Internal environment refers to the fluids that surround the cells in the body. This stable internal environment, called homeostasis, is maintained by the organ systems of the body.
The Circulatory System
The circulatory system consists of the blood circulatory system. It carries nutrients and oxygen throughout the body and collects metabolic wastes for elimination. The lymphatic system has three main functions: transport absorbed fat from the fat, defend the body in the immune system, and return excess interstitial fluid to the blood.
Blood is a thick red liquid that flows inside the blood vessels. Blood's plasma consists of water and dissolved substances. Blood cells, red and white blood cells, and platelets make up the remaining 45% of blood. They are formed in the red bone marrow located inside some bones, such as the femur. White blood cells (leucocytes) are the largest blood cell with a present nucleus. Monocytes protect the body against infection. Red blood cells (erythrocytes) are the most abundant with an absent nucleus and a biconcave disc shape. They contain haemoglobin responsible for its red color and transport oxygen. Platelets (thrombocytes) are cell fragments with an absent nucleus that help blood to clot by forming a plug to close small breaks in the blood vessels.
The Blood Vessels
The blood vessels: Arteries carry blood away from the heart, larger near the heart and thinner and smaller arterioles farther from the heart with thick and elastic walls. Veins carry blood from the organs to the blood, with venules (thin veins) farther from the heart and larger and thicker veins closer to the heart, with thin and not very elastic walls. Valves prevent blood from flowing backwards. Capillaries connect arterioles and venules, with tiny size and walls consisting of one layer of flat cells, the capillary endothelium. The heart walls consist of cardiac muscle called myocardium that pumps the blood. The tricuspid valve connects the right atrium and the left ventricle. The mitral valve connects the left atrium and the left ventricle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the heart.