Endocrine and Reproductive Physiology - Human Anatomy and Physiology

In order for the testes to develop, the only thing necessary is the SRY gene product, which is created from the SRY gene on the Y chromosome. DHT are involved in the development of other male sex traits during early development, such as the penis, while testosterone is involved in both early development and the development of secondary sexual traits during puberty. Wolfian excitatory hormone does not actually exist. Estrogen is not involved in the development of male sex organs.

Estradiol levels in the body are typically controlled with a negative feedback loop; however, the luteal surge results in a positive feedback loop for estradiol. Instead of luteinizing hormone levels decreasing under high estradiol levels, they increase. As a result, estradiol levels continue to rise. This dramatic increase of estradiol and luteinizing hormone is called the luteal surge, and results in ovulation.

The corpus albicans develops from the corpus luteum only after ovulation, and is responsible for regulating progesterone levels after the egg has been released.

A spike in the concentration of luteinizing hormone (LH) leads to ovulation on day 14 of the menstrual cycle. This spike is known as the "LH surge" and is initiated by a positive feedback mechanism involving estrogen.

Follicle-stimulating hormone (FSH) is involved in the maturation of the follicle, but not ovulation. Progesterone functions in maintaining the endometrial tissue after implantation has occurred. Testosterone is not involved in the female reproductive cycle.

Estrogen levels peak towards the end of the follicular phase. This causes a surge in the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts about 24-36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary and swept into the fallopian tubes by the fimbriae.

The oviducts, also known as the fallopian tubes, connect the ovaries to the uterus. When an egg, or ovum, is released once each month, it leaves the ovary and flows down the oviduct into the uterus, awaiting fertilization. If fertilization does not occur, then the uterine lining is shed, taking with it the egg, through the uterus and out the vagina. The urethra is connected to the urinary bladder.

Because the fetal lungs and liver are not fully functional until birth, the fetal circulation uses structures in order to bypass these organs.

The ductus venosus is used to reroute blood from the fetal liver and deposit it into the right atrium.

The ductus arteriosus connects the fetal pulmonary arteries to the aorta to bypass the lungs, while the foramen ovale connects the right and left atria to reduce flow to the pulmonary arteries.

The umbilical vein carries blood from the uterus, where it have been loaded with oxygen from maternal circulation, back to the body.

The ductus arteriosus is used to reroute blood from the pulmonary trunk and place it into the aorta of the fetus, preventing flow to the non-functional fetal lungs. After birth, this structure deteriorates and closes. The ligamentum arteriosum is the vestigial structure that is seen in adults. Instead of connecting the pulmonary arteries and aorta, it helps to stabilize these structures.

The foramen ovale is a fetal window between the right and left atria to help blood bypass the pulmonary circuit. After birth, this window closes to become the fossa ovalis.

Following gastrulation, three primary germ layers form: the ectoderm, the mesoderm, and the endoderm. We can typically predict which structures will form from the germ layers based on the layers of the body. The endoderm will form the digestive tract, the mesoderm will form the skeleton and muscles, and the ectoderm will form the skin and nervous system.

The most common location for the fertilization of the egg is the fallopian tube. The zygote then undergoes several divisions and some development before implanting in the endometrium of the uterus as a blastocyst. Abnormal implantation can occur while the blastocyst is still in the fallopian tube, resulting in an ectopic pregnancy. This event generally requires surgery to rectify and can be extremely dangerous to the mother and developing embryo.

Thyroid hormone is secreted by the thyroid and plays a role in bone growth, neural maturation, and the body's basal metabolic rate. It is released in response to thyroid-stimulating hormone (TSH) from the anterior pituitary, and targets cells throughout the body. Hypothyroidism is characterized by reduced synthesis of thyroid hormones, and can present as weight gain, fatigue, and sensitivity to cold.

Cortisol, a hormone secreted by the adrenal gland, functions to maintain blood pressure by upregulating the number of alpha-1 receptors on arterioles.

Gastrin is a hormone secreted by the G cells of the stomach that acts to increase gastric acid secretion, increases the growth of gastric mucosa, increases gastric motility, and stimulates the secretion of digestive enzymes. In addition to stimulating the release of pepsinogen, gastrin stimulates the pancreas to release digestive enzymes. Bicarbonate, however, is not an enzyme and is stimulated by another hormone.

Secretin is a hormone secreted by the S cells in the duodenum that acts to increase pancreatic bicarbonate secretion and decrease gastric acid secretion.

Remember that hormones are generally slow-acting in the body. Glands will respond to the conditions of the body by elevating hormones in a negative feedback fashion. If a person has high blood glucose levels, it is expected that the body is going to increase hormones that act to lower the blood glucose levels.

Insulin is responsible for stimulating glucose sequestration for use in cell metabolism or storage in the liver as glycogen. As a result, insulin levels would be elevated in a person with high blood sugar levels.

Glucagon is functional antagonist of insulin, and serves to elevate blood glucose levels through glycogenolysis and gluconeogenesis. Cortisol is released by the adrenal cortex in response to adrenocorticotropic hormone (ACTH) and long-term stressors, such as prolonged hunger, thirst, and heat/cold exposure. Thyroxine (T4) is a thyroid hormone that mediates basal metabolic rate and is released in response to thyroid-stimulating hormone (TSH).

While testosterone is associated with the growth spurt during puberty, it is also responsible for the closing of the epiphyseal plates on long bones. This process helps stop the growth in length of the body.

Testosterone is the primary male sex hormone and is secreted by Leydig cells of the testes. Testosterone is a steroid hormone, meaning it can enter cells and act directly on DNA as a transcription factor. In this way, it mediates the production of sperm, anatomical growth, and development of secondary sex characteristics in males.

Hypersecretion, or oversecretion, of the growth hormone during childhood can typically result in gigantism. The person will be very tall and have normal face and body proportions; the growth plates are still present at this time and they all receive a larger amount of the hormone.

Dwarfism would be a result of hyposecretion, or undersecretion, of growth hormone during childhood.

Acromegaly is caused by hypersecretion of growth hormone, but during adulthood as opposed to childhood.

Erythropoietin acts on red bone marrow to increase the amount of red blood cells being produced. Aldosterone is responsible for electrolyte homeostasis via the excretory system. Cortisol is the hormone associated with stress response, and regulates blood glucose levels. Lastly, prolactin is responsible for milk production.

Vasopressin, also known as antidiuretic hormone, is released by the posterior pituitary and targets the kidney for water reabsorption. Milk production is controlled by prolactin. Milk ejection is an effect of oxytocin. Lastly, the thymosin and thymopoietin are responsible for lymphocyte development.

ADH inserts special channels into the collecting ducts of the kidney that allow for water absorption from the fluid that is destined to become urine. An increasing amount of ADH would decrease the amount of urine voided from a person as more water is being absorbed in the distal convoluted tubule and collecting ducts of the kidneys. ADH also constricts arterioles everywhere in the body, which increases the peripheral resistance and arterial pressure.

Erythropoietin is a glycoprotein that increases the production of red blood cells. Erythropoietin is secreted by the kidneys and liver, and activates red bone marrow to induce hematopoiesis.

Oxytocin has many effects in the body. First, it stimulates contraction of the myoepithelial cells which surround mammary alveoli in the mammary gland. This causes milk to be ejected. Oxytocin is also released during labor and works to enhance uterine contractions in order to facilitate birth. Lastly, oxytocin is widely known as the "bonding hormone," and studies show that its presence establishes maternal behavior. Oxytocin does not, however, form and regulate the female reproductive system; this is done by the female sex hormone, estrogen.