Homeostasis and feedback loops

Contraction of the smooth muscle narrows the vessel (vasoconstriction). A narrower vessel resists flow more, so less blood flows through it. Relaxation widens it (vasodilation) and flow rises.

• Correct: The artery narrows and flow falls:
• The artery widens and flow rises:
• The artery narrows and flow rises:

A hormone enters the blood and travels through the body. It changes only target cells with a receptor protein that binds it. Nerves carry action potentials, and ducts belong to exocrine glands.

• Along a nerve as action potentials:
• Through a duct onto a surface:
• Correct: In the blood:

A nerve is a bundle of neuron fibers. Each fiber carries action potentials, the electrical signals of neurons, from one place to another.

• Blood:
• Correct: Action potentials along neuron fibers:
• Hormones in a duct:

The effector is the tissue that carries out the command. Here the smooth muscle in the vessel walls contracts, causing vasoconstriction.

• The cold air of the room: The cold air causes the stimulus, the drop in temperature. It is not part of the body's loop.
• The nerves carrying signals from the brain: Nerves carrying commands from the brain are the efferent pathway, not the effector.
• Correct: Smooth muscle in the walls of the skin's blood vessels: Correct. The smooth muscle does the work that changes the variable, so it is the effector.
• The temperature control center in the brain: The brain region that compares temperature with the set point is the control center.

• Diameter of the blood vessels in your skin: up. The control center sends signals that relax the smooth muscle of skin blood vessels, so they dilate (vasodilation).
• Amount of sweat released onto your skin: up. The control center signals the sweat glands to release sweat; as it evaporates, it carries heat away.
• Heat lost from your skin to the air: up. More warm blood near the surface and more evaporating sweat both increase heat loss, which opposes the rise in temperature.

Labor contractions are driven by positive feedback: stretch of the lower opening of the uterus causes hormone release, which strengthens contractions, which cause more stretch. A positive loop cannot stop itself. Delivery is an outside event that removes the stretch, so the stimulus disappears and the loop ends.

• Negative feedback has lowered the contractions back to their set point: The contractions grew stronger, not steadier, which is positive feedback. There is no set point that they are returned to.
• The uterine muscle has used up all of its ATP: Muscle does not simply run out of ATP within minutes of delivery. The loop stops because its stimulus is gone.
• Correct: Delivery removes the stretch that was driving the loop: Correct. Removing the stretch removes the stimulus, and a positive loop ends when its stimulus is removed from outside.
• The control center has adjusted to a new set point: Positive feedback does not settle at a set point. The loop ends because something outside it, delivery, removes the stimulus.

"Negative" means the response opposes the stimulus, not that it lowers the variable. The stimulus was a fall in pH; the response raises it. Opposing the change back toward the set point is negative feedback.

• Positive feedback: Positive feedback would push the pH further in the direction it was already moving, lower still. Here the response raises it, which opposes the change.
• Correct: Negative feedback: Correct. A fall triggered a rise: the response opposes the change, which is negative feedback. "Negative" means opposing, not lowering.
• Mass balance: Mass balance compares the input and output of a substance. It describes what disturbs a variable, not the loop that corrects it.
• No feedback at all: A response that pulls the variable back toward its set point is a feedback loop at work. Moving outside the normal range is the stimulus that starts it.

Change per day = input − output = 200 − 170 = +30 mmol. Over four days: 30 × 4 = +120 mmol.

• It falls by 120 mmol: The sign is wrong. Input is larger than output, so sodium builds up rather than falling.
• It rises by 30 mmol: 30 mmol is the change for one day. Multiply by four days.
• Correct: It rises by 120 mmol: Correct. +30 mmol per day for four days is +120 mmol.
• It rises by 800 mmol: 800 mmol is the total input over four days. The change in the body is input minus output.

Her sensors, pathways and control center work, so the command to lose heat is sent. Her effectors, the skin blood vessels and the sweat glands, respond weakly, and the heat load of the heat wave is larger than they can oppose. That is a homeostatic imbalance caused by overwhelmed effectors.

• At the sensory receptors, which no longer detect heat: Her nerves and control center work normally, and her body is responding to the heat, so detection is intact.
• At the afferent pathway, which no longer carries signals: The case says her nerves work normally. Signals are reaching the control center.
• At the control center, which has lost its set point: The control center works normally and is sending commands. The problem is how strongly the effectors carry them out.
• Correct: At the effectors, which cannot remove heat as fast as it builds up: Correct. Weak vasodilation and little sweat mean the effectors cannot match the heat load.

• Signals from the stretch-sensitive sensory receptors: down. Lower pressure stretches the artery walls less, so the stretch-sensitive sensory receptors fire fewer action potentials; the control center reads this as a fall.
• Heart rate: up. The control center sends signals along the efferent pathway that speed up the heart, which pumps more blood per minute and raises pressure.
• Diameter of small arteries: down. The control center triggers vasoconstriction of small arteries. Narrower vessels raise resistance, which raises pressure back toward the set point.