Circulatory system

• 1. Introduction

  ›

  The circulatory system in vertebrates carries blood that never leaves the network of vessels containing it. That single fact, that the blood is held inside a closed loop of arteries, veins, and capillaries, separates the human body from much of the animal kingdom. An arthropod does it differently. Its heart pumps a fluid called hemolymph that returns through the body cavity, bathing the organs directly, with no distinction between blood and the fluid between cells. Sponges and comb jellies, the diploblasts, manage with no circulatory system at all. So how did the human version come to be split into two circuits, four heart chambers, and a vessel network long enough to wrap around the planet? Why does a Persian physician writing in 1025 still matter to how doctors understood the pulse? And what did the ancient Egyptians get right, and wrong, when they imagined air travelling from the heart to every limb? The answers run from a 16th-century BCE papyrus to a connection no anatomist could see until 1661.

• 2. Two Circuits

  ›

  The pulmonary circulation begins at the right heart, sending deoxygenated blood to the lungs to be oxygenated and returned to the left heart. Its partner, the systemic circulation, carries oxygenated blood from the left heart out to the rest of the body, then brings deoxygenated blood back to the right heart through the large veins called the venae cavae. The systemic circuit divides again into a macrocirculation and a microcirculation. An average adult holds five to six quarts of blood, roughly 4.7 to 5.7 liters, about 7% of total body weight. That blood is plasma, red blood cells, white blood cells, and platelets, and it carries oxygen, nutrients, hormones, and gases while collecting waste. Several named routes branch off the main system: the coronary circulation to the heart itself, the cerebral circulation to the brain, the renal circulation to the kidneys, and the bronchial circulation to the lungs' airways. The renal circulation alone receives around 20% of the cardiac output. The digestive system feeds this whole arrangement, supplying the nutrients that keep the heart pumping.

• 3. The Vessels

  ›

  The aorta is a massive, thick-walled artery, the first stretch of the systemic circulation as oxygenated blood leaves the left ventricle. It arches, supplies the upper body, then passes through the aortic opening of the diaphragm at the level of the tenth thoracic vertebra to enter the abdomen. Its walls are elastic, and that elasticity holds blood pressure steady; when the aorta takes in almost five litres of blood from the heart, it recoils and produces the pulsating pressure we feel. As the aorta branches into smaller arteries, their elasticity decreases and their compliance increases. Arteries narrow into arterioles, then into capillaries, where the real exchange happens. The total length of muscle capillaries in a 70 kg human is estimated at between 9,000 and 19,000 km. Capillaries then merge into venules, venules into veins, and the venous system drains into two great vessels: the superior vena cava, draining mostly above the heart, and the inferior vena cava, draining mostly below. Both empty into the right atrium. One vessel breaks the usual pattern. The hepatic portal vein gathers blood from capillaries around the gastrointestinal tract, where it absorbs the products of digestion, then branches into a second capillary system in the liver rather than returning straight to the heart.

• 4. The Lymphatic Partner

  ›

  Excess plasma escapes the circulatory capillaries as interstitial fluid, the liquid sitting between cells, and something has to bring it back. The lymphatic system does that work, returning the excess to blood circulation as lymph through a network of lymphatic vessels, lymph nodes, organs, and tissues. Without it, the blood would become depleted of fluid, which is why the source calls it an essential subsystem rather than an optional extra. Yet it works on different terms from the blood circulation. The lymphatic system is open, not closed, and the circulation of lymph takes much longer than that of blood. Some sources describe it as a secondary circulatory system. It also doubles as a defense network, working with the immune system to guard against pathogens. The lymphatic system is found only in animals that already have a closed blood circulation, which ties its existence directly to the vertebrate body plan.

• 5. From Embryo To Birth

  ›

  Vasculogenesis in the embryo starts the whole system, and the arterial and venous halves arise from different places. The arterial system develops mainly from the aortic arches, six pairs that form on the upper part of the embryo beginning in week 4. The first and second arches regress into just the maxillary and stapedial arteries, the fifth regresses completely, and the system proper comes from arches 3, 4, and 6. The dorsal aortae, running along the back of the embryo on both sides, later fuse to form the basis of the aorta, with about thirty smaller arteries branching off to become intercostal, limb, lumbar, and other arteries. The venous system develops mainly from the vitelline veins, the umbilical veins, and the cardinal veins, all emptying into the sinus venosus, during weeks 4 to 8. Fetal circulation begins within the 8th week and skips the lungs, which are bypassed via the truncus arteriosus. Before birth the fetus draws oxygen and nutrients from the mother through the placenta and the umbilical cord. After birth, the umbilical arteries become the internal iliac arteries, one of several conversions the newborn body must complete.

