The "gray matter" of the brain is largely made up of which of the following?

A. Parathyroid hormone (PTH) is secreted by the parathyroid glands and plays a critical role in regulating calcium levels in the blood. It acts on bones, kidneys, and the intestine to increase blood calcium levels when they are low. In the bones, PTH stimulates osteoclasts, which break down bone tissue, releasing calcium into the bloodstream. In the kidneys, it increases the reabsorption of calcium, preventing its loss through urine. Additionally, PTH influences the intestines to absorb more calcium from the food we eat.

B. Norepinephrine is a hormone and neurotransmitter involved in the body's fight or flight response. It does not regulate blood calcium levels but rather affects heart rate, blood pressure, and blood glucose levels.

C. Growth hormone, produced by the pituitary gland, stimulates growth in children and adolescents and helps to maintain muscle mass and bone density in adults. It does not directly regulate calcium levels in the blood.

D. Melatonin is a hormone secreted by the pineal gland, primarily influencing sleep-wake cycles and circadian rhythms. It does not have a role in regulating blood calcium levels.

A. Conversion to carbon monoxide is not a physiological process that occurs in the human body. Carbon monoxide is a toxic gas that can bind to hemoglobin with a higher affinity than oxygen, which can lead to carbon monoxide poisoning.

B. Active transport is a process that requires energy to move substances against their concentration gradient. It is not the primary method for carbon dioxide movement in the lungs.

C. Passive transport using carrier proteins, also known as facilitated diffusion, is a process where substances move across cell membranes with the help of membrane proteins. However, this is not the main mechanism for carbon dioxide transfer in the alveoli.

D. Diffusion down a concentration gradient is the process by which carbon dioxide moves from the blood, where its concentration is higher, into the alveoli, where its concentration is lower. This movement is due to the natural tendency of molecules to spread out evenly in an available space, and it does not require energy. This is the primary method for gas exchange in the lungs, allowing carbon dioxide to be expelled from the body during exhalation.

A. The excretory system is responsible for the removal of waste products from the body's metabolism, including the filtration and excretion of waste material from the blood by the kidneys.

B. The digestive system breaks down proteins into amino acids through the action of enzymes. Proteins are first denatured by stomach acid, then enzymes like pepsin begin the breakdown, which is continued in the small intestine by other enzymes such as trypsin and chymotrypsin.

C. The endocrine system consists of glands that secrete hormones into the bloodstream, which regulate processes such as growth, metabolism, and sexual development. It does not play a direct role in the breakdown of proteins into amino acids.

D. The circulatory system is responsible for the transportation of blood throughout the body, which includes nutrients, gases, and waste products, but it does not break down proteins into amino acids. This process is solely the function of the digestive system.

A. The rate of fall is affected by mass of the object, since object D and C fall at a faster rate than A. However it is important to consider other factors such as shape and size of the object.

B. Although objects A and B have the same mass, they do not fall at the same rate, as evidenced by their different times of fall.

C. The data does not support the conclusion that greater mass leads to a faster fall. Object C, with a mass of 30.0g, falls in 0.5sec, while object D, with a greater mass of 35.0g, falls in a longer time of 1.5sec.

D. A and B have the same mass but have different rates of fall. This indicates that air resistance is greater for A, hence the slower rate of fall.

A. The renal vein is the vessel that drains oxygen-depleted blood away from the kidneys. It connects the kidneys to the inferior vena cava, which then carries the blood back to the heart to be re-oxygenated.

B. The renal artery is responsible for delivering oxygen-rich blood from the heart to the kidneys. It branches off from the abdominal aorta and enters the kidneys to supply them with the necessary oxygen and nutrients.

C. The urethra is a tube that connects the urinary bladder to the urinary meatus for the removal of fluids from the body. It is not involved in the drainage of blood from the kidneys.

