The sympathetic nervous system is one of two divisions of the autonomic nervous system; it is responsible for the "fight or flight" response and is involved in homeostasis. Some sympathetic fibers pass through the paravertebral ganglia of the sympathetic trunk, while other sympathetic fibers synapse there.

Parasympathetic nerves are associated with cranial nerves, and the dilation of blood vessels in skin of the back and limbs (the sympathetic system constricts these vessels). Preganglionic neurons originate in the thoracolumbar region of the spinal cord (T2 to L1) then travel to a paravertebral ganglion or prevertebral ganglion, where they synapse with a postganglionic neruon. The paravertebral ganglion are found throughout the length of the spinal cord, including the cervical, thoracic, lumbar, and sacral areas.

During the normal inspiratory phase of breathing, in other words, when a human is "breathing in," the physiological mechanism used is called "negative-pressure" breathing. Negative-pressure refers to the pressure in the chest cavity as compared to the surrounding environment. The body generates negative-pressure in the chest cavity during breathing by the contraction of the diaphragm muscle (it pulls downward, expanding the thoracic cavity size and space for the lungs to fill), and the outward expansion of the ribcage (which also expands the thoracic cavity size and provides more space for the lungs to fill). With the increased volume of the thoracic cavity generated, this creates the negative pressure that is needed to draw air into the lungs down its gradient of higher pressure (outside the body/thorax) to lower pressure (into the lungs/thorax). Positive-pressure is an incorrect choice because it is the opposite of what occurs during normal human inspiration.

Positive-pressure is sometimes artificially used in the medical setting with machines in patients with obstructive sleep apnea, or in patients who cannot breathe on their own, but is not a part of standard physiological respiration. The Frank-Starling mechanism describes the mechanism by which the heart pumps blood, but does not describe respiration. Glomerular filtration describes the mechanism by which the glomeruli of the kidneys initially filter blood, but does not describe respiration. Idiopathic pulmonary fibrosis is a disease of the interstitium of lung tissue, but does not describe the physiological mechanism used during inspiration.

The correct answer is red marrow.

Hematopoiesis is the formation of blood cells, which is a function of red marrow. Red marrow forms red blood cells, white blood cells and blood platelets. The process beings with a stem cell that proliferates (reproduces rapidly), differentiates (becomes a specialized cell) and replicates (duplicates).

The other choices are all parts of bones, but have different functions/characteristics besides hematopoiesis. While red marrow contains the red and white blood cells and blood platelets, yellow marrow consists of mostly fat cells. Compact bones are hard and contain closely packed osteons that form a solid mass. The periosteum is the connective tissue covering the surface of a bone. Spongy bones are less dense than compact bone and are lighter than compact bones. It contains plates (trabeculae) of bone and cavities that contain the red marrow.

The human kidney has many functions, all of which are important for sustaining life. The kidney controls blood osmolarity by filtering, reabsorbing, and secreting various ions found throughout the body. The kidney controls fluid volume in the body by this same mechanism. Additionally, the kidney produces erythropoietin and renin which stimulate the production of red blood cells and convert angiotensinogen to angiotensin I, respectively.

The correct answer is a negative pressure change within the pleural space. When the diaphragm and intercostal muscles contract, they expand the chest cavity creating a higher volume of space within the pleural space. As volume within this space increased, the pressure responds by decreasing. This drop in pressure within the pleural space causes air from outside the body (high pressure) to low pressure (within the chest and the lungs).

The heart has four chambers. During diastole the atria contract to push blood into the ventricles, which are relaxed, but during systole the atria relax to fill with blood while the ventricles contract. This alternating contraction moves blood through the heart, the pulmonary circulatory path, and eventually out of the heart.

When the atria contract during diastole, blood is moving into the right ventricle and also into the left ventricle. During systole when the ventricles contract, blood is moving from the right ventricle towards the pulmonary circuit and from the left ventricle to the rest of the body.

The voltage-gated channels along the axonal plasma membrane open and close in response to changes in voltage, and may exist in three distinct states: deactivated, activated, and inactivated. While the axon is at rest, these channels are said to be deactivated; they are impermeable to sodium ions since their activation gates are closed. Once the neuron gets depolarized to the threshold of the voltage-gated sodium channels, the activation gates open, allowing the influx of sodium down its concentration gradient into the cell. During this time the channels are in their activated state. At the peak of the action potential the activation gates are still open, but the inactivation gates close, stopping the flow of sodium through the channels. The channels are in the inactivated state due to the cell becoming depolarized. Once the membrane potential drops back down towards resting, the inactivation gates open, and the activation gates close, thereby deactivating the channels again, until another action potential depolarizes the membrane.

Veins always carry blood towards the heart. The blood in veins is mostly deoxygenated, however the pulmonary vein, which goes from the lungs to the left atrium, carries newly oxygenated blood back to the heart for it to be pumped to the rest of the body.

In contrast, arteries always travel away from the heart and usually carry oxygenated blood, with the exception of the pulmonary arteries. Arteries and arterioles have a thick layer of smooth muscle that helps to regulate blood pressure. Veins may have some smooth muscle, but are not nearly as significant in helping to regulate blood flow.

The voltage-gated channels along the axonal plasma membrane open and close in response to changes in voltage, and may exist in three distinct states: deactivated, activated, and inactivated. While the axon is at rest, these channels are said to be deactivated; they are impermeable to sodium ions since their activation gates are closed. Once the neuron gets depolarized to the threshold of the voltage-gated sodium channels, the activation gates open, allowing the influx of sodium down its concentration gradient into the cell. During this time the channels are in their activated state. At the peak of the action potential the activation gates are still open, but the inactivation gates close, stopping the flow of sodium through the channels. The channels are in the inactivated state due to the cell becoming depolarized. Once the membrane potential drops back down towards resting, the inactivation gates open, and the activation gates close, thereby deactivating the channels again, until another action potential depolarizes the membrane.

Neurons are the cells that make up the nervous system. Neurons are large, permanent cells that do not divide during adulthood and spend most of their lives in the G0 phase of the cell cycle. If part of a neuron is damaged, it undergoes Wallerian degeneration, meaning that the neuron degenerates distal to the injury, and does not undergo reactive gliosis in response to injury. Astrocytes, a type of glial cell, do this.