Anatomy and Physiology

Musculoskeletal system

The musculoskeletal system has 3 functions: it gives the body its shape, it protects the vital organs, and it makes movement possible. The bones do more than provide structure, however. Bone marrow also produces blood cells and stores nutrients.

The skull has two parts. The cranium forms the top, back and sides of the head, and the face is made up of the mandible, the maxillae, the nasal bones, the orbits and the zygomatic arches.

The spine consists of 33 vertebrae, and its hollow center houses the spinal cord. From the top down, the vertebrae fall into five regions: 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the upper back where the ribs attach, 5 lumbar vertebrae in the lower back, 5 sacral vertebrae that form the back wall of the pelvis, and 4 vertebrae in the coccyx, or tailbone. The cervical spine is injured easily, because a heavy head sits on a slender neck, and a cervical injury can be fatal because this part of the cord controls the muscles of breathing. The lumbar spine is also vulnerable, since it is not supported by any other part of the skeleton.

The thorax is formed by 12 pairs of ribs and the 12 thoracic vertebrae. 10 pairs of ribs attach to the sternum; the remaining 2 pairs do not, and these are the "floating ribs." The sternum itself has three parts: the manubrium at the top, the body in the center, and the xiphoid process at the bottom tip.

The pelvis is made up of three bones: the ilium, which is superior and whose upper rim is the iliac crest; the ischium, which is inferior and posterior; and the pubis, which is anterior. The hip joint is formed where the head of the femur, the ball, sits in the acetabulum, the socket.

In the lower extremity, the femur is the largest bone in the body, and a fracture of the femur near the pelvis is what is commonly called a "broken hip." Below the femur lies the patella, or kneecap, and then the two bones of the lower leg: the tibia, the larger, medial bone that forms the shin, and the fibula, the smaller, lateral bone. Each of these ends at the ankle in a bony knob called a malleolus; the lateral malleolus belongs to the fibula and the medial malleolus to the tibia. The foot is built from the tarsals at the ankle, the metatarsals in the foot itself, the calcaneus at the heel, and the phalanges of the toes.

The upper extremity begins at the shoulder, where the clavicle, or collarbone, lies anteriorly and the scapula, or shoulder blade, lies posteriorly; the acromion process forms the top of the shoulder. The humerus is the bone of the upper arm. The forearm has two bones: the radius on the lateral, thumb side, where the radial pulse is felt, and the ulna on the medial side. The wrist is made up of the carpals, the hand of the metacarpals, and the fingers of the phalanges.

Joints are classified by the kind of movement they allow. The hip is a ball-and-socket joint, while the elbow is a hinge joint.

Muscle comes in three types, which differ in how they are controlled and where they are found. The table below sets them out.

Table 6.1Musculoskeletal system

Respiratory system

Air follows a fixed path into the lungs. It enters through the mouth or nose and passes into the pharynx, which has two parts, the oropharynx behind the mouth and the nasopharynx behind the nose. At the bottom of the pharynx sits the epiglottis, a flap that blocks food from entering the trachea. Below it lies the larynx, or voice box, which contains the vocal cords, and beneath that the cricoid cartilage. Air then travels down the trachea, which is held open by 16 C-shaped rings of cartilage, and divides into two bronchi, one for each lung. The bronchi branch into progressively smaller bronchioles, which end in the alveoli, the tiny sacs where gas exchange takes place.

The diaphragm is the muscle that divides the chest from the abdomen, and it is the main muscle of breathing; it is driven by the phrenic nerve. Inhalation is an active process. The diaphragm and the intercostal muscles contract, the chest expands, and the negative pressure this creates inside the chest pulls air in. Exhalation, by contrast, is passive. The muscles relax, the chest shrinks, and the resulting positive pressure pushes air out. Two terms that are easily confused describe different parts of this process: ventilation is the movement of gas into and out of the alveoli, while respiration is the exchange of gases between the blood and the cells.

Children differ from adults in ways that matter for airway management. A child's tongue is larger relative to the mouth, and the trachea and cricoid cartilage are narrower and softer, so the airway is more easily obstructed and more easily injured. The rib cage is flatter, and a child in respiratory distress may show a "seesaw" breathing pattern.

Cardiovascular system

The heart has four chambers: 2 atria on top and 2 ventricles below. Blood returns from the body through the venae cavae into the right atrium, passes to the right ventricle, and is pumped through the pulmonary artery to the lungs, where gas exchange takes place. Oxygenated blood returns through the pulmonary vein to the left atrium, moves into the left ventricle, the strongest of the four chambers, and is pumped out through the aorta to the body. One-way valves prevent blood from flowing backward. The heart also has its own electrical system, the cardiac conduction system, which acts as a built-in pacemaker; this ability to generate its own impulses is called automaticity.

The arteries an EMT needs to know, and where each is found or used, are listed in the table below.

