Which of the following respiratory structures is affected by a disease that disrupts gas exchange, such as emphysema?

The parathyroid hormone (PTH) from the parathyroid glands is important in serum calcium regulation. It increases blood calcium levels by stimulating osteoclasts to break down bone and release calcium into the bloodstream. PTH also enhances the absorption of calcium from the intestines and the kidneys.

Melanocytes are specialized skin cells that produce melanin, which is the protective skin-darkening pigment. They are found mainly in the epidermis, though they occur elsewhere, such as in the hair.

Helper T cells are important cells that stimulate the action of other immune cells. For instance, they not only help activate B cells to secrete antibodies and macrophages to destroy ingested microbes, but they also help activate cytotoxic T cells to kill infected target cells.

This is the plate with the most growth despite the presence of antibiotic cover.

The kidneys are the primary organs involved in the excretion of nitrogenous wastes. Nitrogenous wastes are mostly products of protein metabolism in the liver. One of the common nitrogenous waste products is urea, which is excreted through the kidneys.

The experienced mountain climber is replicating the conditions of the past expedition to understand why that earlier attempt was unsuccessful.

The endocardium lines the interior structures of the heart. It is a thin, innermost layer of the heart wall that lines the heart chambers and valves. It is composed of subendocardial connective tissue and endothelial cells that are continuous with blood vessel endothelium.

• Potassium is an alkali metal, which tends to lose one electron to form a cation with a \(\pm1\) charge.
• Likely to become a cation.

Browning of the apple surface is due to the action of the enzyme polyphenol oxidase. Lemon juice is acidic and lowers the pH of the apple's surface. This inactivates the enzyme polyphenol oxidase, which works best at a neutral pH. This in turn reduces the browning action on the apple.

The pH scale is logarithmic, indicating that each whole number change on the pH scale represents a tenfold change in acidity or alkalinity. Therefore, a substance with a pH of 3 is 10 times more acidic than a substance with a pH of 4.