Describe the major function of the endocrine system.
The endocrine system 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 an endocrine gland?
Endocrine glands release chemical messengers called hormones into the bloodstream. These hormones can then affect the functioning of certain organs (termed target organs).
In general, a gland is an organ or other structure that secretes products within or outside the body.
A hormone 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 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.
Negative feedback is occasionally referred to as “feedback inhibition.”
Homeostasis is primarily maintained through which type of feedback mechanism?
Negative feedback maintains homeostasis.
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?
The inhibition of PTH by calcium is an example of 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. Remember, negative feedback occurs when high levels of a biological product act to decrease production of that product.
In the circulatory system, the presence of several clotting factors indirectly stimulates their own production. What type of feedback does this action exemplify?
Blood clotting is an example of 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 is involved in labor.
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 features characterize a peptide hormone?
Peptide hormones are polar molecules composed of amino acids.
What features characterize a steroid hormone?
Steroid hormones are nonpolar molecules derived from cholesterol.
Most steroid hormones are named using the suffixes “-ol,” -one,” or “-en.”
How do peptide hormones interact with their target cells?
Peptides bind to specific receptors on the cell membrane, promoting an intracellular signaling cascade.
As hydrophilic molecules, peptides cannot cross the phopholipid bilayer and never actually enter the target cell.
How do steroid hormones interact with their target cells?
Steroids 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 steroid and peptide hormones differ in their location of synthesis within the cell?
Peptides are synthesized in the rough ER, while 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, while 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.
This molecule’s rings and hydrocarbon tail make it hydrophobic, or insoluble in water.
Ghrelin is a hormone that acts on brain cells to stimulate hunger. It is hydrophilic (water-soluble) and interacts with target cells by binding to membrane receptors. Which type of hormone does ghrelin exemplify?
Ghrelin is an example of a peptide hormone.
Unlike steroids, peptides are hydrophilic, or water-soluble. They also bind to membrane receptors instead of diffusing into the cell.
Which type of hormone requires membrane receptors to act on a target cell?
Peptide hormones, as well as the catecholamines epinephrine and norepinephrine, require membrane receptors.
As polar molecules, these hormones cannot pass through the hydrophobic cell membrane.
Which endocrine gland, located adjacent to the hypothalamus, is known as the “master gland” of the endocrine system?
The pituitary gland is termed the “master gland” because it secretes several hormones that act on organs throughout the entire body.
The pituitary gland has two parts: the anterior (front) pituitary and the posterior (rear) pituitary.
Which hormones are secreted by the anterior pituitary?
The main hormones secreted by the anterior pituitary are follicle-stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), growth hormone (GH), prolactin, and thyroid-stimulating hormone (TSH).
Which hormones are secreted by the posterior pituitary?
The posterior pituitary secretes antidiuretic hormone (ADH) and oxytocin.
Note that these hormones are synthesized in the hypothalamus. Only after synthesis are they transported to the pituitary for release into the blood.
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 hypothalamus serve?
The hypothalamus 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.
What is a portal system, and how does it relate to endocrine function?
A portal system is a circulatory structure in which one capillary bed drains into another. The hypothalamus and pituitary gland are connected by such a system, which allows hormones to be easily transported from the hypothalamus to the pituitary.
Remember, the hormones of the posterior pituitary are made in the hypothalamus and simply “shipped” to the posterior pituitary for later release.
Which endocrine gland secretes ACTH, and on what target organ does this hormone act?
Adrenocorticotropic hormone (ACTH) is secreted by the anterior pituitary and acts on the adrenal cortex.
Specifically, it stimulates the cortex to secrete its own hormones, which include glucocorticoids and mineralcorticoids.
Which endocrine gland secretes prolactin, and what role does this hormone play in the human body?
Prolactin is secreted by the anterior pituitary. It stimulates the production of milk in females who are nursing infants.
Briefly describe the role of follicle-stimulating hormone (FSH) in females.
In keeping with its name, FSH promotes the development of mature follicles (structures that contain egg cells). Its levels are highest before ovulation and low during pregnancy.
Both FSH and LH are peptide hormones secreted from the anterior pituitary.
Briefly describe the role of follicle-stimulating hormone (FSH) in males.
FSH facilitates spermatogenesis, or sperm production, by stimulating Sertoli cells in the testes.
Both FSH and LH are peptide hormones secreted from the anterior pituitary.
Briefly describe the role of luteinizing hormone (LH) in females.
LH promotes ovulation with a peak in concentration known as the LH surge. It also facilitates the development of the corpus luteum.
