Emergency Care Textbook Professional Responders - Part 4 PDF

4 Anatomy and Physiology Key Content Anatomical Terminology........ Body Cavities........................... Body Systems.......................... The Cell................................. Respiratory System............... Circulatory System............... Lymphatic System................. Immunolo...

4 Anatomy and Physiology Key Content Anatomical Terminology........ Body Cavities........................... Body Systems.......................... The Cell................................. Respiratory System............... Circulatory System............... Lymphatic System................. Immunological System......... Nervous System.................... Musculoskeletal System....... Integumentary System......... Endocrine System................. Digestive System.................. Genitourinary System.......... Interrelationships of Body Systems.............................. How Injuries Occur.................. 54 56 56 57 57 59 62 62 64 66 71 72 73 73 74 75 As a professional responder, you need a basic understanding of normal human body structure and function. This information will help you to recognize and understand illnesses and injuries. Body systems do not operate independently; they depend upon one another to function properly. When your body is healthy, your body systems are working well together, but an injury or illness in one body part or system can have effects in others. Knowing the location and function of the major organs and structures within each body system will help you to more accurately assess a patient’s condition and provide the best care. To remember the location of body structures, it helps to learn to visualize the structures beneath the skin. The structures you can see or feel are reference points for locating the others. For example, to locate the carotid pulse on either side of the ANATOMY AND PHYSIOLOGY Introduction 53 neck, you can use the Adam’s apple as a reference point. Using reference points will also help you to describe the location of injuries and irregularities that you find. This chapter provides you with an overview of important reference points, anatomical terminology, and the functions of the major body systems. ANATOMICAL TERMINOLOGY Knowing about key body structures will help you to identify serious illnesses and injuries and accurately communicate with other emergency care personnel about a patient’s condition. To use terms that refer to the body, you must first understand anatomical position. All medical terms that refer to the body are based on the position shown in Figure 4–1, a-b. The simplest anatomical terms are based on an imaginary line running down the middle of the body, dividing it into equal right and left halves. This line is called the midline. In medical terms, right and left always refer to the patient’s right and left, not the responder’s. Other terms related to the midline include lateral and medial. Anything located away from the midline is called lateral. Anything located toward the midline is called medial. Another reference line can be drawn through the side of the body, dividing it into front and back halves. Anything located toward the front of the body is called anterior (ventral); anything toward the back is called posterior (dorsal). When comparing any two structures, such as two body parts, any part toward the patient’s head is described as superior (cephalic). Any part toward the patient’s feet is described as inferior (caudal). Midline Superior (Cephalic) Proximal Medial Lateral Anterior Posterior (Ventral) (Dorsal) Distal ANATOMY AND PHYSIOLOGY Inferior (Caudal) 54 Right a Left b Figure 4–1, a-b a: The anatomical position and medical use of the terms right and left refers to the patient’s right and left. Medial refers to anything toward the midline; lateral refers to anything away from the midline. Proximal and distal are usually used to refer to extremities. b: Anterior refers to the front of the body; posterior refers to the back of the body; superior refers to anything toward the head; inferior refers to anything toward the feet. Two other terms are generally used when referring to the arms and legs. These terms are proximal and distal. To understand these terms, you must think of the chest, abdomen, and pelvis as the areas that make up the trunk of the body. Points on a limb that are closer to the trunk are described as proximal, and points farther from the trunk are described as distal. Figure 4–2 shows other basic terms used for body regions and their specific parts. These terms will be used throughout this text. Head Neck Shoulder Chest Arm Trunk Elbow Abdomen Pelvis Forearm Wrist Hip Hand Special anatomical terms are used for the abdomen, which is the part of the trunk below the ribs and above the pelvis. By drawing two imaginary lines, one from the sternum down through the navel to the lowest point in the pelvis and another horizontally through the navel, you divide the abdomen into four areas called quadrants (Figure 4–3). Referring to the affected quadrant(s) is important when describing injuries to the abdomen because it helps to determine which organs could be affected. Thigh Upper extremity Lower extremity Knee Lower leg Ankle Foot Right Left Figure 4–2: It is important to refer correctly to the parts of the body. FRONT VIEW Spine BACK VIEW Liver Spleen Stomach Pancreas Abdominal cavity Kidneys Large intestine Small intestine Figure 4–3: The abdominal quadrants. ANATOMY AND PHYSIOLOGY Gallbladder 55 BODY CAVITIES A body cavity is a hollow place in the body that contains organs such as the heart, lungs, and liver. The five major cavities that are illustrated in Figure 4–4 are: Cranial cavity: Located in the head, protected by the skull. Spinal cavity: Extending from the bottom of the skull to the lower back, protected by the bones of the spine. Thoracic cavity (also called chest cavity): Located in the trunk between the diaphragm and the neck, protected by the rib cage and the upper portion of the spine. Abdominal cavity: Located in the trunk between the diaphragm and the pelvis. Pelvic cavity: Located in the pelvis, which is the lowest part of the trunk, protected by the pelvic bones and the lower portion of the spine. Cranial cavity Spinal cavity Thoracic cavity Diaphragm Abdominal cavity Pelvic cavity Figure 4–4: The five major cavities of the body. BODY SYSTEMS The human body is a remarkable machine that performs many complex functions. Vital organs are organs such as the brain, heart, and lungs whose functions are essential for life. Plasma cell membrane— regulates what exits and enters the cell Golgi—processes and packages proteins and fats ANATOMY AND PHYSIOLOGY Organs can be either hollow or solid. Hollow organs, such as the stomach and colon, have large spaces within them to allow solids to pass through. Solid organs, such as the liver and kidneys, do not. 56 A body system is a group of organs and other structures that are especially adapted to perform specific body functions (described in this chapter). They work together to carry out a function necessary for life. For example, the heart, blood, and blood vessels make up the circulatory system, which keeps all parts of the body supplied with oxygen-rich blood. Human health depends on these systems working properly and coordinating properly with one another. When any one of them is damaged or impaired, the person’s health will be affected. Pathophysiology is the study of abnormal changes in mechanical, physical, and biochemical functions of