Question

question 42 (1 point)
which process begins the formation of urine in a mammalian nephron?
active transport
countercurrent flow
osmosis
pressure filtration
question 43 (1 point)
the following structures are parts of a nephron. in which of these structures is water re - absorbed?
i. proximal tubule
ii. distal tubule
iii. descending limb of the loop of henle
iv. ascending limb of the loop of henle
v. collecting duct
i only
i and ii only
i, ii, iii, and iv only
i, ii, iii, and v only

Explanation:

• Question 42: The formation of urine in a mammalian nephron begins with pressure filtration. Blood pressure forces fluid and small solutes out of the glomerulus (a tuft of capillaries) into the Bowman's capsule, forming the filtrate. This is the first step in urine formation. Active transport is involved in re - absorption and secretion processes later. Countercurrent flow is related to the concentration of urine (in the loop of Henle and vasa recta), and osmosis is a part of the re - absorption process (e.g., in the descending limb of the loop of Henle and collecting duct) but not the initial step.
• Question 43:
• Proximal tubule: Re - absorbs a large amount of water (along with other solutes like glucose, amino acids, and ions) through osmosis (as sodium is actively transported out, creating an osmotic gradient).
• Distal tubule: Also re - absorbs some water (regulated by hormones like aldosterone and antidiuretic hormone (ADH)).
• Descending limb of the loop of Henle: Permeable to water. Water moves out of the descending limb into the hypertonic medulla (by osmosis).
• Ascending limb of the loop of Henle: Impermeable to water. It actively transports ions (sodium, potassium, chloride) out.
• Collecting duct: Re - absorbs water (regulated by ADH). When ADH is present, it makes the collecting duct more permeable to water, allowing for further concentration of urine.

Answer:

• Question 42: pressure filtration
• Question 43: I, II, III, and V only