• In open circulatory systems, blood is pumped into vessels but leaves them to percolate through spaces called hemocoels. Blood returns to the heart to be drawn up for another circuit.

• The closed circulatory systems, the blood elements remain within vessels.
• The gill breathing crustacean uses its circulatory system to transport oxygen and carbon dioxide, and accordingly the system is more complex than in the insect where it does not have a respiratory function.

Closed Circulatory System

• Annelids have closed circulatory systems, distinct vessels, and five pairs of tubular aortic arches (hearts) with one way valves.

1    Anus     2    Dorso-subneural vessel        4    Dorsal vessel     5    Hearts (aortic arches)
7    Ventral vessel    8    Subneural vessel

Fishes and the Two Chambered Heart

• The fish heart consists of one atrium (auricle) and one ventricle. Blood entering the atrium is first collected in a large vein, the sinus venous, and blood leaving the ventricle enters an enlarge artery, the conus arteriosus.
• Blood leaving the fish heart enters the ventral aorta, which directs it to the gills, where gas exchange occurs. The blood then passes through the head and body. All blood returning to the heart has traveled through at least two capillary beds. Most blood is returned to the heart via the common cardinal veins.

Amphibians, Reptiles, and the Three Chambered Heart

• Terrestrial vertebrates have a second atrium and lungs and back. In both amphibians and reptiles, some mixing of oxygenated and deoxygenated blood occurs in the single ventricle.
• In the frog's circulation, deoxygenated blood from the body entered the sinus venosis, then the right atrium and the single ventricle. At the same time, oxygenated blood from  the lungs and skin enters the left atrium and then the ventricle. Partial separation is provided by flaps and partial valves in the heart and the conus arteriosus, through which blood passes on its way back to the lungs, skin, and body.

• Reptiles have only a partial septum within the single ventricle, so mixing of oxygenated and deoxygenated blood occurs in these animals

The Four Chambered Heart

• The four chambered heart of crocodiles, birds, and mammals includes a right and left atrium and a right and left ventricle, but the conus arteriosus and sinus venosus are absent. The pulmonary circuit is completely separated.

THE HUMAN CIRCULATORY SYSTEM

• The human circulatory system consists of the four chambered heart, arteries, capillaries and veins.

• The muscular arteries carry blood away from the hear, and except for the pulmonary arteries, this blood is oxygenated.

• All exchanges occur in the thin walled capillaries.

• Blood returns to the heart through the veins.

• While arteries, tend to be thick walled and muscular, veins tend to be larger, to have thinner walls, and to contain less smooth muscle.

Circulation Through the Heart

• Deoxygenated blood from the body enters the right atrium from the superior vena cava and inferior vena cava. From the right atrium, blood enters the muscular right ventricle, which pumps it into the lungs for gas exchange.  Backflow into the right atrium is prevented by the tricuspid valve, a one way valve whose thin flaps are held in place by chordae tendineae.  Backflow from the pulmonary artery to the right ventricle is prevented by the pulmonary semilunar valve.  Deoxygenated blood from the pulmonary artery enters the capillaries of the lung, where its gases are exchanged.
• Oxygenated blood returns from the lungs via the pulmonary veins, enters the left atrium, and moves to the thick muscled left ventricle, which pumps it into the aorta.  Backflow into the left atrium is prevented by the bicuspid valve (mitral valve), while backflow into the left ventricle is prevented by the aortic semilunar valve.

Control of the Heart

• While heart muscle has an inherent contractile nature, control of its rate and effort is both extrinsic (external) and intrinsic (internal).  Extrinsic control is through the autonomic nervous system, which accelerates heartbeat via sympathetic nerves and slows it via parasympathetic nerves.  Epinephrine from the adrenal glands also accelerates the heart.
• Intrinsic control originates in the sinoatrial node (SA node), or pacemaker, which sends contractile impulses across the atrial walls, causing contraction there.  The impulse reaches the atrioventricular node (AV node) and is relayed to the bundle of His in the ventricular septum.  The bundle's two branches pass to the base of the ventricles and up their outer walls, giving rise to many branched purkinje fibers.

