involves secreting digestive enzymes like trypsin and amylase through the pancreatic duct, while its endocrine function entails releasing insulin and glucagon into the bloodstream to regulate blood sugar levels.
Within the pancreas, specialized clusters of cells called the Islets of Langerhans contain three significant cell types: β-cells, α-cells, and δ-cells, each responsible for secreting distinct hormones with varying effects.
Insulin, a peptide hormone released by β-cells, responds to high carbohydrate and protein levels in the blood. It prompts the storage of carbohydrates as glycogen in the liver and muscles, while also storing fat in adipose tissue, thereby lowering blood glucose levels.
In contrast, glucagon, also a peptide hormone, originates from α-cells in response to low nutrient levels in the blood. It primarily triggers glycogenolysis (the breakdown of glycogen) and gluconeogenesis (the generation of glucose) in the liver to elevate blood glucose levels.
The peptide hormone somatostatin, released by δ-cells, acts as a regulator. It blocks the action of other hormones like insulin, glucagon, and growth hormones. Its primary function involves slowing down the activity of other endocrine signals and reducing the body's metabolic rate.
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