Both FSH and LH are peptide hormones secreted from the anterior pituitary.
Briefly describe the role of luteinizing hormone (LH) in males.
LH increases the production of testosterone by stimulating Leydig cells in the testes.
Both FSH and LH are peptide hormones secreted from the anterior pituitary.
The anterior pituitary secretes several hormones that stimulate other endocrine organs instead of non-endocrine target cells. What term describes such 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 function is served by antidiuretic hormone (ADH)?
ADH stimulates the kidneys to increase water reabsorption, especially in dehydrated individuals.
This function limits water loss, preventing blood pressure from becoming too low.
Which endocrine gland secretes oxytocin, and what role does this hormone play in the human body?
Oxytocin is secreted by the posterior pituitary. Its main role is to facilitate labor contractions during childbirth.
Oxytocin is also involved in other childbirth-related functions, including facilitating nursing and promoting mother-infant bonding.
What endocrine function does the pancreas serve?
The pancreas produces and secretes insulin and glucagon, two hormones that have opposite effects on plasma glucose levels. These hormones are produced in structures termed islets of Langerhans.
A third hormone released by the pancreas is somatostatin, but the AP Biology exam is unlikely to mention it.
What function is served by insulin?
Insulin removes glucose from the blood and facilitates its storage in the tissues as glycogen. It is released when plasma glucose levels are high, such as after a meal.
Insulin is secreted by pancreatic beta cells.
What function is served by glucagon?
Glucagon facilitates the conversion of stored glycogen to glucose. This process increases plasma glucose levels.
Glucagon is secreted by pancreatic alpha cells.
What endocrine function does the thyroid gland serve?
The thyroid gland releases thyroid hormones, known as T3 and T4 (thyroxine), 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?
The parathyroid glands produce parathyroid hormone (PTH). This hormone increases plasma calcium levels by stimulating the breakdown of bone.
Make sure to be familiar with the difference between PTH and calcitonin, which have opposite functions.
What effect would excess production of parathyroid hormone have on plasma calcium levels?
Plasma calcium levels would increase.
PTH is released from the parathyroid glands when plasma calcium is low. Among other functions, it stimulates the breakdown of bone, which releases calcium into the extracellular fluid.
What effect would excess production of calcitonin have on plasma calcium levels?
Plasma calcium levels would decrease.
Calcitonin is released from the thyroid when plasma calcium levels are high. It stimulates specialized cells to increase bone formation, taking calcium out of the blood in the process.
Regulation of which ion involves both the thyroid and parathyroid glands?
Plasma concentrations of calcium ion (Ca2+) are controlled by both of these glands.
The parathyroid glands secrete parathyroid hormone (PTH), which increases plasma calcium levels. Calcitonin, a product of the thyroid, has the opposite effect.
Name the two major parts of the adrenal gland.
The two main parts are the cortex, or outer region, and the medulla, or inner region.
What are the main hormones secreted by the adrenal cortex?
The adrenal cortex secretes aldosterone and 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?
The adrenal medulla secretes epinephrine and norepinephrine, both catecholamines.
Notably, these products can function as either hormones or neurotransmitters, depending on the location of release.
Which type of hormone does the adrenal cortex produce?
The products of the adrenal cortex are steroid hormones.
For the AP Biology exam, this can be remembered by recalling their broad classification: corticosteroids.
What function is served by aldosterone?
Aldosterone stimulates the kidneys to increase sodium reabsorption; water follows and is passively reabsorbed as well.
This function results in water retention, preventing blood pressure from becoming too low.
Which two steroid hormones are released by the ovaries in females?
Estrogen and progesterone
These hormones play major roles in the development of secondary sex characteristics, regulation of the menstrual cycle, and pregnancy.
Which hormone is produced by the pineal gland, and what role does it serve?
The pineal gland produces melatonin, which helps regulate circadian rhythms.
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?
This pathway is a 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.
A certain individual gets the majority of her sodium intake from uniodized sea salt. Which endocrine gland might this affect?
A lowered consumption of iodine might affect the thyroid gland.
Iodine is required for thyroid hormone synthesis. Most table salt is iodized to protect against goiter, a condition in which the thyroid gland becomes enlarged to compensate for an iodine deficiency.
A student reads that a certain hormone functions to increase bone resorption. Which hormone is likely being described?
Parathyroid hormone (PTH)
It is important to know alternative ways that the AP Biology exam can phrase common terms. “Bone resorption” simply describes the breakdown of bone, which is promoted by PTH.