The Working Heart

• The lub of the lub dup heart sound is that of the tricuspid and bicuspid valves shutting, while the dup is that of the shutting of the aortic and pulmonary valves.  The period of ventricular contraction is systole while the period between contractions is diastole.
• The amount of blood per contraction is the stroke volume, while the cardiac output is the rate output per minute.
• Elasticity of the major arteries maintains blood pressure during diastole.  Clinically, blood pressure is measured with a sphygmomanometer and is recorded as systole over diastolic pressure (for example, 120/80 mm Hg).
• Problems in maintaining pressure arise in arteriosclerosis (loss of elasticity) and atherosclerosis (plaque formation), arterial wall diseases that increase resistance, burdening the heart.
• Arterioles are capable of vasodilation (opening) and vasoconstriction (closing), shunting blood where need is greatest.

Circuits in the Human Circulatory System

• Circuits are circulatory pathways where some special function is performed by the blood.  They include the following:
• pulmonary circuit (gas exchange);
• hepatic portal circuit (food carried from the gut to the liver for storage and distribution via the portal vein, which forms sinusoids in the liver);
• renal circuit (renal arteries and veins bring blood to and from the kidneys where wastes, excess water, and other substances are removed from the blood);
• cardiac circuit (blood supply to heart muscle includes coronary arteries and coronary veins. Arteries from netlike anastomoses, which help limit the effect of coronary thrombosis. The coronary veins empty into the right atrium via the coronary sinus);
• systematic circuit (a catch-all for the rest of the body).

The Role of the Capillaries

• All transport functions are carried out by the capillaries, which are composed of a single thickness of interlocking cells. Blood is directed into or away from capillary beds by smooth muscle precapillary sphincters in arterioles.
• The functioning of capillaries depends on diffusion gradients , hydrostatic pressure, and active transport (commonly pinocytosis).
• Hydrostatic pressure and to a lesser amount, diffusion gradients, account for loses of water, ions, and nutrients at the arteriolar end of a capillary bed.
• Steep osmotic and diffusion gradients bring water and ions back into the capillaries at the venule end.
• The exchange of oxygen and carbon dioxide follows the diffusion gradient, and water not reclaimed is recycled by the lymphatic system..

Work of the Veins

• Blood pressure is lowest in the veins, but although force is reduced, volume in the veins must nearly equal that of the arteries. The onward movement of venous blood is assisted by one-way valves, muscular squeezing, and breathing movements.

The Blood

• Blood (a connective tissue) consists of cells (mainly erythrocytes, but also leukocytes and platelets) and plasma ( a watery matrix with proteins, nutrients, ions, hormones and wastes).
• Plasma proteins include albumins, which aid in transport, globulins, which include antibodies or immunoglobins, and fibrinogen, which functions in blood clotting.

• Red blood cells are constantly replaced by production in the red bone marrow.  Aging cells are phagocytized by macrophages in the spleen and liver.  Hemoglobin is converted to bilirubin, which joins the bile.  Its buildup in the blood produces jaundice (yellow coloring).

• White blood cells include neutrophils, basophils, eosinophils, lymphocytes, monocytes, and macrophages, each of which plays a role in the immune response, which include recognition, phagocytosis, inflammation, and antibody formation.

• Platelets, or thrombocytes, are cellular fragments formed from platelet mother cells (megakaryocytes).

• All blood cells originate from one cell type, the hemocytoblast or stem cell.
• Clotting is quite complex with some 15 steps.  Following damage to a vessel, platelets gather at the wound, form a collagen plug, and release vasoconstrictors and enzymes called thromboplastins.  The latter converts prothrombin to active thrombin.  thrombin cleaves fribrinogen, forming fibrin, which becomes the clot.

The Lymphatic System

• The lymphatic system maintains fluid and ion balances, transports lipids, and cooperattes with the immune system.
• Lymphatic structures include lymph vessels and lymph nodes.  The vessels begin as lymph capillaries, which lead to collecting ducts and then the larger lymphatics.  Fluids move through the squeezing action of muscles and by br3eathing movements.
• Lymph nodes are the sites where disease organisms and cancer cells are destroyed by lymphocytes and other cells of the immune system.  Cancer commonly spreads via the lymphatic system.