Bloodhounds are biologically adapted to trailing their prey. The process by whichthe nose recognizes an odor is not fully understood, but there are apparently specificreceptor sites for specific odors. In one explanation, recognition occurs when a scentmolecule fits into its corresponding receptor site, like a key into a lock, causing a(Line5) mechanical or chemical change in the cell. Bloodhounds apparently have denserconcentrations of receptor sites tuned to human scents.
When a bloodhound trails a human being, what does it actually smell? The humanbody, which consists of about 60 trillion living cells, sheds exposed skin at a rate of 50million cells a day. So even a trail that has been dispersed by breezes may still seem(Line10) rich to a bloodhound. The body also produces about 31 to 50 ounces of sweat a day.
Neither this fluid nor the shed skin cells have much odor by themselves, but thebacteria working on both substances is another matter. One microbiologist estimatesthe resident bacteria population of a clean square centimeter of skin on the humanshoulder at "multiples of a million." As they go about their daily business breaking(Line15) down lipids, or fatty substances, on the skin, these bacteria release volatilesubstances that usually strike the bloodhounds nose as an entire constellation ofdistinctive scents.
45. What does the passage mainly discuss?
(A) Why people choose bloodhounds for
(B) How a bloodhounds sense of smell works(C) How humans compensate for an
underdeveloped sense of smell
(D) The way in which bacteria work on skin cellsand body sweat
46. The author compares a scent molecule with a(A) key
47. In line 7, the word "it" refers to
(B) human being
48. According to the passage, how many cells ofskin does the human body rid itself of every day?
(A) 60 trillion
(B) 50 million
(C) 1 million
(D) Between 31 and 50
49. In line 10, the word "rich" is used to mean that atrail is
(A) paved with precious materials
(B) a profitable business to get into
(C) a very costly undertaking
(D) filled with an abundance of clues
50. Which of the following acts as a stimulus in theproduction of the human scent?
bsp;(B) Dead skin cells
(D) Fatty substances