D. The ureter is a duct that allows the passage of urine from the kidneys to the urinary bladder. Similar to the urethra, it does not carry blood but is part of the urinary system to excrete urine.

A. Blood pumped from the heart first enters arterioles that then merge to form arteries that deliver blood to the organ. This statement is incorrect because the flow of blood from the heart starts in the arteries, not arterioles, and arterioles do not merge to form arteries but rather are smaller branches of arteries.

B. Blood pumped from the heart first enters arteries that then narrow to form veins that deliver blood to the organ. This statement is incorrect as veins do not deliver blood to organs; veins carry blood back to the heart.

C. Blood pumped from the heart first enters arteries that then narrow to form arterioles that deliver blood to capillaries of the organ. This statement is correct. The blood leaves the heart through the arteries, which branch into smaller arterioles. The arterioles further branch into capillaries, where the exchange of gases and nutrients occurs within the tissues of the organ.

D. Blood pumped from the heart first enters veins that then narrow to form venules that deliver blood to arteries of the organ. This statement is incorrect because veins and venules are part of the return pathway to the heart, not the outbound pathway to the organs, and they do not deliver blood to arteries.

A. Both endocrine and exocrine glands produce secretions that affect target cells, but the way they deliver these secretions to the target cells is different. Endocrine glands release hormones directly into the bloodstream, which then travel to the target cells, while exocrine glands secrete their products into ducts that lead directly to the target areas.

B. Nerve innervation refers to the supply of nerves to a particular part of the body or organ.

While nerve innervation can influence the activity of glands, it is not the primary distinguishing feature between endocrine and exocrine glands.

C. The biochemical nature of the secretions can vary widely among different glands, but this is not the main characteristic that differentiates endocrine from exocrine glands. Both can produce a variety of biochemical substances, including enzymes, hormones, and other compounds.

D. This is the key difference between endocrine and exocrine glands. Endocrine glands are ductless and release their hormones directly into the bloodstream, allowing them to be transported throughout the body. In contrast, exocrine glands have ducts through which they release their secretions to the surface of an organ or tissue, or to the outside of the body.

A. Urea is a waste product formed in the liver through the metabolic breakdown of proteins and is excreted by the sweat glands. When proteins are broken down, ammonia is formed, which is toxic to the body. The liver converts ammonia into urea, which is less toxic and can be safely transported in the bloodstream to be eliminated through sweat and urine.

B. Lysozymes are enzymes that are part of the immune system and are found in various body fluids, including sweat. However, they are not specifically related to the breakdown of proteins and the formation of ammonia.

C. Sebum is an oily substance produced by sebaceous glands, not sweat glands. It helps to lubricate and protect the skin but is not involved in the excretion of waste products from protein metabolism.

D. Water is a major component of sweat and is excreted by sweat glands to help regulate body temperature through evaporative cooling. While it is a component of sweat, it is not specifically excreted in response to protein breakdown and ammonia formation.

A. Methane molecule (CH4) consists of one carbon atom forming four covalent bonds with four hydrogen atoms.

B. Ammonia molecule (NH3) involves one nitrogen atom forming three covalent bonds with three hydrogen atoms.

C. Chlorine molecule (Cl2) is made up of two chlorine atoms sharing a single covalent bond between them.

D. Water molecule (H2O) has one oxygen atom forming two covalent bonds with two hydrogen atoms.

A. This option changes the population, which could introduce new variables related to gender differences in metabolism or physical performance.

B. This option introduces another variable by using two different energy drinks, which could have different ingredients and effects.

C. This option changes the distance run, which could affect the endurance aspect rather than the speed.

D. To best support the conclusion that energy drinks increase the speed of running in women, it would be most effective to conduct a controlled experiment where a variable is changed while others are kept constant. By giving half of the women water and the other half the energy drink, researchers can directly compare the effects of the energy drink against a baseline (water), which has no enhancing properties on running speed. This comparison would help to isolate the effect of the energy drink on running performance