body systems or cells caused by injury or illness. Mitochondria— generate energy Nucleus— contains genetic material Ribosome— produces proteins Lysosome— houses destructive enzymes Figure 4–5: The basic human cell. Endoplasmic reticulum— creates, modifies, and transports proteins The Cell The cell is the basic unit of life (Figure 4–5). Among other things, energy, DNA, and enzymes are created within a human cell. Cells combine to form tissues, which in turn make up organs. Respiratory System Nose Mouth Tongue Jaw Lungs Nasopharynx Pharynx Oropharynx Epiglottis Larynx Trachea Bronchi The airway, which is the passage through which air travels to the lungs, begins at the nose and mouth, which form the upper airway. Air passes through the nose and mouth and through the pharynx (the throat), larynx (the voice box), and trachea (the windpipe) on its way to the lungs (Figure 4–6). The pharynx divides into two passageways: the esophagus (which leads to the stomach), and the trachea (which leads to the lungs). When a person swallows, a flap of tissue called the epiglottis protects the opening of the trachea so that food and liquid do not enter the lungs. The epiglottis divides the airway into two sections: the upper airway and the lower airway. Bronchiole Alveolar sacs Alveoli Figure 4–7: The bronchi branch into smaller tubes and end in air sacs called alveoli. Bronchioles Figure 4–6: The respiratory system includes the pharynx, larynx, and trachea. Air reaches the lungs through two tubes called bronchi. The bronchi branch into increasingly smaller tubes (called bronchioles), eventually terminating in millions of tiny air sacs called alveoli (Figure 4–7). Oxygen and carbon dioxide are exchanged with the blood through the thin cell walls of the alveoli and through tiny blood vessels called capillaries. The lungs are protected by the chest. It is formed by 12 pairs of ribs, 10 of which attach to the sternum (breastbone) on the anterior side of the chest and the spine on the posterior side. The other two pairs attach only to the spine in the back and so are sometimes called floating ribs. The rib cage is the cage of bones formed by the 12 pairs of ribs, the sternum, and the spine, and protects the vital organs within (Figure 4–8). When you inhale, the chest muscles and diaphragm contract, expanding the chest and drawing air into the lungs. When you exhale, the chest muscles and diaphragm relax, allowing air to exit the lungs (Figure 4–9, a-b). This ongoing breathing process is involuntary and is controlled by the medulla oblongata at the base of the brain. The intercostal muscles, between the ribs, are referred to as accessory muscles of respiration because they provide assistance to the diaphragm during the respiration process. ANATOMY AND PHYSIOLOGY The respiratory system supplies the body with oxygen through breathing. The body must have a constant supply of oxygen in order to stay alive. When you inhale, air fills your lungs, and the oxygen in the air is transferred to your blood and carried to the cells of the body. This same system removes carbon dioxide, which is transferred from the blood to the lungs. When you exhale, air is forced from the lungs, expelling carbon dioxide and other waste gases. This breathing process is called respiration. 57 Trachea Clavicle Lungs Ribs Sternum (Breastbone) Heart Liver Stomach Spine Pancreas Figure 4–8: The rib cage surrounds and protects several vital organs. Some examples of injuries, disorders, and diseases that affect the respiratory system include: Asthma Bronchitis Pneumonia Pulmonary edema Airway obstruction Hemothorax Chronic obstructive pulmonary disease (COPD) RESPIRATION PROCESS The body requires a constant supply of oxygen, which varies depending on the needs of the body. These are affected by variables such as activity level. A body that is fighting off an illness, even the common cold, uses more energy and oxygen than a body in its healthy state. When a person is ill, the body must carry out all regular functions and also fight the illness. Some tissues, such as brain tissue, are very sensitive to oxygen starvation. Without oxygen, brain cells soon begin to die. Unless oxygen supplies can be restored, other vital organs will also be affected. The brain is the control centre for respiration. It adjusts the rate and depth of breaths according to the oxygen and carbon dioxide levels in the body. Breathing requires the respiratory, circulatory, nervous, and musculoskeletal systems to work together. Injuries or illnesses that affect any of these systems may cause respiratory emergencies. For example, if the body suffers an injury, the body will require more oxygen to respond, therefore increasing the respiration rate. Respiratory emergencies can be caused by: Choking. Illness (e.g., pneumonia). Respiratory conditions (e.g., emphysema and asthma). Electrocution. Shock. Air flows in Air flows out ANATOMY AND PHYSIOLOGY Trachea 58 Lung relaxes Lung expands Diaphragm relaxes Diaphragm contracts a b Figure 4–9, a-b: The chest muscles and the diaphragm a, contract as you inhale and b, relax as you exhale. Drowning. Heart attack or heart disease. Injury to the chest or lungs. Allergic reactions. Drugs. Poisoning (e.g., inhaling or ingesting toxic substances). AGONAL RESPIRATIONS Superior vena cava Aorta Pulmonary arteries Pulmonary veins Veins Arteries Heart Inferior vena cava Agonal respirations are an inadequate pattern of breathing associated with cardiac arrest states. Agonal respirations are not always seen during cardiac arrest. Because they can be confused with ordinary respiration, it is important that professional responders recognize agonal respirations. Agonal respirations originate from lower brainstem neurons as higher brain centres become increasingly hypoxic (oxygen-deprived) during cardiac arrest. In agonal respirations, the diaphragm is still receiving intermittent residual impulses from the brain, resulting in sporadic gasping breaths. Agonal respirations can present as a snorting, gurgling, moaning, or gasping sound, a gaping mouth, or laboured breathing. The duration differs from person to person, from a few minutes to several hours. While normal respirations follow a regular pattern, agonal respirations are irregular and sporadic. It is important to remember that agonal respirations are not sufficient for delivering oxygen to the body. A person experiencing agonal respirations is not breathing and requires immediate interventions. Figure 4–10: The circulatory system. The heart is actually a double pump: One half pumps blood to the lungs while the other half pumps blood to the body. The heart has four chambers, and is separated into right and left halves. The two upper chambers, called atria, have thinner walls and receive blood, which is then passed down to the muscular pumping chambers called ventricles. Lungs The circulatory system works with the respiratory system to carry oxygen to cells throughout the body and to carry carbon dioxide back to the lungs. It also carries other nutrients throughout the body and removes waste from cells. The circulatory system includes the heart, blood, and blood vessels. Figure 4–10 shows the major structures of the circulatory system. The heart is a muscular fist-sized organ behind the lower half of the sternum. The heart is protected by the ribs and sternum in the front and by the spine in the back (Figure 4–11). Ribs Sternum (Breastbone) please supply: 8-2 from page 129 of the previous edition. Figure Heart Figure 4–11: The heart is located in the middle