Table 6.2Cardiovascular system

Blood leaves the heart through the arteries, which branch into smaller arterioles and then into the capillaries, where the exchange of oxygen, nutrients and waste with the cells takes place. From the capillaries, blood collects into venules, then veins, and finally the venae cavae that return it to the heart. Just as the pulmonary artery is the one artery that carries oxygen-poor blood, the pulmonary vein is the one vein that carries oxygen-rich blood. The carotid and femoral pulses are the central pulses, and they remain palpable even when the peripheral pulses have weakened.

Blood itself has four components, each with its own function, as the table below shows.

Table 6.3Cardiovascular system

Blood pressure is recorded as two numbers. The systolic pressure is the pressure in the arteries while the ventricle contracts and pumps; the diastolic pressure is the pressure while the ventricle refills. A reading is written with the systolic over the diastolic, as in "120/80." Perfusion is the adequate delivery of oxygen and nutrients to the cells, together with the removal of their wastes. Hypoperfusion, or shock, is perfusion that is inadequate to meet the cells' needs, and it is life-threatening.

The cells produce their energy through metabolism, which takes one of two forms depending on whether oxygen is available. Aerobic metabolism, which takes place when oxygen is present, is efficient and produces little waste, only carbon dioxide (CO2) and water. Anaerobic metabolism, which the cells fall back on when oxygen is lacking, is inefficient and produces lactic acid. The lactic acid leads to acidosis, which injures cells and impairs the blood's ability to carry oxygen. For gas exchange to work at all, there must be both enough air and enough blood at the alveoli at the same time; this is called V/Q match.

Lymphatic, nervous & other systems

The lymphatic system captures fluid that has escaped from the bloodstream, called lymph, and returns it to the circulation. It also has an immune function, producing the lymphocytes, a type of white blood cell. Its organs are the tonsils and adenoids, the spleen, the thymus and, by some accounts, the appendix. One clinical rule follows directly from this anatomy: never take a blood pressure (BP) on the affected arm of a patient who has had a mastectomy, because lymphatic tissue has been removed from the armpit and the pressure of the cuff is a risk.

The nervous system has two divisions. The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is made up of the sensory nerves, which carry information from the body to the brain, and the motor nerves, which carry commands from the brain to the body. Within the PNS, the autonomic nervous system (ANS) controls involuntary functions, and it in turn has two branches. The sympathetic branch produces the fight-or-flight response: heart rate rises, breathing deepens and the blood vessels constrict. The parasympathetic branch, sometimes called feed-or-breed or rest, does the opposite: it increases blood flow to the digestive and reproductive organs, slows the heart rate and dilates the vessels. A separate structure, the reticular activating system, keeps a person awake and conscious.

Recognizing sympathetic discharge is a useful assessment skill. A patient who is pale and diaphoretic with an elevated heart rate (HR) and respiratory rate (RR) is showing the body's response to a serious problem, and this can flag serious illness even when the classic symptoms are absent, as in the "silent" myocardial infarction (MI) that older women sometimes experience.

The digestive system is a single passage with supporting organs along it. Food travels from the mouth down the esophagus to the stomach, where acid begins breaking it down, then into the small intestine, whose three parts are the duodenum, jejunum and ileum and where nutrients are absorbed, and finally into the large intestine, or colon, which removes water and forms waste. The liver produces bile, detoxifies the blood, stores sugar and synthesizes blood products. The gallbladder stores bile. The pancreas produces insulin and digestive enzymes. The spleen filters old blood cells and serves as a reservoir of blood; it lies in the left upper quadrant (LUQ) and is at risk in trauma. The appendix, in the right lower quadrant (RLQ), is lymphatic tissue whose function is unclear.

The integumentary system, the skin, provides protection, maintains water balance, regulates temperature, excretes wastes and absorbs shock. It has three layers. The outer layer, the epidermis, contains no blood vessels or nerves. Beneath it, the dermis holds the blood vessels, nerves, glands and hair follicles, which is why an injury that reaches the dermis bleeds, hurts and carries a risk of infection. The deepest layer, the subcutaneous tissue, is fat and soft tissue that insulates the body and absorbs shock.

The endocrine system produces hormones. The pancreas secretes insulin, which regulates glucose. The adrenal glands secrete epinephrine and norepinephrine, the hormones of sympathetic activation; acting on beta-2 receptors they dilate the bronchioles, and acting on beta-1 receptors they increase the rate and force of the heartbeat.

The renal system, or urinary system, regulates fluid balance, filters the blood and adjusts its pH. The kidneys filter urea from the blood, regulate sodium and fluid, and produce bicarbonate. Urine passes from the kidneys through the ureters to the bladder, which serves as a reservoir, and leaves the body through the urethra.

The reproductive system differs by sex. In the male, the testes produce sperm and are connected to the urethra by way of the epididymis and vas deferens. In the female, the ovaries, which lie on either side of the lower abdomen, release eggs into the fallopian tubes, where fertilization takes place. A fertilized egg passes to the uterus, a highly vascular organ in the midline of the lower abdomen that houses the fetus, and the vagina serves as the birth canal.