Topics in this lecture include: Organization of living things, organization of the human body, properties of living things, and homeostasis.
ORGANIZATION OF THE HUMAN BODY
The human body can be organized into different levels from the microscopic to the entire organism: chemicals, molecules, cells, tissues, organs, organ systems, and organism (Figure 1.8). Chemicals are the individual elements, such as nitrogen, hydrogen, carbon, and oxygen, that combine to form small molecules, such as glucose, water, amino acids, or nucleotides, that also combine to form macromolecules, such as complex carbohydrates, DNA, proteins, and fats. The small molecules combine in various ways to produce the large macromolecules that form part of every cell of every living organism.
Cells combine to form the various tissues in the body: epithelial, connective, muscle, and nerve. Different combinations of those tissues form organs. For example, the innermost layer of the stomach is composed of epithelial tissue, which is surrounded by strong collagen-containing connective tissue, which binds to three smooth muscle layers that contract to digest food. Neurons that send signals to the smooth muscle cells cause them to contract when food is in the stomach. All organs are composed of different amounts of all four tissue types. Certain organs function with other organs and tissues to form organ systems, such as the digestive system, respiratory system, or cardiovascular system. All organ systems combined make up the entire human organism.
Properties of Living Things
Living organisms have certain characteristics that differentiate them from non-living things. These characteristics are introduced below and covered in detail in this textbook as follows: all living things are made up of the same macromolecules; they have a cellular structure; they grow and metabolize nutrients; they have mechanisms of homeostasis (below); they reproduce at a cellular and organism level; they have DNA that they pass on through inheritance; and all species change over time through a process called evolution.
Macromolecules
Every living thing is made up of the basic elements, carbon, oxygen, hydrogen, and nitrogen. All living things combine these basic chemical elements into small molecules, such as sugars, amino acids, fatty acids, and nucleotides. The small molecules combine into various macromolecules: polysaccharides (complex carbohydrates), proteins, fats, and nucleic acids.
Cellular structure
All living organisms consist of one or more cells, which are composed of phospholipids, proteins, carbohydrate groups, and cholesterol. Single-cell organisms can be prokaryotes or eukaryotes, and complex multicellular organisms are eukaryotic. Viruses have a protein capsule instead of a cell membrane and, therefore, are not technically considered living things, but viruses are significant to all living things and will be discussed in detail in.
Growth and Metabolism
All living organisms grow and undergo metabolism, producing energy in the form of ATP. Depending on the particular species and conditions, ATP can be produced either with oxygen—aerobically—or without oxygen— anaerobically. Humans produce ATP both with and without oxygen, and we can produce ATP from glucose, amino acids, fatty acids, and nucleotides.
Reproduction
All living organisms reproduce at a cellular level through mitosis and at an organism level through meiosis, by producing sperm or eggs, spores, or seeds.
Hereditary Material
Every living organism passes on some of their genetic material to their offspring. Humans have 46 DNA molecules (chromosomes): 23 from one parent and 23 from the other parent. Offspring are similar and yet different from their parents, and certain traits are passed on through generations.
Evolution
All populations of organisms continuously change genetically from one generation to the next. Humans have 46 chromosomes, but the particular genes on those chromosomes vary: such as the genes that encode skin pigmentation, or body height. Although humans today differ from humans 100 000 years ago, the distinctly human DNA remains intact.
Homeostasis
Homeostasis is a continual process that’s necessary for the normal functioning of every living thing. Some factors are strictly regulated within a narrow range for the human body, including oxygen, carbon dioxide, pH, H+, blood glucose, core body temperature, blood pressure, water, and electrolytes (sodium, potassium, chloride, calcium, etc.). When these factors are in balance, cells function properly, and we feel healthy.
