Introduction:

The cardiovascular system includes heart, blood vessels (arterioles, arteries, venules capillaries, and veins) and blood. In vertebrates, system of blood circulation is closed, by which transport of oxygen and nutrients to tissues is performed. Blood vessels form tubular network which allows blood to flow from heart to all cells of body and then back to heart. Arteries carry blood away from heart, while veins return blood to heart. Blood passes from arterial to venous system in capillaries, that are the thinnest and most numerous of blood vessels.

Blood Vessels:

Arteries:

Artery is blood vessel which sends blood from heart to any part of body. Heart pumps blood through main artery known as dorsal aorta. Main artery then splits and branches out in several smaller arteries so that every region of body has its own system of arteries providing it with fresh, oxygen-rich blood. Arteries are rough on outside and smooth on inside. The artery wall really has 3 layers: the outer layer of tissue, muscular middle layer, and inner layer of epithelial cells. Muscle in middle is flexible and very strong. Inner layer is very smooth so that blood can flow effortlessly with no obstacles in path. Muscular wall of artery assists heart pump blood. When heart beats, artery enlarges as it fills with blood. When heart relaxes, arteries contract and exert force which is strong adequate to push blood along through body. This pace in heart and arteries results in the proficient circulation system.

Veins:

Veins are like arteries but, as they transport blood at the lower pressure, they aren't as strong as arteries. Similar to arteries, veins contains 3 layers: the outer layer of tissue, muscle in middle layer, and smooth inner layer of epithelial cells. Though, layers are thinner, having less tissue. Veins receive blood from capillaries after the exchange of oxygen and carbon dioxide has occurred, and veins transport deoxygenated blood back to lungs and heart. It is significant that deoxygenated blood carries on moving in appropriate direction and not be permitted to flow backward. This event is made probable by valves which are situated inside veins. Valves are like gates which only permit this particular kind of blood to go in one direction.

Capillaries:

Arteries branch through body similar to branches of tree, becoming smaller in diameter as they produce farther away from main vessel. Smallest branches are capillaries. Capillaries are microscopic blood vessels where exchange of significant substances and wastes happens. Capillary walls are just one cell thick. This allows simple exchange of materials between blood and body cells, through procedure of diffusion. Such tubes are so tiny that red blood cells move single-file through the vessels. Diameter of blood vessels changes in response to needs of body. For instance, when you are exercising, muscle capillaries will enlarge, or dilate. This increases blood flow to working muscles that brings more oxygen to cells and eliminates additional wastes from cells.

The Vertebrate Hearts:

The Fish Heart:

Development of gills by fishes needed the more proficient pump, and in fishes we see evolution of true chamber - pump heart. Fish heart is, in essence, the tube with 4 chambers arrayed one after other. First two chambers - sinus venosus and atrium - are collection chambers, whereas second two, ventricle and conusarteriosus, are pumping chambers. The sequence of heartbeat in fishes is peristaltic sequence, starting at rear and moving to front, like early chordate heart. First of four chambers to contract is sinus venosus, followed by atrium, ventricle, and lastly conus arteriosus. In fish, electrical impulse which produces contraction is started in sinus venosus; in other vertebrates, electrical impulse is started by the equivalent of sinus venosus. Fish heart is extraordinarily well suited to gill respiratory apparatus and represents one of the main evolutionary innovations in vertebrates. Maybe its greatest benefit is that blood which moves through gills is completely oxygenated when it moves in tissues. After blood leaves conus arteriosus, it moves through gills, where it becomes oxygenated; from gills, it flows from the network of arteries to rest of body; then it returns to heart through veins. This arrangement has one great restriction, though. In passing by capillaries in gills, blood loses much of pressure developed by contraction of heart, so circulation from gills throughout body is slow. This restrict rate of oxygen delivery to rest of body.

Amphibian Heart:

The growth of lungs by amphibians required the important change in pattern of circulation. After blood is pumped by heart from pulmonary arteries to lungs, it doesn't go directly to tissues of body but is instead returned via pulmonary veins to heart. This causes 2 circulations: one between heart and lungs, known as pulmonary circulation, and one between heart and rest of body, known as systemic circulation. If no changes had happened in structure of heart, oxygenated blood from lungs would be mixed in heart with deoxygenated blood returning from rest of body. Amphibian heart contains two structural characteristics which help reduce the mixing. First, atrium is split in 2 chambers: right atrium gets deoxygenated blood from systemic circulation, and left atrium gets oxygenated blood from lungs. Conus arteriosus is partly separated by the dividing wall that directs deoxygenated blood in pulmonary arteries to lungs and oxygenated blood in aorta, main artery of systemic circulation to body. As there is just 1 ventricle in amphibian heart, division of pulmonary and systemic circulations is partial. Amphibians in water, though, can get extra oxygen by diffusion by their skin. This process, known as cutaneous respiration, assists to supplement oxygenation of blood in the vertebrates.

Mammalian Heart:

Mammalian heart is four-chambered muscular organ pumping through body using blood vessels. It is composed of 2 separate atria and 2 separate ventricles. Right atrium gets deoxygenated blood from body and delivers it to right ventricle that pumps blood to lungs. Left atrium gets oxygenated blood from lungs and delivers it to left ventricle that pumps oxygenated blood to rest of the body. This entirely double circulation is powered by two-cycle pump. Both atria fill with blood and concurrently contract, emptying the blood in ventricles. Both ventricles contract simultaneously, pushing blood simultaneously in pulmonary and systemic circulations. Increased efficiency of double circulatory system in mammals and birds is believed to have been significant in evolution of endothermy, as more proficient circulation is essential to support high metabolic rate required. As generally circulatory system is closed, same volume of blood should move through pulmonary circulation as through much larger systemic circulation with every heartbeat. Left ventricle that pumps blood through higher-resistance systemic pathway is more muscular and produces more pressure than does right ventricle. All through evolutionary history of vertebrate heart, sinus venosus has served as pacemaker, site where impulses which start heartbeat originate. Sinoatrial (SA) node is still site where every heartbeat originates.

Cardiac Cycle:

Cardiac cycle is sequence of events in one heartbeat. It is concurrent contraction of both atria, followed the fraction of second later by simultaneous contraction of both ventricles. Heart comprises of cardiac muscle cells which connect with each other when one contracts other relaxes. It gets its rest between beats. The heartbeat contains two stages:

Phase 1:

Systole is term for contraction. This happens when ventricles contract, closing A-V valves and opening Semi-Lunar valves to pump blood in two main vessels leaving heart.

Phase 2:

Diastole is term for relaxation. This happens when ventricles relax, permitting back pressure of blood to close semi-lunar valves and opening A-V valves. Cardiac muscles of heart contracts repeatedly. Contraction spreads over heart like wave, starting in small region of specialized cells in right atrium known as Sino-Atrial Node (SAN). This is hearts natural pacemaker, and it begins each beat. Impulse spreads from SAN through cardiac muscle of right and left atrium, causing both atria to contract about concurrently. When impulse reaches another special area of heart, right in centre of septum, called as Atrio- Ventricular (or AV) Node, impulse is delayed for about 0.2 s. This permits time for ventricles to fill entirely. AV node relays electrical impulse down septum, along Bundle of His, to base of ventricles. Ventricles then contract concurrently, from bottom upwards, therefore permitting them to empty entirely with every beat. Heartbeat is began by Sino-Atrial Node and passes by Atrio-Ventricular Node, remaining at similar rhythm until nerve impulses cause it to speed up or to slow down.

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