AP Biology 4.5: Feedback

Homeostasis

The maintenance of relatively stable internal physical and chemical conditions in a living organism despite changes in the external environment. Homeostatic variables remain within an acceptable range rather than at one perfectly constant value.

Homeostatic range

The interval between the acceptable lower and upper limits of a regulated variable. A variable may be considered homeostatically controlled even when its value fluctuates within this range.

What three components are present in a typical homeostatic control mechanism?

A receptor detects a change, a control center compares the variable with its acceptable range and coordinates a response, and an effector acts to reduce the deviation.

Receptor in a homeostatic system

The sensing component that monitors an internal or external condition and sends information to a control center. Examples include thermoreceptors, mechanoreceptors, chemoreceptors, and baroreceptors.

Control center in a homeostatic system

The component that receives information from a receptor, determines whether the regulated variable is outside its maintenance range, and directs an appropriate response. The respiratory center and renin–angiotensin system are examples.

Effector in a homeostatic system

A muscle, organ, gland, or cell that responds to signals from a control center and changes the regulated variable. Effectors can alter processes such as respiration, ion transport, gene expression, or hormone secretion.

Negative feedback

A regulatory process in which the response to a deviation opposes the original change, reducing the error and eventually decreasing the signal that initiated the response. Most homeostatic mechanisms use negative feedback.

Positive feedback

A regulatory process in which the response reinforces or amplifies the original change. Unlike negative feedback, positive feedback moves a variable farther from its initial state and generally continues until a specific endpoint or external interruption occurs.

How does positive feedback differ from negative feedback in biological systems?

Negative feedback counteracts a deviation and stabilizes a variable near its homeostatic range. Positive feedback strengthens the initial change, producing an escalating response that ends when an endpoint or limiting event is reached.

What is an important limitation of positive feedback as a homeostatic mechanism?

Because positive feedback amplifies deviations rather than correcting them, it usually cannot maintain a stable internal condition by itself. It is most useful for driving a process rapidly to completion.

How can a biological control signal encode the magnitude and direction of an error?

The signal from a receptor to a control center varies in a way that conveys whether the variable is too high or too low and how far it is from the target. The effector response is generally adjusted in the opposite direction and often increases with the size of the deviation.

Why does homeostasis not require a variable to remain exactly constant?

Biological variables naturally fluctuate because of circadian rhythms, activity, diet, and other conditions. Regulation keeps them within a functional range, and the set point itself can sometimes be reset.

How is homeostasis different from chemical equilibrium?

Equilibrium is a condition in which opposing processes occur at equal rates, whereas homeostasis is an actively regulated steady state. Energy and matter may continuously move through a homeostatic system even when its internal conditions remain relatively stable.

Acclimatization

A gradual adjustment to changed environmental conditions, such as altered temperature or reduced oxygen availability at high altitude. It allows an organism to function over a wider range of external conditions.

How does a fever illustrate that a homeostatic set point can change?

During an infection, the temperature-regulating system resets its target temperature upward. The body then activates heat-conserving and heat-producing responses until the new set point is reached.

What happens to heat loss and metabolism when mammalian core temperature falls?

Skin and limb blood vessels constrict, reducing heat transfer to the environment, while blood is routed through deep countercurrent exchange pathways. Metabolism increases first through non-shivering thermogenesis and then, if necessary, through shivering.

How does sweating cool the body?

Sweat absorbs energy as it evaporates from the skin. This removes thermal energy from the skin and from blood flowing near the surface, lowering body temperature.

How does the body respond chemically and physiologically to an increase in blood glucose?

Pancreatic beta cells release insulin, which promotes glucose uptake by muscle and fat cells and favors storage as glycogen or triglycerides. Insulin also suppresses glucose production by the liver.

How does the body restore blood glucose after it falls?

Pancreatic alpha cells release glucagon while insulin secretion decreases. Glucagon stimulates the liver to break down glycogen by glycogenolysis and to synthesize glucose from noncarbohydrate precursors by gluconeogenesis.

Why can liver glycogen help raise blood glucose but muscle glycogen generally cannot?

The liver can convert glycogen-derived intermediates into glucose and release it into the blood. Muscle primarily uses its glycogen locally for cellular respiration during activity.

How can iron deficiency impair oxygen homeostasis?

Iron is required for hemoglobin, the protein that carries oxygen in red blood cells. Even if the blood oxygen partial pressure is adequate, insufficient iron can reduce hemoglobin production and therefore lower total oxygen content.

Blood carbon dioxide equilibrium

The relevant aqueous equilibrium is $\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$. Increasing dissolved $\mathrm{CO_2}$ shifts the equilibrium right and tends to increase $[\mathrm{H^+}]$ and decrease pH.

How do changes in carbon dioxide affect blood pH and breathing?

