40. A neuromuscular junction is the point of contact between a motor neuron and a skeletal muscle cell.

41. Skeletal muscles are joined to bones by tough connective tissues called tendons.

42. Tendons are attached in such a way that they pull on the bones and make them work like levers. The joint functions as a fulcrum – the fixed point around which the lever moves. The muscles provide the force to move the lever. 43.

The Integumentary System

44. The integumentary system is made up of the skin, hair, nails, sweat and oil glands.

45. The integumentary system serves as a barrier against infection and injury, helps to regulate body temperature, removes waste products from the body, and provides protection against ultraviolet radiation from the sun.

46. The epidermis is the outer layer of skin. 47. The epidermis provides a waterproof covering for the body and protects the body from ultraviolet radiation from the sun.

48. The inner layer of the skin is the dermis. 49. The dermis helps regulate body temperature and gets rid of wastes via the sweat glands.

50.

51. The basic structure of human hair and nails is keratin.

52. Hair protects the skin from ultraviolet radiation, provides insulation from cold, and prevents dirt and other particles from entering the body. 53. Hair is produced by cells at the base of hair follicles.

The Circulatory System

54. The function of the circulatory system is to transport to the cells substances they need. 55. The three main parts of the circulatory system are the heart, a series of blood vessels, and the blood that flows through them.

56. The human heart has four chambers. 57. The atria are the upper chambers that receive blood and pump it into the ventricles. The ventricles are the lower chambers that pump blood to the rest of the body.

58. Pulmonary circulation is the movement of blood from the heart to the lungs and back. Systemic circulation is the movement of blood from the heart to the rest of the body and back.

60. Valves are flaps of connective tissue between the atria and ventricles prevent blood from flowing back into the atria.

62. The sinoatrial node is a small group of cardiac muscle cells that start the wave of muscle contraction through the heart (pacemaker). 63. The atrioventricular node is a small group of cardiac muscle cells that continue the wave of muscle contraction from the atria to the ventricles.

The Blood and Lymphatic System

64. The three functions of blood is to collect oxygen from the lungs, nutrients from the digestive tract, and waste products from tissues.

65. Plasma is a straw-colored fluid making up about 55% of blood. Plasma is 90% water, and 10% dissolved gases, nutrients, enzymes, hormones, waste products, and proteins.

66. The cellular portion of blood consists of red blood cells, white blood cells, and platelets.

67. Erythrocytes are red blood cells, and their function is to transport oxygen. 68. Hemoglobin is the iron-containing protein that binds to oxygen in the lungs and transports it to tissues throughout the body.

69. Leukocytes are white blood cells, and they fight against infection, fight parasites, and attack bacteria.

70. Platelets are plasma proteins and cell fragments that make blood clotting possible.

71. The lymphatic system is a network of vessels, nodes, and organs, that collects the fluid that is lost by the blood and returns it back to the circulatory system. 72. Human require a lymphatic system because as blood circulates, some fluid leaks from the blood into the surrounding tissue.

73. The fluid lost from blood that is collected by the lymphatic system is called lymph. 74. Lymph nodes are small groups of cells located along lymph vessels that act as filters, trapping bacteria and other microorganisms that cause disease.

The Respiratory System

75. Cellular respiration, which takes place in the mitochondria, is the release of energy from the breakdown of food molecules in the presence of oxygen.

76. At the level of the organism, respiration means the process of gas exchange. 77. The basic function performed by the human respiratory system is to bring about the exchange of oxygen and carbon dioxide between the blood, the air, and tissues.

79. Air moves from the nose to the pharynx, larynx, trachea, bronchi, and lungs.

80. To keep the lung tissue healthy, air entering the respiratory system must be warmed, moistened, and filtered.

81. Bronchi are the two large passageways in the chest cavity leading from the trachea to the bronchioles. Bronchioles are smaller passageways leading from the bronchi to the alveoli.