of the chest, behind the lower half of the sternum. ANATOMY AND PHYSIOLOGY Circulatory System 59 To upper body Oxygen-poor blood from the body enters the right atrium and passes into the right ventricle. From there it is pumped to the pulmonary vessels and into the small capillaries surrounding the alveoli at the base of the lungs, where gas exchange occurs. Once oxygenated by the respiratory system, the blood returns to the heart through the pulmonary vessels and enters the left atrium. It then passes into the left ventricle, which pumps it into the large aorta, and from there through the arterial system to the rest of the body. One-way valves direct the flow of blood as it moves through the heart’s chambers. After the oxygen in the blood is transferred to the cells, veins carry the oxygen-poor blood back to the heart through veins. The heart pumps this blood to the lungs to pick up more oxygen before pumping it to other parts of the body. This process is called the circulatory cycle. The cross-section of the heart in Figure 4–12 shows how blood moves through the heart to complete the circulatory cycle. ANATOMY AND PHYSIOLOGY The system of blood vessels that carries blood through the body is referred to as the vascular system. It consists of the following key components: Arteries: Large high-pressure tubes that carry oxygenated blood from the heart and lungs to the body. Veins: Large lower-pressure tubes that carry oxygen-poor blood back to the heart and lungs. Capillaries: Small tubes that allow the transfer of gases, nutrients, and waste between the vascular system and the body’s cells. They are the link between arteries and veins. 60 Two exceptions to the points above exist: The pulmonary arteries carry oxygen-poor blood from the heart to the lungs, and the pulmonary veins carry oxygenated blood from the lungs to the heart. Blood in the arteries travels quickly and is under greater pressure than blood in the capillaries and veins. Blood in the arteries moves in pulses as the heart beats; blood in the veins flows more slowly and evenly. From upper body Aorta To lung To lung From lung From lung Left atrium Right atrium Left ventricle Right ventricle From lower body (inferior vena cava) To lower body (descending aorta) n = Oxygen-poor blood pumped from the body to the lungs n = Oxygen-rich blood pumped from the lungs to the body Figure 4–12: The right side of the heart receives blood from the body and sends it to the lungs. The left side of the heart receives blood from the lungs and pumps it out through the body. One-way valves direct the flow of blood through the heart. THE HEART’S ELECTRICAL SYSTEM The pumping action of the heart is called a contraction. Contractions are controlled by the heart’s electrical system, which makes the heart beat regularly. You can feel the heart’s contractions in the arteries that are close to the skin such as the ones at the neck or the wrist. The beat you feel with each contraction is called the pulse. The heart must beat regularly to deliver oxygen to the body’s cells to keep the body functioning properly. To better understand both the limitations of CPR and how defibrillators work, it is helpful to understand how the heart’s electrical system functions. Under normal conditions, specialized cells in the heart produce electrical activity. These electrical impulses are the stimuli that cause the heart muscle to contract and pump the blood out of its chambers and throughout the body (Figure 4–13). The normal point of origin of the electrical impulse is the sinoatrial (SA) node, which is situated in the upper part of the heart’s right atrium. When Atria AV node Bundle of His Bundle branches Ventricles Purkinje fibres Figure 4–13: The conduction system of the heart. it fires, the atria contract. The electrical impulse moves to the atrioventricular (AV) node, which is situated between the atria and ventricles, through conduction pathways within the heart muscle. From the AV node, the electrical signal is sent to the ventricles through other pathways. When the ventricles receive the impulse, they contract to expel the blood throughout the body’s blood vessels. This process normally occurs 60 to 100 times per minute. Cardiac monitors are used to read the electrical impulses in the heart and produce an electrocardiogram (ECG), which is a reading of the conduction of the electrical current through the pathways of the heart. The normal conduction of electrical impulses without any disturbances is called sinus rhythm (SR) (Figure 4–14). Figure 4–14: Sinus rhythm. BLOOD COMPONENTS Blood consists of liquid and solid components and comprises approximately 8% of the body’s total weight, with a volume of approximately 5 litres (5 quarts). Blood has three major functions: 1. Transporting oxygen, nutrients, and waste 2. Protecting against disease by producing antibodies and defending against pathogens 3. Helping to regulate body temperature by transferring heat to different parts of the body Blood appears as a uniform red liquid, but it contains the following major components: Plasma: A clear yellowish fluid; carries blood cells as well as many proteins, minerals, and waste products Red blood cells: Bond with oxygen molecules so that they can be carried to cells White blood cells: Fight infection Platelets: Repair leaks in blood vessels by promoting clotting Plasma makes up just over half of the volume of blood circulating within the body. Composed mostly of water, plasma carries the other blood components through the circulatory system and maintains the blood volume that the circulatory system requires to function effectively. Plasma also contains nutrients essential for energy production and cell maintenance, and carries waste products away from cells. Red blood cells (erythrocytes) make up the majority of the blood’s solid components. They contain hemoglobin, which bonds to oxygen so that it can be carried to the cells that need it. Red blood cells also carry carbon dioxide away from the cells so that it can be exhaled. Red blood cells are produced in the marrow in the hollow centre of large bones, such as the humerus and femur. When blood is rich in oxygen, it appears bright red. Low-oxygen blood appears dark red or purple. This is why arterial blood can usually be quickly identified by its bright colour. White blood cells (leukocytes) are a component of the body’s immune system, defending against invading micro-organisms. They also aid in producing antibodies that help the body resist infection. Platelets (thrombocytes) are disc-shaped cell fragments in the blood. They are a crucial component of blood clotting: When the vascular system is ruptured (e.g., a vein is cut by a knife), platelets bind together on the inside of the damaged vessel to form a clot. This blocks the ANATOMY AND PHYSIOLOGY SA node 61 hole in the blood vessel and prevents blood from escaping until the wound can heal. Some examples of injuries, disorders, and diseases that affect the cardiovascular system include: Myocardial infarction (heart attack). Angina (chest pain caused by reduced blood flow to the heart). Ischemia (a restriction in blood supply to tissues). Aneurysm (dilation or swelling of a blood vessel’s wall, which makes it more likely to rupture). Atherosclerosis (fatty deposits and fibrosis on artery walls). External or internal bleeding. Hypertension (elevated blood pressure). Congestive heart failure (CHF) (inefficient heart contractions, which can cause a backup of blood in the systemic and/or pulmonary circuit). Lymphatic System The lymphatic system performs three interrelated functions: 1. Removal of excess fluids (lymph) from body tissues 2. Absorption of fatty acids and subsequent transport of fat to the circulatory system 3. Formation of white blood cells (WBCs) and initiation of immunity through the formation of antibodies, creating specific resistance to pathogens ANATOMY AND PHYSIOLOGY The lymphatic system acts as a secondary circulatory system, transporting lymph throughout the body as the circulatory system transports blood. It collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses, or bacteria. 