Preface

Daniel Schott

A master of disguise, the octopus silently glides from the sand to take a perch on a dark coral head. Instantly, its skin turns from white to a mottled brown, blending in so perfectly that the octopus is invisible to predators… and to prey.

Photos: Left, Juuyoh Tanaka (田中十洋) https://www.flickr.com/photos/tanaka_juuyoh/ CC BY 2.0; Right, Steven van Tendeloo https://commons.wikimedia.org/wiki/File:Octopus_vulgaris_france.jpg CC BY-SA 3.0.

Much of physiology is about the flow of information (Latin, informare: to tell). What do we mean by the word “information”? Look at the example of the of the octopus: A change in the octopus’s environment – the colour and pattern of light – results in a chemical and electrical change in photoreceptor cells (Greek, photos: light) in its eyes, which, in turn, results in a change in the chemical environment of nerve cells next to the photoreceptors. A change in one cell triggers a change in another cell, which triggers changes in yet more cells. These changes are a cascade of chemical and electrical signals through many different cells of the nervous system, to muscle cells in the octopus’s skin. Then, those muscle cells contract to expand pigment sacs called chromatophores (Greek, chroma: colour; phoros: carrying), or relax to shrink them.

Thus, a change in the octopus’s surroundings results in a change in the colour and pattern of the octopus’s skin. Information is a choice among possible states, for example, a choice between a shrunk or an expanded state of a chromatophore. Life could not exist without the ability to sense information about the environment and respond to it.

This book introduces the concept of communication (Latin, communis: shared, common) between cells and between organs in the animal body. What do we mean by the word “communication”? Communication is when a change happening in one place results in a change in a different place. In other words, communication is a transfer of information. Communication is indispensable for teamwork, so it shouldn’t be any surprise that the survival and reproduction of multicellular forms of life depend on sophisticated mechanisms of communication among cells and organs.

This teamwork among cells, tissues, and organs is central to every aspect of animal physiology.

For example, the gut is in constant communication with the brain. When you see food, and especially when you smell food, signals from your eyes and nose tell your brain to prepare for eating. The brain sends electrical impulses down nerves to blood vessels in your salivary glands, telling the blood vessels to open up. The increased blood flow increases the production of saliva. Other nerves carry electrical impulses to electrically active cells in the walls of your stomach. In response, those cells produce chemical signals that tell cells of the stomach lining to produce acid. Once plenty of food has entered your stomach, other electrically active cells in the stomach wall detect the stretching, and send signals up a nerve to your brain to tell it that your stomach is full.

Teamwork among cells, tissues, and organs isn’t essential only for responses to the body’s surroundings: It’s also essential for keeping the physical and chemical conditions inside the body constant within the narrow range that’s suitable for living cells. Blood sugar, oxygen supply, body temperature, and many other factors need to be kept constant in order for cells to stay healthy. Keeping conditions the same is called “homeostasis” (Greek, hómoios: in like manner; stasis: standing still).

For example, let’s look at blood sugar. The most important and abundant sugar in blood is glucose (Greek, gleûkos, from glukús: grape juice, sweet taste). Glucose is an energy source for all living cells in the body. If the level of glucose in the blood drops too low, cells throughout the body have to deal with a shortage. The brain is especially sensitive to lack of glucose, because nerve cells have an absolute need for glucose. If your blood glucose gets low, the first thing the people around you might notice is that you become confused and clumsy. You might not notice it yourself, because glucose starvation impairs your brain! If the level of glucose in the blood gets too high, that’s dangerous too, in a different way – since the effects might not be noticed until years later. Blood glucose that’s too high for too long causes lasting damage to tissues throughout the body, especially to blood vessels. Hundreds of millions of people today are suffering long-term health problems due to high blood glucose.

As a result, the body has many different ways to keep glucose within a two-fold range of concentrations. Blood glucose in a healthy person may briefly spike or plummet, but several mechanisms spring into action to bring the blood glucose concentration back to a steady state. When you’re digesting a starchy meal, your intestine dumps a large amount of glucose from the food into your bloodstream. An organ called the pancreas detects the increased glucose, and reacts by releasing a small protein called insulin into your blood^[1]. The insulin tells your liver and your muscles to take up glucose, and store it, thus returning your blood glucose to a moderate level. If you haven’t eaten in a long time, your blood glucose level drops because the cells throughout your body use it up. Your pancreas detects the change in glucose concentration, and releases a different small protein, called glucagon, which tells your liver to release the glucose that it stored after your meal. If your liver has run out of stored glucose, the glucagon tells the liver to make new glucose from other chemicals.

Notice that your pancreas doesn’t put out insulin and glucagon constantly. That wouldn’t be useful. Why? Because unchanging amounts of insulin and glucagon wouldn’t transmit any information. Instead, your pancreas senses the amount of glucose in your blood, and responds to changes in glucose with changes in the amount of insulin and glucagon produced. This communicates information to the liver about how much glucose it needs to release or absorb in order to keep the concentration of glucose in the blood within a healthy range.

There are many other ways the body acts to control blood glucose levels. You can learn more about homeostasis in Chapter 9, and about blood sugar in Chapter 12. A theme throughout this book is that cells and organs are constantly talking with each other. It’s a fascinating conversation that’s going on inside our bodies, and we hope that you enjoy discovering the many ways that the parts of the body communicate with each other.