Human Circulatory and Lymphatic System Functions

The Human Circulatory System

The circulatory system is responsible for distributing food and oxygen around the body and collecting waste products from cells. It consists of blood, the heart, and blood vessels (arteries, veins, and capillaries).

Blood Tissue and Its Components

Blood is a tissue formed by a liquid, plasma, and a set of cells: red blood cells, white blood cells, and platelets.

• Plasma: Composed of 90% water and dissolved substances such as nutrients, salts, waste products, hormones, etc.
• Red Blood Cells (RBC): Responsible for transporting oxygen.
• White Blood Cells: Responsible for phagocytizing microbes and producing antibodies.
• Platelets: Essential for blood clotting.

Heart Anatomy and the Cardiac Cycle

The heart muscle contracts to empty (systole) and dilates (diastole) rhythmically to pump blood throughout the body. The period between one contraction and another is known as the cardiac cycle. The heart has four cavities: the left atrium, right atrium, left ventricle, and right ventricle. Between the left atrium and left ventricle is the mitral valve; between the right atrium and right ventricle is the tricuspid valve.

Systemic and Pulmonary Blood Circulation

Blood circulation is divided into two circuits:

• Systemic Circulation (Major): Blood leaves the left ventricle through the aorta artery, which branches into arteries, capillaries, and arterioles. Capillary exchange takes place with cells for gases, nutrients, and waste substances. From the capillaries, blood is directed back to the heart by venules and veins. It enters the right atrium through the inferior vena cava and the superior vena cava.
• Pulmonary Circulation (Minor): Blood passes from the right atrium to the right ventricle and then into the pulmonary arteries, leading to the lungs. It is oxygenated by the lung capillaries and returns via the pulmonary veins back to the left atrium, ending the double circuit.

The Lymphatic System and Immune Function

The lymphatic system is in charge of returning intercellular plasma to the circulatory blood. It transports fats absorbed from the intestines and produces lymphocytes, cells that originate antibodies. It comprises:

• Lymph: A fluid that circulates inside the lymphatic vessels; it is derived from plasma and contains interstitial fluid and lymphocytes.
• Lymphatic capillaries: Fine-walled vessels that start blindly and absorb plasma.
• Lymphatic vessels: Conduits formed by the confluence of many capillaries, featuring semilunar valves. The most important are the thoracic duct and the right lymphatic vein, which drain into the subclavian vein.
• Lymph nodes: Small thickenings located in the lymphatic vessels, inside of which are lymphocytes.

Functions of Major Blood Vessels

• Aorta: Brings oxygenated blood from the heart to the organs.
• Pulmonary artery: Transports oxygen-poor blood to the lungs.
• Vena cava: Carries oxygen-poor blood to the heart.
• Iliac vein: Features an intermediate thin layer of smooth muscle.
• Iliac artery: Features an intermediate layer of thick smooth muscle.
• Pulmonary vein: Carries blood rich in oxygen from the lungs.