62 Unlike the circulatory system, the lymphatic system is not closed and has no central pump. The lymph moves slowly and under low pressure due to peristalsis, which is the operation of semilunar valves in the lymph veins, and the milking action of skeletal muscles. Like veins, lymph vessels have one-way semilunar valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls can also help draw fluid into the lymphatic capillaries. This fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body). These ducts drain into the circulatory system at the right and left subclavian veins. LYMPH Lymph originates as blood plasma that leaks out of the capillaries of the circulatory system and becomes interstitial fluid (fluid in the space between individual cells of tissue). Plasma is forced out of the capillaries by hydrostatic pressure, and as it mixes into the interstitial fluid, the volume of fluid increases slowly. Approximately 90% of this plasma is reabsorbed into the capillaries through osmosis, but the remaining 10% accumulates as overfill. The excess interstitial fluid is collected by the lymphatic system by diffusion into lymph capillaries and is processed by lymph nodes prior to being returned to the circulatory system. Once within the lymphatic system, the fluid is called lymph and has almost the same composition as the original interstitial fluid. Immunological System The immune system is a network of organs, cells, and proteins that identify and destroy harmful foreign substances in the body. These can be grouped into three types of defense: innate defence, non-specific responses to infection, and specific defence. It is only when all three lines are breached that infection and disease can occur. Protecting the body is a difficult task, since pathogens range from viruses to parasitic worms and must be detected with absolute specificity as they are hidden amongst normal cells and tissues. It is important that the body identify pathogens correctly so that it destroys as many as possible without attacking its own cells. This is further complicated by the fact that most pathogens are constantly evolving and mutating, increasing their chance of avoiding detection and successfully infecting their hosts. The body’s innate defences include physical and chemical barriers that prevent pathogens from entering or establishing themselves in the body. One of the primary barriers against bacteria and other harmful organisms is the skin, which keeps pathogens from entering the body. In addition to the physical barrier, skin creates a chemical barrier: Sebaceous glands in the skin produce sweat and sebum, which contain antiseptic molecules (primarily lysozyme) that break down bacterial cell walls. The body’s mucous membranes form a second physical barrier. Mucous membranes line various body cavities that are exposed to the external environment and also line internal organs. Major mucous membranes can be found in the nose, mouth, ears, genital area, and anus. Mucous membranes function by secreting mucus that traps bacteria and other foreign debris. Some mucous membranes are ciliated, meaning that they are covered with cilia, thin tail-like projections extending approximately 5 to 10 micrometers from the surface. The main function of cilia is to move things across the surface of the membrane. Cilia move potentially harmful substances towards the outside of the body where they can be harmlessly expelled. Innate defence includes other chemical defenders as well. Tears and saliva also contain lysozyme, and glands in the stomach lining produce hydrochloric acid (HCl), which kills most pathogens that are ingested. NON-SPECIFIC RESPONSES TO INFECTION Non-specific immune responses are generalized responses initiated by the body when infection is detected. They include inflammation and fevers. This line of defence also includes the deployment of white blood cells (WBCs). Any break in the skin can allow bacteria to enter the body. When an injury occurs, cells rupture, releasing histamine. This causes the capillaries to dilate, becoming more permeable and leaking fluid into these tissues. This reaction by the body is called an inflammatory response. Inflammation is characterized by swelling, redness, heat, pain, and dysfunction of any organs involved. During an infection, white blood cells may release a chemical that changes the thermostat setting in the hypothalamus. When the thermostat is set to a higher temperature, the current body temperature registers as too cold. To increase the temperature to the new level, the body shivers to generate heat. This can result in a fever. The increased temperature makes the body less hospitable to pathogens. The hypothalamus may subsequently lower the thermostat, in which case the person will suddenly feel overheated and start to sweat as the body attempts to cool off. A person may alternate between these heating and cooling responses during the course of an infection. When the skin is broken, WBCs arrive and attempt to engulf and destroy invading pathogens. Chemicals produced by the WBCs are carried by the blood to bone marrow, where they stimulate the production and release of additional WBCs to support the immune response. During the response, some WBCs die and become mixed with other cells such as dead tissue, bacteria, and living WBCs. This forms a thick, yellow-white fluid called pus. SPECIFIC DEFENCE Specific defences directly target invading pathogens by responding to antigens. Antigens are proteins found on the membranes of cells that the body recognizes as non-self (e.g., microbes, foreign cells, and cancer cells). Normally, the body’s immune system does not respond to its own antigens; however, if it does, this is referred to as autoimmune disease. Sometimes a person will develop an immune response to a harmless antigen, such as pollen or cat dander; this is an allergic response. Lymphocytes Specific immunity is dependent upon two special types of WBCs called lymphocytes: B cells and T cells. Their names are based on where in the body they mature. B cells mature in the bone marrow, and T cells mature in the thymus gland. B- and T-cell lymphocytes are capable of ANATOMY AND PHYSIOLOGY INNATE DEFENCE 63 recognizing an antigen because they have specific receptor molecules on their surface that exactly fit individual antigens as a key fits into a lock. B Cells B-cell lymphocytes are responsible for antibodymediated immunity (also called humoral immunity). They produce antibodies that bind with and neutralize specific antigens. Antibodies do not directly kill bacteria but mark them for destruction. When antibodies bind to viruses, they can prevent the viruses from infecting cells. When antibodies bind to toxins, they can neutralize them. This allows people to be immunized against specific toxins, such as tetanus. ANATOMY AND PHYSIOLOGY As B cells mature, they develop surface receptors that allow them to recognize specific antigens. They then travel through the bloodstream, distributing throughout the lymph nodes, spleen, and tonsils. Once B cells reach their destination, they remain inactive until they encounter a foreign cell with an antigen that matches their particular receptor site. When such an encounter occurs, the B cell’s receptors will attach it to the antigen. The B cell is then turned on and actively secretes antibodies that will bind with the invading antigen. Although these cells only live a few days, their antibodies remain and circulate in the blood and lymph, gradually decreasing in number. 