Organs and Organ Systems - Biology

Insulin is an example of an endocrine hormone because it is secreted into the blood and transported to distant areas of the body. Insulin is released from the pancreas, but acts on numerous regions of the body, including the liver and muscle cells.

Autocrine and paracrine signaling involve signaling close to the cell that secreted the hormone. In paracrine signaling, molecules are secreted by one cell and bind to receptors on an adjacent cell to elicit a response. In autocrine signaling, the secreted compounds bind to receptors on the surface of the same cell from which they were released, eliciting a response from the same cell. Exocrine secretions are released into ducts designed to connect outside of the body, such as the digestive tract or sweat glands. This is in contrast to endocrine secretions, which enter the blood or interstitium.

The thyroid gland is responsible for secreting thyroid hormones (T3 and T4), which are responsible for setting the basal metabolic rate (BMR). The concentrations of these hormones are what tell the cells which metabolic pathways to undergo. Since body heat is a byproduct of metabolism, thyroid hormone also determines body temperature.

The adrenal medulla secretes epinephrine (adrenaline), which is involved in the body's "fight of flight" response. Epinephrine is released in response to direct neural stimulation during periods of short-term stress and acts to stimulate the sympathetic nervous system.

The pituitary gland is known as the "master" endocrine gland because it secretes several hormones that control other endocrine glands. These are known as tropic hormones. Adrenocorticotropic hormone, thyroid-stimulating hormone, and follicle-stimulating hormone are some examples of pituitary tropic hormones.

Steroid hormones are the only ones derived from cholesterol and are always characterized by a four ring molecular structure. Some examples include sex hormones such as androgens and estrogens as well as some adrenal hormones like cortisol (a glucocorticoid) and aldosterone (a mineralocorticoid).

Increased potassium levels will stimulate aldosterone to increase potassium secretion. The main regulators of aldosterone are potassium and the renin-angiotensin pathway.

The spleen is like a giant lymph node, and it is organized in a somewhat similar manner. Although it can be surgically removed if it is damaged, such patients are at life-long risk of death from fairly ordinary infectious processes. The spleen is a reservoir of immune competence. Blood passes through the spleen for exposure to white blood cells. When the white blood cells detect antigens or foreign particles in the blood, they initiate the immune response. The spleen is essentially a screening center to check the blood for contaminants.

Adaptive immunity occurs when antibodies are produced as a result of exposure to a pathogen or immunization. These antibodies are specific for the particular microorganism and memory cells are produced. Cell-mediated immunity is a direct form of defense based on the action of lymphocytes to attack foreign cells and destroy them. Congenital immunity is immunity one is born with. This may result from antibodies received from the mother's blood. Innate immunity is not pathogen-specific and includes the secretion of proteins and the activities of natural killer cells. Passive immunity involves the introduction of preformed antibodies into an unprotected individual. This may occur through infusion of immune globulin or antibodies that pass from the mother to the fetus through the placenta.

VDJ recombination occurs during early B- and T-cell maturation, resulting in diverse antibodies and T-cells. This DNA recombination occurs between the V, D, and J segments of the antibody or T-cell before transcription occurs. As a result, a unique sequence is generated, transcribed, and then translated to a functional protein. This recombination is responsible for creating the unique series of antibodies that the body is capable of producing in order to detect the various antigens represented by foreign pathogens.

B cells and T cells are both part of the adaptive immunity. B cells secrete antibodies that bind to foreign antigens. Upon binding to a specific antigen, B cells can be activated by T cells, which facilitate the synthesis of specific antibodies for the antigen. This enhances the antibody-antigen binding and allows for a better immune response. T cells have receptors on their surface that detect antigens. Once they detect the antigen, T cells can activate B cells and other immune system cells (such as macrophages and neutrophils) to eliminate the foreign antigen. B cells do not play a role in the activation of T cells.

The question states that the immune complex has antibodies bound to antigens. Recall that B cells eliminate pathogens by secreting antibodies. These antibodies bind to antigens and release factors called cytokines. Cytokines recruit phagocytic cells like macrophages and neutrophils that kill the infected cell. They also activate a part of the innate immune system called the complement, which aids in the elimination of the pathogen. This type of immune response is called a humoral immune response. Elimination of the pathogen using T cells is called a cell-mediated immune response.

Note that both the humoral and the cell-mediated immune responses are very specific responses that are part of the adaptive immunity. Innate immunity involves non-specific immune responses via macrophages, granulocytes (neutrophils, eosinophils, and basophils), complement system, and NK cells.

