1 Brief overview of body parts and functions

1.1 Introduction

It is well known that body parts and functions are controlled by chemical and biochemical processes. What is perhaps less obvious is that our bodies also follow strict physical principles. However, this becomes clear when we examine different parts of our body. Every movement requires forces, moments, and mechanical stability. The body’s energy balance follows the principles of thermodynamics. The sensory system, including hearing and vision, is based on acoustic and optical principles. The respiratory system, circulatory system, and kidneys all obey the laws of diffusion, hydrostatics, and hydrodynamics. The propagation of signals along nerve fibers can be described by electrical circuits. Chapters 2–12 are devoted to the description of some organs, sensors, and systems and emphasize their connection to fundamental physical principles. Attempting such a description inevitably leads to simplification. However, the aim of this textbook is not to underpin the complexity of biological systems but rather to draw attention to the connections with physical principles. Therefore, the chapters in this volume are not a substitute for textbooks on physiology.

In this first chapter we give a very brief overview of the main organs and systems of the human body, which will be deepened in later chapters. Most of these organs are not specific to humans but can be found in all mammals and even in most vertebrates.

1.2 Overview

1.2.1 Cell

Cells with an average size of 20 μm are the fundamental building blocks of living matter [1, 2, 3]. The human body consists of about 3 × 1013 eukaryotic cells [2], i.e., cells that contain a nucleus. They are stacked together like bricks of a house. The cross section of a typical cell is schematically shown in Fig. 1.1. All cells have a wall to distinguish between inside and outside. The chemical and electrical gradients across the wall are essential for cell functioning and intercell communication. Inside the cells, there is another smaller nucleus that holds the genetic material in the form of a double-stranded helical chain, known as deoxyribonucleic acid (DNA). Each DNA molecule contains a sequence of paired nucleobases, which read like letters of a book that constitutes the genetic code. Genes are like chapters in this book. About 1000 pairs constitute one chapter. Estimates are that 80% of about 25 000 genes in the DNA encode about 2 × 104 different functional proteins in the human body that do their daily job [4]. Proteins are made up of 21 different amino acids arranged in chains and folded up to complex three-dimensional structures. They build ion channels and molecular motors; form receptors, enzymes, and hormones; take care of oxygen transport; strengthen tissues and bones in the body; regulate water and ion concentrations; and are responsible for many more tasks. Proteins vary greatly in size; some hold more than 100 000 atoms [5]. Only about 5000 proteins have been described at an atomic resolution so far [6]. Although almost all cells contain complete and identical genetic information, they specialize in different tasks. Muscle cells develop a surprising tensile force, liver cells specialize in performing important tasks for food metabolism, and nerve cells transmit electrical signals as fast as 100 m/s. We can distinguish between about 210 different cell types with specific tasks [7]. Only stem cells are not specialized. The specialization of all other cells for particular tasks is only possible through a high degree of self-organization and communication between the cells.

Fig. 1.1: Schematics of a human cell. The cell membrane separates the cytoplasm from the extracellular space. The cytoplasm contains, among many other parts, the genetic information inside the nucleus and the mitochondria with its genetic information, acting as a powerhouse for the cell functions. The cell membrane is a double lipid layer perforated by many ion channels to maintain an electrical potential across the cell membrane or change it by depolarization upon a stimulus (adapted from OpenStax Anatomy and Physiology, 2016, © creative commons).

Comparing cells with bricks is, after all, a gross oversimplification. When engineers build a complex system, they start with simple building blocks that fit together to form something more complex. In contrast, in organisms, the complexity does not begin at the cell level but already at the molecular level of DNA, ribosomes, lysosomes, and many more. This incredible complexity organizes life. First, cells organize to form tissues: epithelial tissue, connective tissue, muscular tissue, and nervous tissue. In turn, tissues assemble to form organs with characteristic and distinct shapes and functions like the liver and the kidneys. Several organs work together in a system, such as the digestive system that involves 10 different organs. The body contains 11 distinct systems: 1. cardiovascular/circulatory system; 2. respiratory system; 3. digestive system/excretory system; 4. endocrine system; 5. lymphatic system/immune system; 6. sensory system; 7. locomotor system; 8. nervous system; 9. renal system/urinary system; 10. integumentary system; and 11. reproductive system. All 11 systems interact and are responsible for the human body’s life. These interactions take place under the promise of constancy in a variable environment. For instance, core body temperature, arterial blood pressure, and partial oxygen/carbon dioxide pressures of blood are kept constant by an active negative feedback system, like a thermostat. This control mechanism is known as homeostasis, according to Bernard,1 who first stated this principle [8, 9]. Body temperature homeostasis is one example that is further discussed in Chapter 4, and further homeostatic systems are described in the following chapters.

Finally, cells do not live forever. Cells have a life cycle, comprising phases of growth, rest, cell division, and cell death. Understanding the cell cycle’s controlling mechanisms is essential for coping with cancer and treating tumors through radiation therapy. These topics are considered in Chapters 8–10 of Vol. 2.

1.2.2 Circulation

For maintaining all body functions, blood circulation is essential. As the name implies, blood circulation is a closed circuit, where the flow is mechanically powered by the heart acting as a pump. A dense mesh of blood vessels penetrates any body part. Blood circulation is a transport system for oxygen, nutrients, and heat to their destinations, including muscles, bones, and all other organs. Oxygen is taken up from the surrounding air during inhalation and binds to hemoglobin by diffusing through membranes in the lung. In return, during expiration, carbon dioxide, as a residue of combustion, is exhaled. Figure 1.2 shows a simplified schematic of blood circulation. It has an oxygen-rich and an oxygen-poor part, along with a high-pressure part and a low-pressure part. The color coding (red: oxygen-rich, blue: oxygen-poor) originates from our visual perception. Oxygen-rich blood has a much brighter red color than oxygen-poor blood. Circulation is discussed in more depth in Chapter 8.

Fig. 1.2: Schematic of the human circulatory system. The right ventricle of the heart (1) takes in oxygen-poor blood from the extremities (9, 10) and pumps it into the lungs (2, 3) for oxygen enrichment. After returning from the lung into the left atrium and ventricle of the heart (4, 5), the blood is ejected into the periphery (6–8) to supply all body tissues with oxygen. Blood pressures in the left circulation (red) are higher than in the right circulation (blue) at similar locations. The left ventricle muscle is also stronger than the right ventricle muscle (reproduced from http://www.online-sciences.com/, © creative commons).

1.2.3 Heart