64 At the time of activation, some of the cells become memory B cells, which have a longer lifespan. Memory B cells record the information about the foreign antigen so that antibodies can be made more quickly, and in greater quantities, if a second exposure occurs. This is the principle behind vaccination: By creating cells in the body that are keyed to detect a specific pathogen, the vaccination gives the body the tools to quickly attack and destroy that pathogen if it is introduced again. The activated T cell rapidly multiplies into a large group of cytotoxic T cells. These cytotoxic T cells migrate to the site of the infection or disease and produce chemicals that directly kill the pathogens. A portion of these activated T cells also become memory T cells, recording information about the foreign antigen so that T cells can respond more quickly and with greater power if a second exposure occurs. ALLERGIC AND INFLAMMATORY RESPONSES An allergy is an inflammatory immune response to a foreign antigen. On its own, the antigen is not harmful to the body, but in a person who is sensitive to the antigen, the body produces an inflammatory response designed to get rid of it. This inflammatory response also creates antibodies to respond to future exposures to the antigen. Allergic inflammatory responses can have effects ranging from mild to fatal. The immune response to an allergen is classified as either a sensitivity or a hypersensitivity. During a hypersensitivity reaction, the inflammatory response is more intense. When the allergen enters the body, it binds to the antibodies that are already present. These antibodies sit on mast cells (specialized WBCs) located in connective tissues throughout the body. When an allergen binds with the antibody, the mast cell triggers the immediate release of histamine and other mediators that cause allergy symptoms. The severity of the reaction varies, ranging from localized reactions near the site where the allergen entered the body (e.g., a rash) to life-threatening allergic reactions called anaphylaxis. In an anaphylactic reaction, a massive release of histamine causes widespread vasodilation, circulatory collapse, and severe bronchoconstriction. Unless treated promptly, anaphylaxis can result in death. T Cells Defending the body against intracellular pathogens is the role of T cells. Macrophages, a type of white blood cell, seek out and ingest invading microbes. Antigens from the microbe turn on, or stimulate, the T cell. Nervous System The nervous system is the most complex and delicate of all body systems. The brain and spinal cord form the central nervous system. The brain regulates all body functions, including the respiratory and circulatory systems. The primary functions of the brain can be divided into three categories: 1. Sensory functions 2. Motor functions 3. Integrated functions such as responsiveness, memory, emotion, and use of language The brain transmits and receives information through a network of nerves. Figure 4–15 shows the nervous system. The spinal cord, a large bundle of nerves, extends from the brain through a canal in the spine. Nerve branches extend to various parts of the body through openings on the sides of the vertebrae. Nerves transmit information as electrical impulses from one area of the body to another. Some nerves conduct impulses from the body to the brain, allowing you to see, hear, smell, taste, and Nerve cells cannot regenerate or grow back. Once brain cells are damaged or killed, they are not replaced. Nerve cells may die due to disease or injury. When a particular part of the brain is diseased or injured, a person may permanently lose the body functions controlled by that area of the brain. For example, if motor control centres are damaged, the person may experience permanent paralysis. Peripheral nervous system The spinal column is a strong, flexible column that supports the head and the trunk, and it encases and protects the spinal cord. The spinal column, which extends from the base of the skull to the tip of the coccyx, consists of a series of small articulated bones called vertebrae. The vertebrae are separated from one other by cushions of cartilage called intervertebral discs (Figure 4–16, a). This cartilage is an elastic tissue that acts as a shock absorber, cushioning against the effects of everyday movement. The spinal column is divided into five regions (Figure 4–16, b): 1. The cervical or neck region 2. The thoracic or mid-back region 3. The lumbar or lower back region 4. The sacrum 5. The coccyx, which consists of three to four small fused vertebrae at the lower end of the spinal column ANATOMY AND PHYSIOLOGY Spinal cord Central nervous system Figure 4–15: The nervous system. Illness or injury to the brain may change a person’s level of responsiveness (LOR) and/or affect specific psychological functions such as memory, emotion, and language. Injuries to the nervous system can also affect sensation and the ability to move (motor function). One of the functions of the brain, particularly of the brain stem, is maintaining responsiveness. When the brain stem is fully functioning and oxygenated, the person is responsive. Decreased oxygen supply results in a decreased level of responsiveness, where the person may only respond to tactile or painful stimuli. The level of responsiveness may further degrade to a point of unresponsiveness, where there is no response to any stimulus. Brain Nerves to and from the spinal cord feel. These are the sensory functions. Other nerves conduct impulses from the brain to the muscles to control motor functions. 65 C1 C2 C3 Cervical C4 C5 C6 C7 T1 T2 T3 T4 T5 T6 Thoracic T7 T10 T11 T12 L1 L2 L3 Intervertebral disc Lumbar L4 a b Vertebra L5 S1 S2 Spinal cord S4 S5 S3 b Sacrum 5 fused vertebrae Coccyx 4 fused vertebrae Figure 4–16, a-b: a, Vertebrae are separated by cushions of cartilage called intervertebral discs; b, the spine is divided into 5 regions. ANATOMY AND PHYSIOLOGY Injuries to the spinal column may include fractures and dislocations of the vertebrae, sprained ligaments, strained muscles, and compression or displacement of the discs between the vertebrae. 