Plasma cells are circulating cells that form part of adaptive immunity that secrete antibodies to specific antigens. These cells arise from naïve B cells. Broadly specific naïve B cells have the ability to bind to several antigens. Once bound, these naïve B cells differentiate into plasma cells that secrete antibodies that are very specific to the antigen. T cells facilitate this differentiation, but only B cells give rise to plasma cells.

A naïve T cell has the ability to differentiate into three kinds of cells. First, it can differentiate into a helper T cell. These cells facilitate the activation of other immune cells such as B cells, macrophages, and granulocytes. Second, a naïve T cell can differentiate into a cytotoxic T cell. These cells bind to infected cells and induce their death. Third, a naïve T cell can differentiate into a regulatory T cell. These T cells bind to the same antigens as the first two cells; however, instead of initiating an immune response, they regulate it by suppressing the activity of T cells.

HIV is a virus that likes to reside inside helper T cells. A person infected with HIV will have a decreased helper T cell count, which makes the person more susceptible to other opportunistic infections (infections that only occur in immune-compromised individuals). A patient with very low helper T cell count develops AIDS and often passes away due to these opportunistic infections.

There are three kinds of T cells: helper T cells, cytotoxic T cells, and regulatory T cells. All T cells have glycoproteins on their surfaces that act as receptors. CD4 and CD8 are two glycoproteins that can be found on T cells. Helper T cells and regulatory T cells have CD4 glycoproteins, whereas cytotoxic T cells have CD8. These glycoproteins serve as markers to distinguish between T cell types.

The question is asking about CD8, or cytotoxic, T cells. Recall that cytotoxic T cells bind to infected cells and induce their death. Typically, cytotoxic T cells bind to infected cells that have the pathogen inside them (meaning intracellular pathogens). Intracellular pathogens include viruses and intracellular bacteria; therefore, T cells that attack these cells will be CD8 cells. In addition, cytotoxic T cells also attack cancer cells; therefore, these T cells will also be CD8 cells.

Extracellular bacterial cells do not infect host cells; therefore, these bacteria are eliminated via the helper T cells. These T cells bind to the bacteria and activate other immune cells such as B cells, macrophages, and granulocytes that eliminate the bacteria.

Eggs, or oocytes, are produced in the ovaries. During ovulation, the egg is released from the ovary into the fallopian tube. If fertilized, the egg transitions into a zygote and is implanted into the uterus for gestation and development.

The spleen is involved in filtering the blood and lymph, and is not involved in reproduction.

The epididymis is responsible for storing sperm in order for it to mature. Upon ejaculation, it will be propelled into the vas deferens and out of the urethral opening.

The ovaries release eggs into the fallopian tubes, which carry the egg to the uterus.

Fimbriae are protrusions from the fallopian tube that receive the egg from the ovary before transferring it to the fallopian tube itself. The uterus houses the endometrium and, if fertilization occurs, the developing embryo. The cervix is the structure that separates the uterus from the vaginal canal.

An ectopic pregnancy occurs when the egg gets implanted into the fallopian tube rather than the uterus. This is painful, and leads to surgical procedures in order to protect the mother.

The uterus is a muscular organ in the pelvic cavity. It is also called the womb. The uterus is divided into three regions: the upper region (the fundus), the midportion (the body), and the cervix. The ovaries are almond-shaped organs located in the pelvic cavity. The fallopian tubes connects the ovaries and the uterus. A fallopian tube is divided into three sections: the infundibulum, the ampulla, and isthmus. The vagina, or birth canal, is a muscular tube connecting the cervix to the vulva. The epididymis is a twisted tube that is located on and beside a testis.

The prostate gland is located immediately below the bladder and anterior to the rectum, and it surrounds the urethra. The gland secretes a fluid that becomes part of the seminal fluid. The epididymis rests on and beside the posterior surface of a testis. It is the first part of the secretory duct of each testis. The vas deferans is a continuation of the epididymis and transports sperm from each testis to the ejaculatory duct, which empties into the prostatic urethra. The ovaries are part of the female reproductive system and located in the pelvic cavity attached to the uterus. The ovaries produce the female reproductive cell, the ovum, and hormones. The uterus is located in the pelvis between the sacrum and pubic symphysis. The uterus contains and nourishes the embryo and fetus.

The ovaries contain 1-2 million eggs at birth. This number slowly declines throughout the female's life. The quality of eggs remaining after age 35 is low, whereby years of spontaneous mutation have occurred.