4. Sensation

This chapter examines sensation, which is a catch-all term for monitoring any state and feeding it into a physiological process. When people talk of their ‘senses’ they usually mean the five senses by which they consciously monitor features of the outside world. These senses—vision, hearing, smell, taste, and touch—provide rich flows of information and most make use of specialized organs. In all five cases, the sensory system combines two functions: measurement of a stimulus and encoding it in a way that can be transmitted via a nerve into the brain. In addition, the brain may signal back to the sensing system to modulate the way that it works.

Concluding remarks

This concluding chapter highlights one physiological principle that shines through the discussion of human physiology: the emergence of control through the cooperative, collective action of different entities. Studying physiology is about more than gaining facts. It is also about gaining an outlook that can see, analyse, and appreciate the action of cooperative systems. It is an outlook that fully acknowledges the importance of understanding at the molecular level, but that can still retain a focus on what the molecules are for. In this respect, physiology connects more than most with our deepest questions about what it is to be human.

6. From thought to action

This chapter addresses muscles. The ultimate result of sensation and thought is usually some kind of action, be it moving the whole body; manipulating an object with the hand; or moving diaphragm, mouth, tongue, and voice-box to speak. All of these depend on muscles which, in their various forms, provide a nearly universal means for the nervous system to control the body and the world. Muscle cells are highly adapted for turning chemical energy into mechanical force. The chapter then looks at skeletal muscle and the musculoskeletal system. Some muscles are arranged circumferentially around a cavity. Two examples of this are the heart and the gut.

7. Defence

This chapter describes how the human body protects its internal conditions against micro-organisms and the environment. The body’s first line of defence is the mechanical barrier provided by the skin, a part of the integumentary system. When the integument is breached, the body’s immediate priority is to seal the hole by coagulation of the leaking blood. The next line of defence is chemical: the secretions that cover the surfaces of eyes and the inside of the nose contain a variety of proteins that attack bacteria. Within the blood and fluids that bathe internal tissues are proteins of the complement system. The chapter then considers the innate and the adaptive immune systems.

8. Reproduction

This chapter explores the physiology of reproduction. It spans the making of gametes, making a potential home for an embryo in the mother, arranging that fertilization takes place, supporting the developing embryos, making a supply of milk (lactation) in time for the birth, and, of course, developing from a single cell into an adult capable of repeating the whole process. The chapter then looks at the determination of biological sex in humans. The scientific understanding of reproductive physiology now gives people choices. People can now choose to limit their reproduction by measures such as sexual abstinence, condoms, and oral contraceptive pills.

2. Energy

This chapter focuses on energy as just one aspect of body function. The chapter describes the energy-harvesting chemical reactions which happen in all human cells. For them to be possible, these cells must have access to glucose and oxygen, and must be able to dispose of the waste products, carbon dioxide and water. In a large organism such as a human, even these apparently simple requirements are challenging and are met by a complex set of physiological systems, in particular the digestive, respiratory, cardiovascular, and excretory organs of the body.

5. Reacting and thinking

This chapter assesses the nervous system. In the trunk of the body and the neck, the central nervous system (CNS) is called the spinal cord; in the head, it is called the brain. The CNS is dominated by two cell types: neurons and glia. The neurons form a vast network in which information is split, combined, and somehow processed. Examples of this processing include reflex arcs, the ‘circuitry’ that detects features such as edges in images coming from the eyes, and simple types of learning and memory. However, most other things in the brain, especially thinking and feeling, are not yet understood at all well.

3. Homeostasis

This chapter discusses the importance of maintaining a stable internal environment, which is itself an active physiological process. This process is called ‘homeostasis’. Control systems, whether natural or artificial, can be divided into two broad classes: open-loop and closed-loop. The closed-loop type is clearly best suited to homeostasis, and it is not surprising that it is widespread in physiology. The chapter then considers the control of the concentration of potassium, calcium, and glucose in the blood. Many drives—such as desires for water, food, warmth, sleep, or a lavatory—can be linked very clearly to individual physiological homeostasis.

1. Human physiology

This chapter defines human physiology and how it is studied. Human physiology is the science of how the body works. As well as telling us how our bodies work, physiological knowledge is important to keeping them working in the event of injury or disease. As in all of the biological sciences, research in human physiology uses observation, inference, imaginative proposing of hypotheses, and the testing of these hypotheses by experiment. Many experiments on human physiology are done directly on human volunteers: data obtained directly from humans are obviously of the highest relevance to human biology. The chapter then looks at imaging technologies and the nature of explanations in human physiology.