HEART

HEART

The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system.^[1]Blood provides the body with oxygen and nutrients, as well as assists in the removal of metabolic wastes.^[2] In humans, the heart is located between the lungs, in the middle compartment of the chest.^[3]

In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles.^[4][5] Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart.^[6] Fish, in contrast, have two chambers, an atrium and a ventricle, while reptiles have three chambers.^[5] In a healthy heart blood flows one way through the heart due to heart valves, which prevent backflow.^[3] The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium.^[7]

The heart pumps blood with a rhythm determined by a group of pacemaking cells in the sinoatrial node. These generate a current that causes contraction of the heart, traveling through the atrioventricular node and along the conduction system of the heart. The heart receives blood low in oxygen from the systemic circulation, which enters the right atrium from the superior and inferior venae cavae and passes to the right ventricle. From here it is pumped into the pulmonary circulation, through the lungs where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta to the systemic circulation−where the oxygen is used and metabolized to carbon dioxide.^[8] The heart beats at a resting rateclose to 72 beats per minute.^[9] Exercise temporarily increases the rate, but lowers resting heart rate in the long term, and is good for heart health.^[10]

Arteries
The arteries are the blood vessels that deliver oxygen-rich blood from the heart to the tissues of the body. Each artery is a muscular tube lined by smooth tissue and has three layers: The intima, the inner layer lined by a smooth tissue called endothelial.

Veins

Veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart. In contrast to veins, arteries carry blood away from the heart.

Capillaries

Capillaries (/ˈkæpɪlɛriz/ in US; /kəˈpɪləriz/ in UK) are the smallest of a body's blood vessels (and lymph vessels) that make up the microcirculation. Their endothelial linings are only one cell layer thick. These microvessels, measuring around 5 to 10 micrometres (µm) in diameter, connect arterioles and venules, and they help to enable the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste substances between the blood and the tissues^[3] surrounding them. Lymph capillaries connect with larger lymph vessels to drain lymph collected in the microcirculation.

During early embryonic development^[4] new capillaries are formed through vasculogenesis, the process of blood vessel formation that occurs through a de novo production of endothelial cells which then form vascular tubes.^[5] The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already present endothelial which divides.^[6]