Increased carbon dioxide promotes formation of carbonic acid and hydrogen ions, lowering pH. Chemoreceptors detect the change and stimulate deeper or faster breathing, which removes more carbon dioxide and shifts the equilibrium toward lower hydrogen-ion concentration.

How do the lungs and kidneys complement one another in blood pH regulation?

The lungs rapidly alter blood carbon dioxide by changing ventilation. The kidneys provide slower, longer-term compensation by changing the excretion or retention of hydrogen ions and bicarbonate.

How is high-altitude acclimatization related to blood pH and oxygen status?

Reduced atmospheric oxygen can stimulate hyperventilation, increasing oxygen uptake but lowering blood carbon dioxide. The kidneys compensate for the resulting tendency toward higher pH by excreting hydrogen ions and bicarbonate.

What is the relationship between partial pressure of oxygen and total blood oxygen content?

Partial pressure measures the tendency of dissolved oxygen to escape into the gas phase, whereas total oxygen content depends largely on the amount of hemoglobin available to bind oxygen. Thus, a person with anemia can have adequate oxygen partial pressure but reduced total oxygen-carrying capacity.

How does the body respond to chronically low blood oxygen content?

Oxygen-sensitive kidney cells release erythropoietin, or EPO, which stimulates red bone marrow to produce more red blood cells. Increased hematocrit and hemoglobin concentration raise the blood's oxygen-carrying capacity.

Baroreceptor

A pressure-sensitive receptor that detects stretching of arterial walls, especially in the aortic arch and carotid sinus. Its signals help coordinate changes in heart rate and arteriole diameter to oppose changes in arterial blood pressure.

Why can small changes in arteriole diameter strongly affect blood flow and blood pressure?

Arterioles are the main resistance vessels of the arterial system. Because resistance depends strongly on vessel radius, vasoconstriction greatly increases resistance while vasodilation greatly decreases it.

How does the body respond to a sudden decrease in arterial blood pressure?

Reduced baroreceptor stimulation causes increased sympathetic activity, raising heart rate and constricting arterioles. If the decrease is severe, the adrenal medulla releases epinephrine, which further promotes tachycardia and vasoconstriction.

How does atrial natriuretic peptide help lower elevated blood pressure?

Stretch of the heart atria promotes release of atrial natriuretic peptide, or ANP. ANP inhibits renin and aldosterone effects, increasing sodium excretion; water follows sodium osmotically, reducing blood volume and pressure.

How is ionized calcium concentration regulated in the blood?

A decrease in plasma $\mathrm{Ca^{2+}}$ stimulates parathyroid hormone, or PTH, which promotes calcium release from bone, reduces phosphate in the blood through urinary excretion, and increases calcitriol production. Calcitriol increases intestinal calcium absorption; elevated calcium stimulates calcitonin, which favors calcium deposition in bone.

Why does PTH-induced phosphate excretion tend to increase free calcium concentration?

Phosphate and calcium can combine to form relatively insoluble salts. Removing phosphate from the blood reduces the amount of calcium tied up in these salts, increasing the concentration of free ionized $\mathrm{Ca^{2+}}$.

How does calcitriol affect calcium homeostasis?

Calcitriol is produced by the kidneys in response to PTH and increases calcium absorption by epithelial cells of the small intestine. This supplies calcium to the blood when plasma ionized calcium is low.

Renin–angiotensin system

A regulatory system that responds especially to reduced renal sodium delivery or reduced arterial pressure. Renin initiates formation of angiotensin signaling molecules that promote responses such as sodium retention, water retention, and blood-pressure restoration.

How do the kidneys indirectly sense plasma sodium concentration?

Cells of the juxtaglomerular apparatus assess sodium concentration in tubular fluid after it has passed through portions of the nephron, rather than measuring plasma sodium directly. They also respond to renal blood flow, which provides information related to arterial pressure.

What happens when renal sodium delivery or arterial pressure decreases?

Juxtaglomerular cells release renin. The resulting hormonal cascade promotes restoration of sodium and water balance and helps raise arterial blood pressure.

Why does water often follow sodium during homeostatic regulation?

Dissolved ions contribute to osmotic pressure. Retaining sodium increases the osmotic tendency of the extracellular fluid, causing water retention; excreting sodium generally promotes accompanying water loss.

How can nuclear receptors participate in homeostasis?

Nuclear receptors can alter gene expression by increasing or decreasing transcription of target genes. These changes adjust the amounts of proteins and enzymes that carry out longer-term negative-feedback responses.

What is retrograde signaling in the example of endocannabinoid regulation?

Postsynaptic neurons synthesize lipid-derived signaling molecules such as anandamide and 2-arachidonoylglycerol, which travel backward to presynaptic CB1 receptors. Their binding reduces neurotransmitter release and helps prevent excessive signaling.