The human cardiovascular system comprises the blood, the heart, and a dual-circuit system of blood vessls that serve as conduits between the heart, the lungs, and the peripheral tissues of the body. The respiratory system and renal system are intricately interrelated to cardiovascular function; it is generally held that these systems must be understood in relation to each other, if they are to be understood at all.The blood consists of a yellow fluid called 'plasma,' in which are suspended erythrocytes leukocytes . Blood is the medium by which oxygen and nutrients are delivered to the peripheral tissues of the body, and by which carbon dioxide and other metabolic wastes are removed. Furthermore, the blood is a chief delivery mechanism for the endocrine and immune systems. The blood also serves as a chemical buffer, maintaining the body's overall pH within a desirable range.
The heart is the muscular organ which pumps the blood via its inherent contractile activity. The heart can be viewed as two separate pumps--the right-sided pump serving the pulmonary circulation and the left-sided pump serving the systemic circulation Simply stated, each side of the heart has a receiving portion ad a pumping portionThe vascular system is made up of arteries, veis, and capillaries. Arteries are blood vessels that carry blood away from the heart. Veins are blood vessels that return blood o the heart. Capillaries are the smallest blood vessels, and are the locus of nutrient and gas exchange between the blood plasma and the peripheral tissues of the The cardiac cycle and cardiovascular circuitry
The cardiac cycle is classically divided into seven discrete phases, but will be radically simplified here. For a more detailed description, please refer to the cardiac cycle.

The cardiac cycle consists of alternating periods of relaxed filling and active ejection of blood. These actions are attributable to the inherent contractile activity of the heart which is, after all, made of specialized muscle fibers. Cardiac contractio is coordinated by a specialized pacemaker-and-conduction system see te heart for more details. During the diastolic phase of the cardiac cycle, the ventriles are filling with blood. During the systolic phase, the rightand left ventricles are contacting, forcing their collecte blood into the pulmonary and systemic circuits, respectively. Although it is a gross oversimplification, the atria can be hought of acontinually filling.The pulmonary and systemic circuits are routes-in-sequence. Blood is pumped from the right heart to the lungs, returns to the left heart, then is pumped to the periphery, then returns to the right heart to eginthe sequence anew. This enables a perpetual cycle of oxgenation, systemic delivery, metabolic deoxygenation, nd return.he primary purpose of the pulmonary circulation is to oxygenate the blood. Deoxygenated blood returning from the periphral tissues of the body collects in the right atrium of the heart, fills the right ventricle during diastole, and iejected into the pulmonary arteries duing systole. These arteries carry the blood to the lungs, where it passes through a capillary network close to air-filled alveoli. This enables the release of carbon dioxide and the uptake of oxygen from the air.

The now oxygenated blood returns to the left atrium via the pulmonary vein, fills the left ventricle during diastole, and is ejected into the aorta, the major artery which suplies blood to the body via its numerous brnches. This is the starting point for the systemic circulation, which consists of the intricate network of arteries, arterioles, capillary beds, venules, and veins that service the peripheral tissues of the body whih include the brain and other organs, the skeletal muscles, etc.The major functions of the renal system are to filter metabolic wastes from the blood passing through the peripheral circulation and to mediate systemic blood pressure via the renin-angiotensin-II-aldosterone sysem. Compromise of renal function, as occurs in widely-varying conditions, inherently disposes the patient to toxic states and/or cardiovascular pathologies.

The circulatory system is often subdivided into various functional circuitsSplanchnic circulation
Also called visceral circulation, the splanchnic circulation is the part of the systemic circulation that supplies the digestive organs. The major arteries of the splanchnic circulation branch directly off the aorta and include the celiac artery superior mesenteric artery,
Portal circulationThere are two exceptions to the system of double circulation.The deoxygenated blood from the capillaries of the gastrointestinal tract drains into the portal vein which, instead o going directly back to the heart, leads to th liver. This allows the liver to take up the nutrients that were extracted by the intestines from food. The liver also neutralizes some toxins taken up by the intestines. Blood from the liver drains via the hepatic veins into the inferior vena cava and then the right side of the heart.There is also a small portal flow from the hypothalamus to the anterior pituitary gla
Fetal circulationThe circulatory system of the fetus is different, as the fetus does not use its anatomical changes, including closure of the ductus arteriosus and foramen ovale. See also: coronary circulation.

mechanism used to transfer some component of a fluid from one flowing current of fluid to another across a permeable barrier between them. It is used extensively in biological systems for a wide variety of purposes (it is also a key concept in chemical engineering thermodynamics). For example, fish use countercurrent exchange in their gills to transfer oxygen from the surrounding water into their blood, and birds use a countercurrent heat exchanger between blood vessels in their legs to keep heat concentrated within their bodies.

Countercurrent exchangeThe diagram to the right presents a generic representation of a countercurrent exchange system, with two parallel tubes containing fluid separated by a permeable barrier. The substance to be exchanged, whose concentration is represented by the darkness of the orange shading, transfers across the barrier in the direction from greater concentration to lesser concentration. With the two flos moving in opposite directions, the countercurrent exchage system is able to maintain a constant cocentration gradient between the two flows over their entire length, and can result in almost all of the substance being transferred.Concurrent exchangeContrasting with the countercurrent exchange system is the concurrent exchange system, in which the two fluid flows are in the same direction. As the diagram to the left shows, a concurrent exchange system is only capable of moving half of the substance across from one flow to the other, no matter how long the exchanger is.