Question

mammals regulate blood pressure by changing the diameter of blood vessels. when blood pressure decreases, cells of the kidney secrete an enzyme called renin. this enzyme binds a substrate called angiotensinogen and releases a product called angiotensin. angiotensin binds to cells of blood vessels, causing them to constrict. the decreasing diameter of blood vessels raises the blood pressure.

the top figure shows a set of linear relationships among three variables involved in regulating blood pressure.

the bottom figure shows four path models (a through d) that represent possible homeostatic systems that regulate blood pressure. the regulated variable is represented by a dashed box. other variables are represented by a solid black box. an arrow connecting one box to another indicates a relationship between two variables. the sign above an arrow (+ or -) indicates whether the variables are positively or negatively related.

which path model accurately describes the relationships among the three variables?
a. model a
b. model b

Explanation:

Analyze the biological relationships

Using the Homeostasis and Homeostatic Regulation knowledge points:

• Blood pressure to Renin/Angiotensin: "When blood pressure decreases, cells of the kidney secrete... renin... releases... angiotensin." This means a decrease in blood pressure leads to an increase in angiotensin concentration. Thus, the relationship from Blood pressure to Concentration of angiotensin is negative (-).
• Angiotensin to Vessel Diameter: "Angiotensin binds to cells of blood vessels, causing them to constrict." Constriction means a decrease in diameter. Thus, an increase in angiotensin concentration leads to a decrease in the diameter of blood vessels. This relationship is negative (-).
• Vessel Diameter to Blood Pressure: "The decreasing diameter of blood vessels raises the blood pressure." This means a decrease in diameter leads to an increase in blood pressure (and an increase in diameter would lead to a decrease in blood pressure). Thus, the relationship from Diameter of blood vessels to Blood pressure is negative (-).

Match relationships to path models

Using the Path Models and Linear Relationships knowledge points:

• **Blood pressure \(

ightarrow\) Concentration of angiotensin**: Negative (-) arrow.

• **Concentration of angiotensin \(

ightarrow\) Diameter of blood vessels**: Negative (-) arrow.

• **Diameter of blood vessels \(

ightarrow\) Blood pressure**: Negative (-) arrow.

Let's examine the four options shown in the top-left diagrams:

• Model A:
• Blood pressure \(

ightarrow\) Concentration of angiotensin: (-)

• Concentration of angiotensin \(

ightarrow\) Diameter of blood vessels: (-)

• Diameter of blood vessels \(

ightarrow\) Blood pressure: (-)
All three arrows are labeled with a minus sign (-). This perfectly matches our derived relationships.

• Model B: Shows positive (+) relationships for some paths.
• Model C: Shows positive (+) relationships for some paths.
• Model D: Shows positive (+) relationships for some paths.

Therefore, Model A is the correct path model.

Answer:

• (A) Model A (Correct answer)
• (B) Model B
• (C) Model C
• (D) Model D