introduction to the human body exchange of substances between the blood and body tis- larger than those of the blood capillaries. Lymph is tissue sues,e.g. nutrients, oxygen and cellular waste products. fluid containing large molecules, e.g. proteins, fragments Blood vessels form a network that transports blood to: of damaged tissue cells and microbes. It is transported s the lungs(pulmonary circulation) where oxygen is along lymph vessels and is returned to the bloodstream absorbed from the air in the lungs and at the same There are collections of lymph nodes situated at various time carbon dioxide is excreted from the blood inte points along the length of the lymph vessels. Lymph is the air filtered as it passes through the lymph nodes, and m cells in all parts of the body(general or systemic microbes, noxious substances and some waste materials The lymphat stem provides the sites for formation Heart. The heart is a muscular sac. It pumps the blood and maturation of lymphocytes, the white blood cells round the body and maintains the blood pressure in the involved in immunity lungs and general circulation. This is essential for life The heart muscle is not under conscious(voluntary) Internal communication control. At rest, the heart contracts between 65 and 75 times per minute. The rate may be greatly increased dur Communication and the nervous system The nervous system is a rapid communication syst needs of the muscles moving the limbs are increased, and (Ch 7). The main components are shown in Figure in some emotional states The rate at which the heart beats can be counted by The central nervous system consists of taking the pulse. The pulse can be felt most easily where the brain, situated inside the skull an artery lies close to the surface of the body and can be the spinal cord, which extends from the base of the pressed gently against a bone. The wrist is the site most skull to the lumbar region and is protected from Lymphatic system The peripheral nervous system is a network of nerve The vessels, which begin as blind-ended tubes in the spaces between the blood capillaries and tissue cells(Fig. 1. 7). sensory or afferent, providing the brain with'input' Structurally they are similar to veins and blood capilar- motor or efferent, which convey nerve impulses ies but the pores in the walls of the lymph capillaries are carrying'output from the brain to effector organs the muscles and glands Spinal cord -Lymph nodes Peripheral nerves Central nervous system Peripheral nervous system Figure 1.7 The lymphatic system: lymph nodes and vessels Figure 1. 8 The nervous systemIntroduction to the human body 9 exchange of substances between the blood and body tis￾sues, e.g. nutrients, oxygen and cellular waste products. Blood vessels form a network that transports blood to: • the lungs (pulmonary circulation) where oxygen is absorbed from the air in the lungs and at the same time carbon dioxide is excreted from the blood into the air • cells in all parts of the body (general or systemic circulation). Heart. The heart is a muscular sac. It pumps the blood round the body and maintains the blood pressure in the lungs and general circulation. This is essential for life. The heart muscle is not under conscious (voluntary) control. At rest, the heart contracts between 65 and 75 times per minute. The rate may be greatly increased dur￾ing physical exercise, when the oxygen and nutritional needs of the muscles moving the limbs are increased, and in some emotional states. The rate at which the heart beats can be counted by taking the pulse. The pulse can be felt most easily where an artery lies close to the surface of the body and can be pressed gently against a bone. The wrist is the site most commonly used for this purpose. Lymphatic system The lymphatic system (Ch. 6) consists of a series of lymph vessels, which begin as blind-ended tubes in the spaces between the blood capillaries and tissue cells (Fig. 1.7). Structurally they are similar to veins and blood capillar￾ies but the pores in the walls of the lymph capillaries are larger than those of the blood capillaries. Lymph is tissue fluid containing large molecules, e.g. proteins, fragments of damaged tissue cells and microbes. It is transported along lymph vessels and is returned to the bloodstream. There are collections of lymph nodes situated at various points along the length of the lymph vessels. Lymph is filtered as it passes through the lymph nodes, and microbes, noxious substances and some waste materials are removed. The lymphatic system provides the sites for formation and maturation of lymphocytes, the white blood cells involved in immunity. Internal communication Communication and the nervous system The nervous system is a rapid communication system (Ch. 7). The main components are shown in Figure 1.8. The central nervous system consists of: • the brain, situated inside the skull • the spinal cord, which extends from the base of the skull to the lumbar region and is protected from injury by the bones of the spinal column. The peripheral nervous system is a network of nerve fibres, which are: • sensory or afferent, providing the brain with 'input' from organs and tissues, or • motor or efferent, which convey nerve impulses carrying 'output' from the brain to effector organs: the muscles and glands. Figure 1.7 The lymphatic system: lymph nodes and vessels. Figure 1.8 The nervous system