The myelin sheath surrounds the axon of neurons and speeds up the propagation of action potentials by allowing them to skip along the axon.
Of the glands in the answer choices, only the salivary glands do not release any hormones into the bloodstream. They release saliva into the mouth by way of ducts.
Only arteries have muscular walls that can regulate blood flow. Because arterial blood pressure tends to be high compared to blood pressure in the veins, arteries do not contain valves to prevent backflow of blood. Veins do contain valves, however, because blood flows relatively sluggishly through them.
The four-chambered heart is an evolutionary advance over the two-chambered heart. By splitting up oxygenated and deoxygenated blood, the four-chambered heart ensures that more highly oxygenated blood reaches the cells that need it. The four-chambered heart does not raise or lower blood pressure. Neither does it increase blood flow to the lungs or the brain.
Because carbon dioxide is dissolved in the blood as carbonic acid, when levels of carbon dioxide in the blood are high, the pH tends to drop (the blood becomes more acidic). When carbon dioxide levels are low, the blood can become slightly more basic. Respiratory rate is adjusted to control for these factors. A safe pH level is extremely important for the operation of proteins, hormones, and enzymes.
After the follicle releases the ovum during ovulation, it changes form to become the corpus luteum. The corpus luteum remains in the ovary producing progesterone and begins to disintegrate if fertilization does not take place.
When a newly hatched bird first sees another moving object, it instinctually believes that the object is its mother and will follow it. This phenomenon is called imprinting. Tropisms occur in plants. Conditioning is a learned rather than instinctual behavior, as is associative learning. The sympathetic nervous system operates on the body in emergency situations.
Carbon dioxide, a waste product of cell respiration, diffuses from cells into the circulatory bloodstream, and is expelled from the body through the lungs of the respiratory system.
Most of photosynthesis takes place in the palisade layer of the leaf, so the cells of the palisade layer have the highest concentration of chloroplasts.
Chloroplasts make carbohydrates during the Calvin cycle, which occurs in the intermembrane space of the chloroplast called the stroma. The light reaction creates the ATP and NADPH that power the Calvin cycle. Oxygen is a by-product of photosynthesis, not a raw material. Grana are the location of the light reaction.