A 14-year-old boy suffers from Type I diabetes. Administration of which pancreatic hormone would exacerbate (worsen) his condition?
Glucagon would make this boy’s condition worse.
Type I diabetes involves a lack of insulin, a hormone that reduces plasma glucose levels. Thus, this boy would likely have high blood sugar. Glucagon, the hormone with the opposite function of insulin, would raise his blood glucose levels further.
What is the function of the reproductive system?
The reproductive system generates new offspring from parent organisms, facilitating the survival of the species.
Some reproductive methods also increase genetic variation. This allows a species to adapt to changes in its environment.
What is the difference between sexual and asexual reproduction?
Sexual reproduction involves two organisms and results in offspring that genetically differs from both of them. Asexual reproduction is carried out by a single organism and produces genetically identical offspring.
All prokaryotes use asexual reproduction, though they have alternative methods of gene transfer. Eukaryotic species can reproduce sexually, asexually, or through both mechanisms.
A gamete is a haploid cell that combines with another gamete during sexual reproduction. Together, the fused cells form a zygote that develops into a new organism.
In humans, the male gamete is the sperm, while the female gamete is the egg or ovum.
The gonads are endocrine organs that produce gametes.
The female gonads are the ovaries; the male gonads are the testes.
Name the major male and female sex hormones.
The most important sex hormones are testosterone in males and estrogen and progesterone in females.
While these hormones are steroids, their production is closely related to that of the peptides FSH and LH.
What is an androgen?
An androgen is a general term for a hormone that influences male development.
The most important androgen to know is testosterone, a steroid hormone.
What is oogenesis, and where and when does it begin?
Oogenesis is the production of the female gametes, ova, in the ovaries.
Oogenesis begins before birth. Specifically, all of a female’s oocytes are produced and arrested at prophase I by the time she is born.
Order the following terms from least to most mature: primary oocyte, ovum, oogonium, secondary oocyte.
The least mature cell is the oogonium, which is the original parent cell in oogenesis. It is followed by the primary oocyte, secondary oocyte, and ovum.
Both meiosis and mitosis are involved in this diffferentiation process.
From birth until puberty, all of a female’s oocytes are arrested at which stage of oogenesis?
All of a female’s oocytes are primary and are arrested in prophase I of meiosis.
Meiosis I corresponds to which step of oogenesis?
In oogenesis, meiosis I involves the division of a primary into a secondary oocyte.
Meiosis II, which is only completed if the egg is fertilized, involves the division of this secondary oocyte into a mature ovum.
How many cells are produced in the differentiation of one primary oocyte into one mature ovum?
Oogenesis produces three cells per primary oocyte: one ovum and two polar bodies.
In meiosis I, the primary oocyte divides into one secondary oocyte and one polar body. This first polar body generally does not divide further. If fertilized, the secondary oocyte undergoes meiosis II, where it divides into one ovum and a second polar body.
Label the parts of the sperm cell below.
What is spermatogenesis, and when and where does it begin?
Spermatogenesis is the production of the male gametes, sperm, in the seminiferous tubules of the testes.
Spermatogenesis begins at puberty.
Order the following terms from least to most mature: spermatid, spermatogonium, spermatozoon, secondary spermatocyte, primary spermatocyte.
The least mature cell is the spermatogonium, which is the original parent cell in spermatogenesis. It is followed by the primary spermatocyte, secondary spermatocyte, and spermatid. A fully mature sperm cell is known as a spermatozoon.
Both meiosis and mitosis are involved in this differentiation process.
Around what age in men does sperm production cease?
Spermatogenesis never ceases; it continues throughout a man’s adult life.
In contrast, oogenesis does not occur as a steady process. Virtually all of a woman’s eggs are partially differentiated by birth, and all differentiation ceases completely at menopause.
Name two differences between spermatogenesis and oogenesis.
- Spermatogenesis produces four equally functional gametes, while oogenesis produces one mature ovum and two nonfunctional polar bodies.
- Spermatogenesis takes place continuously from puberty to death, while oogenesis begins before birth and ends at menopause.
During which step of spermatogenesis do cells become haploid?
Developing cells become haploid during meiosis I, which is the division from primary to secondary spermatocyte. After this transition, the gamete will be haploid but with replicated chromosomes.
Meiosis II, on the other hand, refers to the division from secondary spermatocyte to spermatid. After this step, the gamete will be haploid with only one copy of each chromosome, as in its final state.
Puberty is the period of the human lifespan during which the reproductive system becomes mature.
In females, this process is marked by menarche, or the first menstrual period. In males, spermatogenesis begins at puberty.
