Endocrine System Flashcards
The MCAT requires an in-depth understanding of hormones, from the difference between peptides and steroids to their effects on other organ systems. Use these cards to master endocrine organs, specific hormones, and the complex interactions involved.
Describe the major function of the endocrine system.
It controls cellular, tissue, and organ processes through the release of chemical signals.
These processes mainly relate to growth, metabolism, homeostasis, and reproduction.
What is the function of endocrine glands?
They release chemical messengers called hormones into the bloodstream. These hormones can then affect the functioning of target organs.
In general, a gland is an organ or other structure that secretes products within or outside the body.
Define:
hormone
It is a molecule that is released into the bloodstream by an endocrine gland. Hormones act as chemical messengers and bind to receptors on their target organs.
Two large classes of hormones are peptides and steroids.
What features characterize peptide hormones?
They are polar molecules composed of amino acids.
What features characterize steroid hormones?
They are nonpolar molecules derived from cholesterol.
Most steroid hormones are named using the suffixes “-ol,” “-one,” or “-en.”
Some hormones are classified as neither steroids nor peptides. From which amino acid are most of these hormones derived?
Tyrosine
Tyrosine derivatives include the thyroid hormones T3 and T4. The catecholamines, namely epinephrine and norepinephrine, are also part of this category.
With regard to solubility, do tyrosine derivatives resemble peptide or steroid hormones?
Some tyrosine-based hormones resemble peptides, while others resemble steroids.
Like peptides, epinephrine and norepinephrine are water-soluble. However, the thyroid hormones are lipid-soluble, like steroids.
How do steroid and peptide hormones differ in their location of synthesis within the cell?
- Peptides are synthesized in the rough ER.
- Steroids are modified from cholesterol in the smooth ER.
ER stands for “endoplasmic reticulum,” a membrane-bound organelle.
How do steroid and peptide hormones differ in their method of travel within the blood?
- Peptides can travel freely in the bloodstream.
- Steroids generally must be bound to carrier proteins.
Like most differences between the hormone types, this can be explained by solubility characteristics. Peptides are hydrophilic (water-soluble), while steroids are hydrophobic.
Name the four-ringed biological molecule that acts as the precursor for all steroid hormones.
Cholesterol
This molecule’s rings and hydrocarbon tail make it hydrophobic, or insoluble in water.
How do steroid hormones interact with their target cells?
They diffuse through the cell membrane, travel to the nucleus, and alter transcription of certain DNA sequences.
As hydrophobic molecules, steroids can easily pass through the phospholipid bilayer. However, they must be bound to a specific receptor when in the cytoplasm.
How do peptide hormones interact with their target cells?
They bind to specific receptors on the cell membrane, promoting an intracellular signaling cascade.
As hydrophilic molecules, peptides cannot cross the phospholipid bilayer and never actually enter the target cell.
Since peptides cannot cross the cell membrane, they rely on signaling molecules inside the target cell to effect a change. What term describes this kind of pathway?
second messenger system
The first messenger is the hormone itself; the second is an intracellular ion or molecule, like Ca2+ or cAMP. These systems commonly involve G protein-coupled receptors, or GPCRs.
Ghrelin is a hormone that acts on brain cells to stimulate hunger. It is hydrophilic and interacts with target cells by binding to membrane receptors. Which type of hormone does ghrelin exemplify?
peptide hormone
Unlike steroids, peptides are hydrophilic, or water-soluble. They also bind to membrane receptors instead of diffusing into the cell.
The anterior pituitary secretes several hormones that stimulate other endocrine organs instead of non-endocrine target cells. What term describes such hormones?
tropic hormones
One example is TSH, or thyroid-stimulating hormone. Instead of directly altering a cellular process on its own, it stimulates the thyroid to produce its own hormones.
What is the difference between negative and positive feedback?
- In negative feedback, formation of the product of a certain process decreases the rate of that process.
- In positive feedback, product formation increases the rate of the process that produces it.
On the MCAT, negative feedback is occasionally referred to as “feedback inhibition.”
Homeostasis is primarily maintained through which type of feedback mechanism?
negative feedback
Homeostasis relates to an organism’s need to maintain fairly stable conditions within its body and cells. Since negative feedback acts to prevent product concentrations from becoming too high, it helps preserve this stability.
When plasma calcium levels are high, the release of parathyroid hormone is inhibited. What type of feedback does this action exemplify?
negative feedback
Parathyroid hormone (PTH) functions to increase plasma levels of calcium ion. When plenty of calcium is already present, PTH activity decreases; this prevents Ca2+ levels from becoming too high.
In the circulatory system, the presence of several clotting factors indirectly stimulates their own production. What type of feedback does this action exemplify?
positive feedback
One example is thrombin, an enzyme. Thrombin activates platelets and other factors, which leads to additional thrombin production. This allows the clotting process to occur quickly.
What type of feedback loop is seen during labor contractions?
positive feedback
When a pregnant female begins labor contractions, the process must be continued until birth. Oxytocin acts to stimulate these contractions, which feed back to stimulate more oxytocin release.
What is a portal system, and how does it relate to endocrine function?
It is a circulatory structure in which one capillary bed drains into another. The hypophyseal portal system allows hormones to be transported from the hypothalamus to the pituitary gland.
A similar example is the hepatic portal system. However, its main function is digestive, not endocrine.
What endocrine function does the hypothalamus serve?
It secretes chemicals that control the release of hormones from the anterior pituitary. It also produces the two posterior pituitary hormones, ADH and oxytocin.
The hypothalamus is connected to the pituitary via the hypophyseal portal system.
Both the anterior and posterior pituitary secrete important hormones. How do these hormones differ in their location of synthesis?
Hormones that are secreted from the anterior pituitary are also synthesized there. In contrast, posterior pituitary hormones are synthesized in the hypothalamus.
What endocrine function does the thyroid gland serve?
It releases thyroid hormones (namely T3 and T4), which control metabolism. It also secretes calcitonin.
The endocrine activity of the thyroid is determined by thyroid-stimulating hormone (TSH), which is secreted from the anterior pituitary.
Which hormone is produced by the parathyroid glands, and what role does it serve?
- Parathyroid hormone (PTH)
- This hormone increases plasma calcium levels by stimulating the breakdown of bone.
Many MCAT questions deal with the difference between PTH and calcitonin, which have opposite functions.
What endocrine function does the pancreas serve?
It secretes insulin and glucagon, two hormones that have opposite effects on plasma glucose levels.
A third hormone released by the pancreas is somatostatin, but the MCAT mentions it much less frequently.
Which hormone is produced by the pineal gland, and what role does it serve?
The pineal gland produces melatonin, which helps regulate circadian rhythms.
Name the two major parts of the adrenal gland.
- cortex, or outer region
- medulla, or inner region
What are the main hormones secreted by the adrenal cortex?
- aldosterone
- cortisol
More broadly, the cortex produces glucocorticoids (such as cortisol and cortisone) and mineralocorticoids (such as aldosterone). It also synthesizes some sex hormones.
What are the main hormones secreted by the adrenal medulla?
- epinephrine
- norepinephrine
Both are catecholamines.
Notably, these products can function as either hormones or neurotransmitters, depending on the location of release.
A patient exhibits lowered production of a certain kidney enzyme, leading to chronic low blood pressure. In which enzyme is the patient likely deficient?
renin
Renin is an enzyme vital to the release of aldosterone. Specifically, renin activates angiotensinogen, the first step that promotes aldosterone secretion.
While other deficiencies (in ADH, for example) could cause low blood pressure, renin is the only example that is released from the kidney itself.
