Homeostasis: active recall

It regulates internal conditions within ranges compatible with normal function, despite changes inside or outside the organism. A regulated quantity can fluctuate; homeostasis does not imply that every measurement is held at one mathematically exact value at every moment.

The resulting response tends to oppose the change in the regulated quantity, reducing the stimulus that initiated it. Receptors, control processes and effectors contribute to the loop. “Negative” describes the direction of feedback rather than an undesirable effect.

A receptor detects a relevant signal or change. An effector produces a response that can alter the regulated condition. Information may be communicated through nerves, hormones or other signals, so identifying both roles helps explain the sequence rather than merely naming structures.

Target cells have suitable receptors and downstream response machinery. Binding can initiate changes in cell activity; merely being reached by the blood does not make every cell respond identically. Different receptor types and cellular contexts can also produce different responses.

Pancreatic beta cells can increase insulin release. Its actions include promoting glucose uptake in relevant tissues and favouring storage pathways such as glycogen synthesis. The effect is a coordinated change in metabolism, rather than insulin chemically turning into glucose.

Pancreatic alpha cells can release glucagon, which acts particularly on the liver to promote glucose output, including through glycogen breakdown and gluconeogenesis. Describe the tissue and metabolic process; glucagon is not itself a carbohydrate store.

Pressure drives water and small solutes from the blood into the nephron through the filtration barrier. Cells and most large plasma proteins remain in the blood. The resulting filtrate contains useful solutes as well as substances that may be excreted, so further processing is needed.

Reabsorption returns substances from the nephron filtrate to the blood. Transport processes in the tubule recover useful solutes, with water movement linked to the conditions they establish. Filtration occurs earlier and is not the same directional process.

ADH is made in neurosecretory cells in the hypothalamus. It is transported along their axons and stored and released from the posterior pituitary. Distinguishing synthesis from release avoids treating the pituitary as the site where this hormone is initially made.

Osmoreceptors associated with the hypothalamus detect the change and influence neurosecretory activity. More ADH can be released from the posterior pituitary. This links detection of a more concentrated internal fluid environment to a response that conserves water.

Binding to suitable receptors initiates signalling that promotes insertion of aquaporin water channels into the relevant cell membrane. Increased permeability permits more water to move down the established water-potential gradient. ADH does not create water or directly pump it through the channels.

Less water remains in the fluid that is excreted, so urine volume generally falls and its solutes are more concentrated, under comparable conditions. Restoring the internal water balance reduces the original stimulus through negative feedback.