Give two structural differences between the male and female reproductive systems.
- Male reproductive organs are mainly found outside of the body, while female organs are located internally.
- In males, the urethra serves both reproductive and urinary purposes, while in females, it is only involved in urination.
On the diagram below, label the ovaries, Fallopian tubes, uterus, and vagina.
In which structure of the female reproductive system does the fertilized zygote implant?
The zygote implants in the endometrium (lining) of the uterus.
Immediately after ovulation, this lining begins to thicken to provide a favorable environment for the potential zygote.
What is another term used for the Fallopian tubes?
Each Fallopian tube connects a single ovary to the uterus.
On the diagram below, label the testes, epididymis, vas deferens, prostate gland, and urethra.
What is the function of the scrotum?
The scrotum, which holds the testes, sits outside the body. This keeps the testes at a temperature slightly lower than 37º C, facilitating spermatogenesis.
What process occurs in the epididymis?
In the epididymis, sperm become fully mature and motile. This structure also stores sperm after they are made.
Name two male reproductive glands and describe their functions.
The prostate gland secretes a basic solution that protects the sperm from acidic conditions. The bulbourethral or Cowper’s glands produce a lubricating fluid prior to ejaculation.
A polar body is a haploid cell produced by meiosis I or meiosis II of oogenesis. Though they possess normal amounts of genetic material, polar bodies contain little cytosol and few organelles.
Oogenesis is thus characterized by uneven division of cytoplasm. Most of the cytoplasm is allocated to the functioning oocyte, while little is given to the polar body.
What is the length of an average menstrual cycle?
Name the two simultaneous cyclical processes involved in the menstrual cycle.
The menstrual cycle involves the ovarian cycle and the uterine cycle.
In general, the ovarian cycle relates to the development of the follicle within the ovary. The uterine cycle describes the buildup and degradation of the endometrial lining.
What are the phases of the ovarian cycle?
The ovarian cycle contains three phases or events: the follicular phase, ovulation, and the luteal phase.
The follicular phase roughly corresponds to the first half of the menstrual cycle, with the luteal phase comprising the other half. Ovulation occurs between these two phases.
What physiological changes occur during the follicular phase?
Both LH and FSH are released at fairly high levels, stimulating the development of the follicle. The follicle itself starts to secrete estrogen.
Remember, both LH (luteinizing hormone) and FSH (follicle-stimulating hormone) are peptides released from the anterior pituitary.
What are follicles, and in which female reproductive structure are they located?
Follicles are multicellular structures located within the ovaries. Each follicle surrounds and nourishes a single oocyte.
During ovulation, the follicle bursts and develops into the corpus luteum.
Once every 28 days, one follicle ruptures inside the ovary, releasing an oocyte into the Fallopian tube. What term is used to refer to this process?
The release of an oocyte from the ovary is called ovulation.
Describe the conditions that trigger ovulation.
Estrogen levels increase due to its release from the follicle. At this time only, estrogen exerts a positive feedback effect on LH secretion, causing a spike known as the LH surge. The follicle then ruptures, releasing the oocyte.
The active ingredients of many birth control medications include synthetic estrogen and progesterone. What effect would such medications have on ovulation?
Ovulation would be inhibited.
The presence of both estrogen and progesterone mimics the condition of pregnancy, in which FSH and LH are inhibited. Without these hormones, no LH surge can be initiated and ovulation cannot occur. In pregnancy, this effect is important to prevent multiple pregnancies from simultaneously occurring.
What physiological changes occur during the luteal phase?
The follicle, now called the corpus luteum, begins to secrete progesterone. LH and FSH fall to very low levels.
If fertilization occurs, implantation of the zygote would occur during this phase. Menstrual cycles, then, would temporarily cease due to pregnancy.
The corpus luteum is the structure that arises from the burst follicle after ovulation. If the ovum is not fertilized, it is degraded; otherwise, it persists during early pregnancy.
Since the corpus luteum secretes progesterone, it technically functions as an endocrine gland.
What are the three phases of the uterine cycle, and what major events do they contain?
- The first phase, menstruation, is marked by the degeneration of the endometrial lining.
- The second or proliferative phase involves regrowth of the endometrium, promoted by estrogen.
- The final or secretory phase is affected by progesterone from the corpus luteum. This causes the uterine lining to release nutrients.
How do the phases of the uterine cycle match up with the ovarian cycle?
The follicular phase encompasses both menstruation and the proliferative phase, while the luteal phase aligns perfectly with the secretory phase.
Prior to ovulation, what type(s) of feedback is/are exhibited by estrogen?
