Basophils are classified as granulocytes. Their granules release enzymes during an allergic response. Baspophils are similar to mast cells and release histamine and heparin. Eosinophils play an important role in parasitic infections. Neutrophils are the body's first line of defense against invading bacteria. They search out, kill, and phagocytize bacteria that are involved in infection. Platelets are fragments of a larger cell called a megakarocyte. Platelets help form blood clots and prevent blood loss. Lymphocytes are involved in specific immune responses and include natural killer cells, b-lymphocytes, and t-lymphocytes.

The lymphatic system is part of the circulatory system and carries fluid called lymph, via lymphatic vessels, toward the heart. The lymphatic system functions to remove interstitial fluid from tissues, transport lipids as chyle from the digestive system, and defend the body against infection and the spread of tumors.

The circulatory system transports oxygen and nutrients to body tissues. Most nutrients are absorbed directly into the blood from the small intestine. Fats are the exception, and are transported into the lymph via lacteals.

The correct answer is returns excess interstitial fluid to blood. The other choices are functions of the urinary system, not the lymphatic.

The lymph capillaries in the lymphatic system pick up excess interstitial fluid and proteins and becomes lymph. If excess fluid is built up in the tissue spaces, blood volume and pressure will decrease. The lymph is returned to the blood in the veins (venous blood) to prevent this from occurring and also to minimize edema (swelling) from the fluid build-up.

The other choices are functions of the urinary system. Red blood cell production is controlled with the secretion of the hormone erythropoietin. Blood pressure is regulated by the secretion of the enzyme renin. The volume and composition of body fluids is maintained within normal limits by regulating the amount of water excreted in urine. The urinary system also acts to maintain concentrations of electrolytes in fluids and normal pH of blood.

Lymph flow is controlled by the contraction of skeletal muscle, smooth muscle cells around lymphatics, respiration, and gravity.

Osmosis is used to transport fluids into the lymph, but cannot be used to control lymphatic flow.

There are three main groups of lymph nodes that provide lymphatic drainage in the thoracic area. There are the sternal nodes, intercostal nodes, and phrenic nodes.

The sternal nodes, also known as the parasternal nodes, drain the areas of the medial part of the breast, the intercostal areas, the diaphragm, and the area above found above the umbilicus. They drain into the meeting point of the internal jugular and subclavian veins. The intercostal nodes also drain the area of the intercostal spaces and the pleura, emptying into the thoracic duct. The phrenic nodes collect lymphatic fluid from the pericardium, liver, and diaphragm. They empty into the posterior mediastinal lymph nodes. The axillary nodes drain into the apical nodes, which then empty into the subclavian vein.

Normal inspiration typically involves the flattening (contraction) of the diaphragm in order to increase the volume of the thoracic cavity, and can be done unconsciously. In order to increase the amount of inhaled air, other muscles such as the external intercostals and the sternocleidomastoids are included by conscious control. Both of these muscles aim to raise and expand the thoracic cavity in order to assist in inhalation.

The rectus abdominis is involved in the opposite action of forced exhalation. The rectus abdominis aims to decrease the volume of the thoracic cavity by contracting. This assists in forced exhalation.

Unconscious breathing is controlled by the pons and the medulla oblongata, both of which are parts of the brain stem. This unconscious breathing can be consciously controlled by using the cerebral cortex, which manages most voluntary actions.

It helps to remember that the brain stem is responsible for unconscious control of the body: breathing, heart rate, blood pressure, etc. It is the addition of the cerebral cortex that allows humans to have conscious control over actions, such as breathing, and override the unconscious controls. For example, the cerebral cortex is used to consciously stop breathing when diving underwater.

At rest the diaphragm is slightly curved superiorly such that it makes this sort of shape:      When it contracts, it flattens out, with the middle of the muscle being pulled down until the muscle is roughly horizontal. Remembering that the diaphragm separates the thoracic and abdominal cavities, if it contracts, it physically increases the volume of the thoracic cavity. Now, remembering your fluid physics, an increase in volume is accompanied with a decrease in pressure. We know that high pressure flows to low pressure spontaneously. The atmospheric pressure is now higher than the intrapleural (or thoracic cavity) pressure, causing air to flow into the lungs.

Note that the external intercostals aid in inspiration and the internal intercostals aid in expiration.

Tidal volume is, by definition, the amount of air inspired/expired during normal breathing. The maximum volume of air that can be inspired after a normal expiration is the inspiratory capacity. The maximum volume of air that can be expired after a maximal inspiration is the vital capacity. The volume of air still in the lungs after a maximal expiration is the residual volume. The maximum volume of air that can be inspired after a normal inspiration is the inspiratory reserve volume.

During a respiratory cycle, the diaphragm contracts and moves downward. When this occurs the pressure in the alveoli falls. This pulls air into the lungs. At the same time external intercostals muscles contract, raising ribs and sternum and enlarges the cavity even more. During exhalation the diaphragm relaxes (moves up) and air is foced out of the body. A hiccup is a muscular spasm of the respiratory muscles including the diaphragm. A pneumothorax is a "hole" in the lungs that causes air to accumulate in the pleural space.