66 Some examples of injuries, disorders, and diseases that affect the nervous system include: Parkinson’s disease. Multiple sclerosis (MS). Amyotrophic lateral sclerosis (ALS). Spinal cord injury (This can be related to trauma to the musculoskeletal system.) Brain tumors. Concussion. Stroke/transient ischemic attack (TIA). Musculoskeletal System The musculoskeletal system is made up of the body’s muscles, the bones that form the skeleton, and connective tissues such as tendons and ligaments. Together, these structures support the body, protect internal organs, allow movement, store minerals, produce blood cells, and generate heat. MUSCLES The body has more than 600 muscles. Most are skeletal muscles that attach to bones. Skeletal muscles account for most of a person’s lean body weight (body weight without excess fat). FRONT VIEW BACK VIEW Face muscles Neck muscles Neck muscles Deltoid Deltoid Biceps Flexors and extensors of wrist and fingers Quadriceps Chest muscles Back muscles Abdominal muscles Gluteus maximus Groin muscles Flexors and extensors of foot and toes Triceps Flexors and extensors of wrist and fingers Hamstring muscles Calf muscles Achilles tendon Body movements result from skeletal muscles contracting and relaxing. Figure 4–17 shows the major muscles of the body. Because skeletal muscle actions are under your conscious control, skeletal muscles are also called voluntary muscles. Skeletal muscles protect underlying structures such as bones, nerves, and blood vessels. Most skeletal muscles are anchored to a bone at each end by tendons, which are strong cordlike tissues. Muscles and their adjoining tendons extend across joints. When the brain sends a command to move, electrical impulses travel from the brain down through the spinal cord and nerve pathways to the individual muscles and stimulate the muscle fibres to contract. This shortens (contracts) the muscle, pulling the ends of the muscle closer together, which in turn pulls the attached bones, causing motion at the joint the muscle crosses. The contraction and relaxation of muscles also produces heat. Most movement is caused by muscle groups working together (Figure 4–18). For instance, the hamstring muscles are a group of muscles at the back of the thigh, and the quadriceps are a group of muscles at the front of the thigh. When the hamstring muscles contract, the knee bends (flexion). When the quadriceps contract, the knee straightens (extension). Generally, when one group of muscles contracts, another group of muscles relaxes (Figure 4–19). Even simple tasks, such as tossing a ball, involve a complex series of movements in which different muscle groups contract and relax. Muscle actions can be voluntary or involuntary. Involuntary muscles, such as the heart and diaphragm, are automatically controlled by the brain. You don’t have to think about them to make them work. Voluntary muscles, such as leg and arm muscles, are most often under your conscious control. You are aware of telling them ANATOMY AND PHYSIOLOGY Figure 4–17: Major muscles of the body. 67 to move, and they usually don’t move unless you want them to. Injuries to the nervous system, brain, spinal cord, or peripheral nerves can affect muscle control. A complete loss of muscle control is called paralysis. When an isolated injury to a muscle occurs, the adjacent muscles can sometimes assume some of the function of the injured muscle (Figure 4–20). Skeleton Figure 4–18: Muscles work together to produce movement. Contract Relax An adult skeleton is comprised of over 200 bones of various sizes and shapes (Figure 4–21). These bones shape the skeleton, giving each body part a unique form. Prominent bones that can be seen or felt beneath the skin provide landmarks for locating less visible internal structures. The skeleton protects vital organs and other soft tissues (Figure 4–22, a-c). The point where two or more bones come together is a joint. Ligaments, which are fibrous bands that hold bones together at joints, give the skeleton stability and, with the muscles, help maintain posture. Bones Contract Relax Figure 4–19: Movement occurs when one group of muscles contracts and an opposing group of muscles relaxes. Adjacent muscle ANATOMY AND PHYSIOLOGY Injured muscle 68 Figure 4–20: Adjacent muscles can often assume the function of an injured muscle. Bones are hard, dense tissues. Their strong, rigid structure helps them to support the body and withstand stresses that cause injuries. The shape of each bone depends on its function and the stresses imposed on it. For example, although they are similar in appearance to the bones of the arm, the bones of the leg are much larger and stronger because they support the body’s weight. In addition to supporting and protecting the body, bones aid movement. The bones of the arms and legs work like a system of levers and pulleys to position the hands and feet. Bones of the wrist, hand, and fingers are progressively smaller to allow for fine movements, such as writing. The small bones of the feet enable the body to walk smoothly, and together they work as shock absorbers while walking, running, or jumping. Bones have many blood vessels and nerves. FRONT VIEW BACK VIEW Skull Cranium Face Clavicle Scapula Thorax Ribs Thorax Sternum Spinal column Spinal column Humerus Radius Ulna Pelvis Coccyx Femur Patella Tibia Fibula a b c Figure 4–22, a-c a: The immovable bones of the skull protect the brain; b, the rib cage protects the heart and lungs; and, c, the vertebrae protect the spinal cord. ANATOMY AND PHYSIOLOGY Figure 4–21 The skeleton. 69 Bones are filled with a spongey tissue called marrow, which produces blood cells and supplies them to the circulatory system. Bones are classified as long, short, flat, or irregular: Long bones are longer than they are wide. These include the bones of the upper arm (humerus), the forearm (radius and ulna), the thigh (femur), and the lower leg (tibia and fibula). Short bones are about as wide as they are long. Short bones include the small bones of the hands (carpals) and feet (tarsals). Flat bones have a relatively thin, flat shape. They include the sternum, the ribs, the shoulder blades (scapula), and some of the bones that form the skull. Irregular bones are those that do not fit into one of the other three categories. They include the vertebrae, bones of the face, and the sesamoid bones (tiny bones embedded in other tissues). Bone injuries are usually very painful and can bleed excessively. The bleeding can become lifethreatening if not properly cared for. Bones heal by forming new bone cells. Bones become more brittle with age. Bones in young children are more flexible than in adults, so they are less likely to break. An older adult’s bones are less dense and more likely to give way under everyday stresses. spontaneously, with little or no aggravation, trauma, or force. For example, the person may be taking a walk or washing dishes when the fracture occurs. Some hip fractures thought to be caused by falls are actually spontaneous fractures that cause the person’s fall. Repeated fractures are also a sign of osteoporosis. Osteoporosis is a leading cause of bone and joint injuries in older people. It is much more common in women. Joints A joint is a structure formed by the ends of two or more bones coming together. Figure 4–23 shows a typical joint. Most joints allow motion. However, the bones at some joints are fused together, which restricts motion. Fused bones, such as the bones of the skull, form solid structures that protect their contents (Figure 4–24). Joints are held together by tough, fibrous connective tissues called ligaments. Ligaments resist joint movement, so joints surrounded by ligaments have restricted movement, while joints that have fewer ligaments move more freely. For instance, the shoulder joint, which has fewer ligaments, has a greater range of motion than the hip joint, although their structures are similar. Each joint is surrounded by a capsule that secretes synovial fluid to lubricate the joint. Femur ANATOMY AND PHYSIOLOGY Osteoporosis is a degenerative bone disorder that occurs when the amount of calcium in the bones decreases, causing low bone mass (in addition to the natural deterioration of bone tissue). 