Estrogen exhibits positive feedback, triggering the LH surge. At other times, however, estrogen negatively feeds back on LH and FSH production.
Estrogen’s activity prior to ovulation is one of the few examples of positive feedback that you may see on the AP Biology exam.
In females, which process involves a decrease in sex hormones and the halting of ovulation?
Specifically, menopause is the period when menstrual cycles stop occurring and a female can no longer become pregnant.
How many sperm cells typically fertilize a single ovum?
One sperm fertilizes each ovum, assuming fertilization occurs at all.
Ova use specific mechanisms to avoid polyspermy, or fertilization by multiple sperm. The details of these processes are unlikely to be tested on the AP Biology exam.
Which gamete contributes mitochondria to the developing zygote?
A zygote’s mitochondria are contributed by the ovum.
While sperm contain mitochondria, they are destroyed by the ovum shortly after fertilization. As a result, mitochondrial diseases are inherited maternally.
The zygote is the diploid cell created when a sperm cell fertilizes an ovum.
Note that a zygote is single-celled; once it undergoes a single division, it will be called an embryo.
By which process does an embryo increase its number of cells, but not its overall size?
The early divisions of the embryo are referred to as cleavage. In this process, the cells divide very quickly without growing between divisions.
The regular cell cycle includes two rest or “gap” phases, G1 and G2. During cleavage, dividing cells skip these phases.
Trace the path of an zygote from the point of fertilization to the point of implantation. What structures does it encounter?
Fertilization occurs in the Fallopian tubes. From there, the zygote travels directly to the uterus and implants in the endometrium, or uterine lining.
The morula is the name given to the embryo after multiple cleavage divisions. This structure is extremely dense.
The morula later develops into the blastula.
What is a blastula, and from what other structure does it arise?
A blastula is a hollow cell arrangement that develops from the morula. A blastula contains a central blastocoel, a dense inner cell mass, and a surrounding trophoblast.
What is gastrulation, and which germ cell layers does it produce?
Gastrulation is the differentiation of the blastula into three distinct layers: the ectoderm, the mesoderm, and the endoderm.
The endoderm is the innermost germ layer formed during development.
Structures arising from the endoderm include internal organs such as the pancreas and urinary bladder. The endoderm also gives rise to the linings of the digestive tract and lungs.
The ectoderm is the outermost germ layer formed during development.
Structures arising from the ectoderm include the eyes, skin, hair, nails, and nervous system.
The mesoderm is the middle germ layer formed during development.
Structures arising from the mesoderm include blood vessels, gonads, and the muscular and skeletal systems. Note that the kidneys also derive from mesoderm, not endoderm.
A student asserts that the endoderm gives rise to internal organs like the pancreas, lungs, kidneys, and urinary bladder. About which structure was the student incorrect?
The student was wrong about the kidneys, which develop from the mesoderm.
The remaining organs (pancreas, lungs, and urinary bladder) do arise from the endoderm.
What process involves the differentiation of ectodermal tissue into the structures that will become the nervous system?
This differentiation process is called neurulation.
The final product of neurulation is the neural tube, which develops into the central nervous system. Peripheral nervous structures arise from cells of the neural crest.
Briefly describe the steps involved in neurulation.
- The notochord, a long, cylindrical structure related to the backbone, forms.
- The neural plate (ectoderm lying above the notochord) begins pinching inward to create neural folds.
- The neural folds meet, forming the neural tube.
- Neural crest cells move away to form peripheral structures.
After what amount of time has the zygote progressed into an embryo? Into a fetus?
The zygote is considered an embryo after its first cell division. The embryo is classified as a fetus during the first trimester, specifically after the eighth week following fertilization.
Which hormone, commonly detected in pregnancy tests, prevents the corpus luteum from degrading after implantation?
Human chorionic gonadotropin (hCG), a classic sign of pregnancy, maintains the corpus luteum.
Note that hCG is a peptide hormone.
Which hormone, released from the anterior pituitary, promotes lactation after birth?
Prolactin, a peptide hormone, promotes lactation.
A woman is midway through her second trimester of pregnancy. Which of her hormones will be present at high levels?
Estrogen, progesterone, and hCG exist at high levels during pregnancy.
In early pregnancy, progesterone and some estrogen are secreted by the corpus luteum. Within the third month, the placenta becomes the main source of these hormones, as well as hCG.
The placenta is the organ through which a fetus obtains nutrients and performs gas exchange.
The placenta also performs endocrine functions, secreting estrogen, progesterone, and hCG.