Organs and Organ Systems - Biology

Insulin stimulates the reuptake of glucose from the blood into the cells. Thus, the glucose levels in the blood decrease, as the glucose is taken into cells. The cells may either store it as glycogen (in liver and skeletal muscle) or use it in glycolysis to make ATP.

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.

Osteogenic cells are a type of stem cell that differentiate into osteoblasts, which allow bone to form. Eventually, osteoblasts will become enveloped into the bone matrix and differentiate into osteocytes. Osteoclasts have the opposite function of osteoblasts, and are responsible for the resorption of bone matrix. This releases calcium into the bloodstream by breaking down bone.

Red bone marrow is found in the epiphyses, or ends of long bones. It is the site of hematopoiesis, or red blood cell development.

Yellow bone marrow is primarily composed of fat. Osteons are the functional units of bone, and house the cells that build and break down the bony matrix. Cartilage is found on the articular surfaces of bone, and helps provide support for joints.

The skeletal system has a variety of functions, including protecting internal organs, storing minerals and energy molecules, and assisting in movement.

The production of heat, however, is a function of the muscular system.

Progesterone ("pro-gestation") is responsible for the maintenance of a pregnancy. Initially, the corpus luteum secretes progesterone in preparation for pregnancy. Then, after fertilization, human chorionic gonadotropin (hCG) is released which causes the corpus luteum to keep secreting progesterone. The placenta takes over it's function after 6 weeks gestation.

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 sodium potassium-pump is used in order to establish the negative resting potential in neurons. Since both sodium and potassium ions are positively charged, there needs to be more ions leaving the cell compared to ions entering. The pump accomplishes this by pumping three sodium ions out of the cell, while pumping two potassium ions into the cell. This loss of positive charge inside the cell results in the negative resting potential of neurons.

Thank about the net transfer of ions. Three positive sodium ions out of the cell for every two positive potassium ions into the cell is the same as one positive ion leaving the cell. When positive ions leave, the inside of the cell becomes more negative, helping the cell reach its resting potential of around –70mV.

In the beginning of an action potential voltage-gated sodium channels begin to open, allowing sodium ions to rush into the cell. This influx of positive ions results in a change in the polarity of the cell, making the voltage become positive inside the cell. This process is called depolarization.

Hyperpolarization comes after depolarization, and is caused by potassium ions leaving the cell interior. The removal of these positive ions causes the cell to become more negative than the resting potential.

Repolarization is the final process to return the cell to its resting potential. The sodium-potassium pump brings potassium ions back into the cell and removes the sodium ions, returning the cell to its normal resting state.

Action potentials are electrical signals transmitted by neurons. When a neuron is stimulated, a signal is transmitted down the axon. This signal is the action potential.

An action potential in a neuron can help to stimulate a muscle to contract, but the muscle itself will not conduct an action potential.

The axon is wrapped in fatty bundles called myelin sheaths that promote fast transmission of an electrical signal. The other structures listed here are not myelinated.

The refractory period occurs when the cell repolarizes/hyperpolzarizes beyond the resting potential; that is, the membrane potential drops to a value more negative than when it is at rest. This prevents the firing of another action potential immediately after one has been fired. The other values represent the resting potential (), the threshold (), and values that are more positive, and are therefore incorrect.

As an action potential begins, there's a rapid influx of sodium in to cell, causing the cell's membrane potential to rapidly increase, depolarizing the cell. Once the cell has reached its action potential peak, the sodium channels begin to close. This closing activates the potassium channels. These channels allow potassium to leave the cell. Since potassium is a positive ion, as it leaves, the cell's membrane potential becomes more negative, repolarizing. The slight dip in the action potential curve, labeled as hyperpolarization, is result of the potassium channels lagging to close, and potassium loss is "overshot". As a result, too much potassium lost from the cell will cause the cell's potential to become more negative relative to its normal potential.

Action potentials are largely due to the movements of potassium and sodium across a membrane. While other ions and neurotransmitters can affect action potential firing, the movements of these two ions have the greatest effect on a neuron firing.

As an action potential is essentially an electrical current, it makes sense for it to open voltage-gated channels. Specifically, voltage-gated calcium channels are opened to allow calcium ions to flow into the cell and bind to synaptic vesicles.

The sympathetic nervous system is responsible for stress responses, while the parasympathetic nervous system is responsible for resting responses. The sympathetic nervous system causes increased heart rate, pupil dilation, suppressed digestion, inhibited salivation, and dilated bronchi.