• 6. When It Fails

  ›

  Atherosclerosis is the precursor to many cardiovascular diseases, building small atheromatous plaques in the walls of medium and large arteries until they grow or rupture and block the vessel. It is a risk factor for acute coronary syndromes, marked by a sudden deficit of oxygenated blood to heart tissue, and it is linked to aneurysm and to the splitting, or dissection, of arteries. Clots tell a second story. A thrombus can form in a vein or artery, and deep venous thrombosis, usually in the legs, often follows long periods of staying still. Such clots may embolise, travelling elsewhere to cause a pulmonary embolus, transient ischaemic attacks, or stroke. Many of these are called lifestyle diseases because they develop over time and track with exercise, diet, and smoking. Others are congenital, like heart defects or persistent fetal circulation, in which the changes meant to happen after birth never do. To find such problems, doctors take a pulse for heart rate, read blood pressure with a sphygmomanometer, and listen for murmurs with a stethoscope. More invasive tools follow: angiography injects dye to map an arterial tree, and during the same procedure blockages can be opened with stents and bleeds plugged with coils. In the United States, only 28% of cardiovascular surgeries were performed in an ambulatory care setting, a sign of how serious these operations remain.

• 7. Hearts Across The Tree Of Life

  ›

  Annelids like earthworms and cephalopods like squids and octopuses share the closed system of vertebrates, keeping blood enclosed in chambers or vessels. But within the vertebrates the heart itself shows stages of evolution. A fish runs a single circuit, its blood pumped through the gill capillaries and on to the body, with a heart that is one pump of two chambers. Amphibians and most reptiles use a double circulatory system, though the heart is not always fully split; amphibians have a three-chambered heart. In reptiles the ventricular septum is incomplete and the pulmonary artery carries a sphincter muscle, which can divert blood away from the lungs and back to the body, a trick useful to cold-blooded animals regulating temperature. Mammals, birds, and crocodilians complete the split into four chambers, and the four-chambered heart of birds and crocodilians is thought to have evolved independently from that of mammals. Other animals dispense with circulation entirely. Flatworms, flattened dorso-ventrally, let nutrients and oxygen diffuse directly to every cell from a branched digestive system, while jellyfish use a gastrovascular cavity that both digests and circulates. The blood vascular system itself probably first appeared in an ancestor of the triploblasts over 600 million years ago, overcoming the time-distance limits of diffusion, while endothelium evolved in an ancestral vertebrate some 540 to 510 million years ago.

• 8. A Closed Loop, Slowly Understood

  ›

  The Ebers Papyrus, an Egyptian medical text of the 16th century BCE with over 700 prescriptions, holds the earliest known writings on the circulatory system and connects the heart to the arteries. The Egyptians thought air entered through the mouth, reached the lungs and heart, then travelled to every member through the arteries, a picture only partially correct. In the 6th century BCE, the Ayurvedic physician Sushruta in ancient India knew of vital fluids moving through the body and described arteries as channels. Plato, in the Timaeus, argued blood circulated by the same rules governing the elements and placed little importance on the heart. Galen, in 2nd-century Rome, believed venous blood formed in the liver and arterial blood in the heart, each consumed at the body's edges with no return, the heart sucking blood in rather than pumping it out. That error survived for centuries. In 1025, Avicenna's Canon of Medicine accepted the false hole in the ventricular septum, yet gave the first correct explanation of pulsation, writing that every beat comprises two movements and two pauses. In 1242, Ibn al-Nafis described pulmonary circulation more accurately than anyone before, insisting the septum had no pores and predicting small communications between the pulmonary artery and vein more than 400 years before capillaries were seen. Michael Servetus, the first European to describe pulmonary circulation, near 1546, buried it in a theological treatise, Christianismi Restitutio, most copies burned in 1553. Realdo Colombo offered a better-known account in 1559. Then William Harvey, a pupil of Hieronymus Fabricius, published his Exercitatio Anatomica de Motu Cordis in 1628, arguing the heartbeat drove a continuous circulation through minute connections at the body's extremities. Harvey never saw those connections. Marcello Malpighi discovered the capillary system joining arteries and veins in 1661, completing the loop that everyone before him had only inferred.