70 Bone-building cells constantly repair damage that occurs as a result of everyday wear and tear, keeping bones strong. When the calcium content of bones decreases, the bones become frail, less dense, and less able to repair themselves after incurring stress and damage. The loss of density and strength leaves bones more susceptible to fractures (especially of the hips, vertebrae, and wrists). Instead of being caused by tremendous force, fractures may now occur Patella Ligaments Ligaments Tibia Fibula Tendon Figure 4–23: A typical joint consists of at least two bones that are held together by ligaments. Some examples of injuries, disorders, and diseases that affect the musculoskeletal system include the following: Sprains Strains Dislocations Fractures Carpal tunnel syndrome Osteoporosis Integumentary System Figure 4–24: Fused bones, such as those of the skull, form solid structures that protect their contents. Joint motion also depends on the bone structure. Joints that move more freely have less natural support and are therefore more prone to injury. However, there is a normal range of movement for each joint. When a joint is forced beyond its normal range, ligaments stretch and tear. Stretched and torn ligaments permit too much motion, making the joint unstable. Unstable joints can be disabling, particularly when they are weight-bearing joints (e.g., the knee or ankle). Unstable joints are also more prone to injury and often develop osteoarthritis, which is an inflammatory condition of the joints that generally occurs in older adults. The integumentary system consists of the skin, hair, and nails (Figure 4–25). Most important among these is the skin because it protects the body. The skin is the largest single organ, and without it the human body could not function. It helps keep fluids inside and prevents infection by keeping pathogens from entering the body. The skin also helps make vitamin D and stores minerals. The skin is made of tough, elastic fibres that can stretch without tearing, protecting it from injury. The skin has two layers. The outer layer of skin, the epidermis, provides the barrier to bacteria and other organisms that can cause infection. The deeper layer, called the dermis, contains the important structures of the nerves, glands, and blood vessels. Hair Skin Epidermis Subcutaneous layer Nerves Glands Fatty tissue Figure 4–25: The integumentary system. ANATOMY AND PHYSIOLOGY Dermis 71 The outer surface of the skin consists of dead cells that are continually rubbed away and replaced. The skin contains the hair roots, oil glands, and sweat glands. Oil glands help keep the skin soft, supple, and waterproof. Sweat glands and pores help regulate body temperature by releasing sweat. The nervous system monitors blood temperature and causes the body to sweat if blood temperature rises even slightly. Small amounts of sweat are often released to the skin’s surface without the person’s awareness. Blood supplies the skin with nutrients and helps provide its colour. When blood vessels dilate (become wider), the blood circulates close to the skin’s surface. This increases heat loss, cooling the blood, and makes the skin feel warm. It also makes some people’s skin appear flushed and red. The reddening may not appear on darker skin. When blood vessels constrict (become narrower), less blood is close to the skin’s surface. This causes the skin to feel cool and appear pale, and reduces heat loss. Nerves in the skin also absorb information about the environment. Nerves make the skin very sensitive to touch, pain, and temperature, so the skin is also an important part of the body’s sensory communication network. Beneath the skin lies a layer of fat, which helps to insulate the body, aiding in thermoregulation. The fat layer also stores energy. The amount of fat in this layer varies from person to person and between different areas of the body. Some examples of injuries, disorders, and diseases affecting the integumentary system include the following: Skin cancer Eczema Psoriasis Burns Lacerations, contusions, and abrasions Endocrine System The endocrine system is one of the body’s two regulatory systems. Together with the nervous system, it coordinates the activities of other systems. The endocrine system consists of several glands (Figure 4–26). Glands are organs that release fluid and other substances into the blood or onto the skin. Some produce hormones, chemical messengers that enter the bloodstream and influence tissue activity in various parts of the body. For example, the thyroid gland makes a hormone that controls metabolism, the process by which all cells convert nutrients into energy. Pineal gland Hypothalamus Pituitary gland Thyroid ANATOMY AND PHYSIOLOGY Adrenal glands 72 Ovaries Testes Figure 4–26: The endocrine system. Some examples of injuries, disorders, and diseases that affect the endocrine system include: Traumatic brain injuries (TBIs), which can damage the pituitary gland or hypothalamus. Thyroid cancer. Type 1 and 2 diabetes. Digestive System The digestive system, also called the gastrointestinal (GI) system, consists of organs that work together to break down food and eliminate waste from the body. Figure 4–27 shows the major organs of the digestive system. Food enters the system and is broken down into smaller components that the body can use. This occurs both mechanically (e.g. when food is chewed) and chemically (e.g. when food is broken down molecularly by stomach acid). As food passes through the system, the body absorbs nutrients that can be converted for use by the cells. The unabsorbed portion continues through the system and is eliminated as waste. Mouth Esophagus Liver Stomach Gallbladder Since most digestive system organs are in the unprotected abdominal cavity, they are very vulnerable to injury. Damaged organs may hemorrhage internally, causing severe loss of blood, or spill waste products into the abdominal cavity, causing infection. Large intestine (colon) Pancreas Small intestine Rectum Anus Figure 4–27: The digestive system. Examples of injuries, disorders, and diseases that affect the digestive system include the following: Bowel obstruction Gastrointestinal bleed Appendicitis Abdominal injuries Genitourinary System Kidneys Ureters Urinary bladder Urethra Figure 4–28: The urinary system. ANATOMY AND PHYSIOLOGY The genitourinary system is made up of two organ systems: the urinary system and the reproductive system. The urinary system consists of organs that eliminate waste products filtered from the blood (Figure 4–28). The primary organs are the kidneys and the bladder. The kidneys are located behind the abdominal cavity just beneath the chest, one on each side. They filter waste from the circulating blood and form urine. Urine is then stored in the bladder, which is a small muscular sac. The bladder expands as it fills and then contracts when the urine is released. 