Functions of the Circulatory and Excretory Systems

The Lymphatic System

The lymphatic system is a network of vessels and structures distributed throughout the body.

• Lymph capillaries: They collect excess interstitial fluid.
• Lymphatic vessels: They return excess lymph to the circulatory system.
• Lymph nodes: They contain defense cells that locate and fight microorganisms that could cause infection.

Composition and Function of Blood

Blood is a thick red liquid that flows inside the blood vessels.

• Blood plasma: A yellowish liquid that makes up 55% of blood.
• Blood cells: Red blood cells, white blood cells, and platelets make up the remaining 45% of blood.
• White blood cells (leukocytes): Protect the body against infection and foreign substances.
• Red blood cells (erythrocytes): Transport oxygen and pick up carbon dioxide for elimination.
• Platelets (thrombocytes): Help blood to clot by forming a plug to close small breaks in the blood vessels.

Types of Blood Vessels

• Arteries: They carry blood away from the heart.
• Veins: They carry blood from the organs to the heart; valves prevent blood from flowing backwards.
• Capillaries: They connect arterioles and venules.

The Three Stages of the Cardiac Cycle

1. Atrial systole: Both atria contract, pushing blood through the tricuspid and mitral valves into the ventricles. Both ventricles fill with blood.
2. Ventricular systole: Both ventricles contract, the pulmonary and aortic valves open, and blood is pushed through the pulmonary artery and aorta. The mitral and tricuspid valves close, preventing blood from flowing back into the atria.
3. Diastole: The atria and ventricles relax, the aortic and pulmonary valves are closed, the atria fill with blood, the atrioventricular valves open, and the ventricles fill with blood.

The Process of Excretion

Excretion is the process by which the waste products from cell metabolism are eliminated from the blood.

• Kidneys: They eliminate toxic substances resulting from cell metabolism through the urine.
• Lungs: They expel carbon dioxide from cellular respiration.
• Liver: It produces a liquid called bile that helps to eliminate waste from digestion through feces.
• Sweat glands: Exocrine glands of the skin responsible for sweat excretion. Sweat is a liquid that is very similar to urine, but more diluted.