If homeostasis is not maintained for a short time, you will feel certain symptoms. For example, if your blood sugar decreases and your liver cells are not efficient at breaking down fat to produce glucose (gluconeogenesis) and if you don’t consume food right away to increase your blood sugar, you might feel tired, weak, or perhaps dizzy. If homeostasis cannot be maintained at all, you will have a disease, and without medical intervention death could occur. For example, if your pancreas was not able to make insulin, then cells would not be able to take up blood sugar and therefore make ATP, and blood sugar levels would be too high; this describes diabetes, which is fatal without injections of insulin.
Every organ system is involved in homeostasis in some way, and this will be discussed in each of the textbook’s chapters on organ systems.
Components of Feedback Systems
Homeostasis regulates most of the factors discussed above through a process of negative feedback (Figure 1.26), which brings about a reverse in the level of a controlled factor. As well, a process of positive feedback sometimes occurs, in which a response caused by an initial stimulation continues to increase.
All of the body’s feedback systems consist of the following components: receptors that detect the level of some factor (such as temperature); an integrating centre that compares information from the receptors to a standard set point (such as 37°C for normal body temperature); and effectors, which respond to signals from the integrating centre: for example, sweat glands. The receptors constantly send information about the body’s internal environment to the integrating centre, which constantly signals the effectors to increase or decrease a specific response.
Receptors The receptors are either extensions of a sensory neuron that sends information to the integrating centre about a specific stimulus, or individual receptor cells that communicate directly with a sensory neuron. The following list gives the names and functions of the common types of sensory receptors: chemoreceptors detect chemical concentrations such as neurotransmitters, drugs, or hormones; osmoreceptors detect changes in osmolarity, that is, water and ion concentrations; tactile receptors detect touch, pressure, and vibration; baroreceptors detect blood pressure; photoreceptors detect light and are found only in the retina; mechanoreceptors detect stretching (e.g., spindle fibres in muscles); proprioceptors detect body position; nociceptors detect pain; and thermoreceptors detect temperature.
Integrating Centre The integrating centre interprets the signals coming from the receptors and determines when any deviation occurs from the standard set point. Certain parts of the brain act as an integrating centre, and certain glands also act as an integrating centre: for example, the hypothalamus assesses temperature, thirst, hunger, and sleep; the medulla oblongata regulates breathing, blood pressure, and heart rate; and pancreas determines blood glucose levels.
Effectors The effectors include any part of the body that responds to stimulation from the integrating centre and brings an altered factor back to normal. They include skeletal muscles (e.g., shivering when cold); smooth muscles (e.g., vasodilation, bronchodilation, increased gastric motility); cardiac muscle (e.g., change in heart rate or its force of contraction); and the endocrine glands.
Negative Feedback Mechanism
Negative feedback means there is a reverse in the level of a controlled factor. For example, exercise causes the body temperature to increase, which is then regulated by the production of sweat that cools the body. The higher the increase in body temperature, the more sweat is produced in an effort to maintain the normal body temperature. When normal body temperature cannot be maintained in extreme conditions, illness and possibly death will occur.
The following example describes what’s involved in a negative feedback mechanism to regulate body temperature:
stimulus (cold external environment)—receptors (thermoreceptors)—integrating centre (hypothalamus determines and signals that body temperature has dropped below the 37°C set point)— effectors (skeletal muscles shiver and produce heat)—new stimulus (increased body temperature)—receptors (thermoreceptors)— hypothalamus (stops signaling that body temperature is below normal)—effectors (skeletal muscles stop shivering).
Positive Feedback Mechanism
A positive feedback mechanism continues to increase a response caused by an initial stimulation (Figure 1.27).
There are very few examples of positive feedback systems in the human body. One example is childbirth, where the stimulus is pressure of the head of the fetus, the receptors are mechanoreceptors in the uterus, and the integrating centre is hypothalamus, which produces oxytocin that signals the effectors— the uterine muscles—to contract; this response leads to further stimulation of the mechanoreceptors, which signals the hypothalamus to produce more oxytocin.