73 The kidneys are partially protected by the lower ribs. However, the kidneys may be damaged by a significant blow to the back just below the rib cage or a penetrating wound such as a stab or gunshot wound. Because of the kidney’s rich blood supply, such an injury may cause severe hemorrhaging. The bladder is injured less frequently than the kidneys, but injuries to the abdomen can rupture the bladder, particularly when it is full. Bone fragments from a fracture of the pelvis can also pierce or rupture the bladder. The male and female reproductive systems include the organs for sexual reproduction (Figure 4–29, a-b). Because these organs are close to the urinary system, injuries to the abdominal or pelvic area can injure organs in either system. The female reproductive organs are smaller than many major organs and are protected by the pelvic bones. The soft tissue of the external structures are more susceptible to injury, although such injury is uncommon. The male reproductive organs are located outside the pelvis and are more vulnerable to injury. The external reproductive organs, called genitalia, have a rich supply of blood and nerves. Injuries to these organs may cause a hemorrhage but are rarely life-threatening. Injuries to the genitalia are usually caused by a blow to the pelvic area or by sexual assault. Some examples of injuries, disorders, and diseases that affect the genitourinary system include the following: Urinary tract infection (UTI) Pelvic inflammatory disease (PID) Ectopic pregnancy Breech birth Post-partum bleeding Interrelationships of Body Systems Each body system plays a vital role in survival. Body systems work together to help the body maintain a constant healthy state. When conditions change, the body’s systems adapt to the new conditions. For example, because the musculoskeletal system works harder during exercise, the respiratory and circulatory systems must also work harder to meet the body’s increased oxygen demands. The body’s systems also react to the stresses caused by illness or injury and to environmental factors such as the ambient temperature. Body systems do not work independently of each other. The impact of an injury or a disease is rarely restricted to one body system. For example, a broken bone may result in nerve damage that may impair movement and feeling. Injuries to the ribs can make breathing difficult. If the heart stops beating for any reason, breathing will also stop. In any significant illness or injury, multiple body systems may be affected. Generally, the more body systems that are involved in an emergency, the more serious the emergency. Fallopian tubes Ovaries ANATOMY AND PHYSIOLOGY Uterus 74 Duct system Urethra Vagina Penis a Testicles Figure 4–29, a-b: The reproductive systems: a, male; b, female. b The body has a natural resistance to injury. However, injuries occur when external forms of energy produce forces that the body cannot tolerate. Mechanical forms of energy and the energy from heat, electricity, chemicals, and radiation can damage body tissues and disrupt normal body function. Some tissues, such as the soft tissues of the skin, have less resistance and are at a greater risk of injury when exposed to trauma than the deeper, stronger tissues of muscle and bone. Some organs, such as the brain, heart, and lungs, are better protected by bones than other organs, such as those in the abdominal cavity. Understanding the forces that cause injury and the kinds of injury that each force can cause will help you recognize certain injuries that a patient may have. Mechanical Energy Mechanical energy produces the following forces: direct, indirect, twisting, and contracting. Figure 4–30 shows how these forces can result in injuries. A direct force is the force of an object striking the body and causing injury at the point of impact. Direct forces can be either blunt or penetrating. For example, a fist striking the chin (blunt force) can break the jaw, or a bullet (penetrating force) can injure structures beneath the skin. Direct force can cause internal and external bleeding, head and spinal injuries, fractures, and other problems, such as crushing injuries. An indirect force is the force of a blunt object striking one part of the body and causing injury to another. For example, falling onto an outstretched hand may result in an injury to the arm, shoulder, or collarbone. Figure 4–30: Four forces—direct, indirect, twisting, and contracting—cause the majority of all injuries. ANATOMY AND PHYSIOLOGY HOW INJURIES OCCUR 75 ANATOMY AND PHYSIOLOGY In a twisting injury, one part of the body remains stationary while another part of the body turns. A sudden or severe twisting action can force body parts beyond their normal range of motion, causing injury to bones, tendons, ligaments, and muscles. For example, if a ski and its binding keep the lower leg in one position while the body falls in another position, the knee may be forced beyond its normal range of motion. Twisting injuries are not always this complex. They more often occur as a result of simple actions, such as stepping off a curb or turning to reach for an object. 76 Sudden or powerful muscle contractions often result in injuries to muscles and tendons. These commonly occur in sports activities—for example, when someone throws a ball without properly warming up or preparing his or her muscles. Occasionally, powerful muscle contractions can suddenly pull a piece of bone away from the point at which it is normally attached. These four forces, products of mechanical energy, cause the majority of injuries. Soft tissue injuries and injuries to muscle and bone (musculoskeletal injuries) are most often the result. SUMMARY Midline Head Neck Shoulder Proximal Medial Chest Arm Trunk Elbow Lateral Abdomen Distal Forearm Pelvis Wrist Hand Hip Thigh Upper extremity Knee Lower leg Ankle Left Foot Lower extremity Cranial cavity Right Left MAJOR BODY CAVITIES Cranial Cavity Located in the head, protected by the skull Spinal cavity Spinal Cavity Extending from the bottom of the skull to the lower back, protected by the bones of the spine Thoracic cavity Thoracic (Chest) Cavity Located in the trunk between the diaphragm and the neck, protected by the rib cage and the upper portion of the spine Abdominal cavity Abdominal Cavity Located in the trunk between the diaphragm and the pelvis Pelvic Cavity Located in the pelvis, protected by the pelvic bones and the lower portion of the spine Diaphragm Pelvic cavity ANATOMY AND PHYSIOLOGY Right 77 SUMMARY BODY SYSTEMS System Major Structures Primary Functions Respiratory system Airway and lungs Supplies the body with oxygen through breathing Circulatory system Heart, blood, and blood vessels Transports nutrients and oxygen to body cells and removes waste products Lymphatic system Lymph, lymph nodes Removes excess fluid (lymph) from body tissues; absorbs fatty acids  and transports fat to the circulatory system; aids in formation of  white blood cells (WBCs); initiates immunity through the  formation of antibodies Nervous system Brain, spinal cord, and nerves Transmits messages to and from the brain Musculoskeletal system Bones, ligaments, muscles, and tendons Provides the body’s framework; protects internal organs and other underlying structures; allows movement; produces heat; manufactures blood Integumentary system Skin, hair, and nails An important part of the body’s communication network; helps prevent infection and dehydration; assists with temperature  regulation; aids in production of certain vitamins Endocrine system Glands Secretes hormones and other substances into blood and onto skin Digestive system Mouth, esophagus, stomach, intestines Breaks down food into a usable form to supply the rest of the body with energy Genitourinary system Uterus and genitalia Performs the processes of reproduction Kidneys, bladder Removes wastes from the circulatory system and regulates water balance MECHANICAL ENERGY CAUSING INJURY ANATOMY AND PHYSIOLOGY Type of Energy 78 Description Direct force An object strikes or is struck by the body. It can be blunt or penetrating. Indirect force Blunt force on one part of the body causes injury to another part. Twisting force Part of the body remains stationary while another part turns. Contracting force A sudden or powerful muscle contraction injures muscles and/or tendons.