Lecture 9: Circulation

1. Some invertebrates have no circulatory system. Small invertebrates have a gastrovascular cavity that functions in circulation as well as digestion. Diffusion through the body of flatworms allows for gas exchange. Fluids in a pseudocoelom act in circulation.

2. Many invertebrates have an open circulatory system. Blood and interstitial fluid intermingle and are called hemolymph. In the molluskan open circulatory system, the heart has two atria (receive blood)and one ventricle (pumps out). Hemolymph is pumped from the gills to the atria, then to theventricle. Hemolymph in the ventricle is pumped to sinuses collectively called the hemocoel, then back to the gills. In arthropods, a tubular heart pumps hemolymph into vessels which connect to the sinuses of the hemocoel. In insects, the circulatory system has a minimal respiratory function

3. Some invertebrates have a closed circulatory system. The proboscis worms have a primitive closed circulatory system, but lack a heart. Pumping is sluggish and is based on movement of the worm, and contractile vessels. Annelids have a complex closed circulatory system. The dorsal blood vessel carries blood anteriorly, the ventral vessel carries blood posteriorly. Both vessels are connected by lateral vessels in each segment. Five contractile vessels ("hearts") pump blood. Earthworms have hemoglobin dissolved in the plasma. Cephalopods have a closed circulatory system, including accessory "hearts" at the bases of the gills. In closed systems, arteries carry blood away from the hearts and veins return blood to the heart. Tiny vessels called capillaries convey blood between these two systems.

4. The closed circulatory system of vertebrates is adapted for several functions. All vertebrates have a ventral muscular heart pumping blood in a system of closed vessels. The functions of the vertebrate circulatory system includes:

1. Transportation of nutrients

2. Transportation of respiratory gases

3. Transportation of wastes

4. Transportation of hormones

5. Maintenance of fluid balance

6. Internal defense

7. Thermoregulation

5. Vertebrates have three main types of blood vessels. Arteries carry blood away from the heart, and typically carry oxygenated blood. They are characterized by branching, forming smaller arterioles. Arterioles, in turn, ultimately form microscopic capillaries. Capillaries merge to form venules, and then veins. The thick walls of arteries and veins prevent passage of materials; the walls of capillaries are so thin that materials pass relatively easily. Smooth muscle in the walls of arteries results in vasoconstriction or vasodilation which maintain appropriate blood pressure and blood supplies. Veins contain blood under pressure, but they may have valves to prevent backflow of blood. They are under less direct pressure from the heart, and there are a number of factors including the process of breathing, skeletal muscles and valves which push blood along the venous system.

6. The human heart is well adapted for pumping blood. The heart, beating over 2 billion times in an average lifetime, can vary its output from 5 to more than 20 L of blood. The heart is surrounded by the pericardium, composed of dense connective tissue. The wall of the heart is composed of cardiac muscle fibers and collagen fibers.

Valves prevent backflow of blood. Atrioventricular valves are located between the atria and the ventricles. The right AV valve is called the tricuspid valve as it has three cusps or flaps. The left AV valve is called the bicuspid or mitral valve. AV valves are closed during ventricular contraction. Semilunar valves are found between the ventricles and the great vessels of the heart. The pulmonary semilunar valve is located between the pulmonary artery and the right ventricle. The aortic valve is located between the aorta and the left ventricle. Congenital valve deformities, or deformities resulting from rheumatic fever or syphilis may be replaced with artificial valves

Each heartbeat is initiated by a natural pacemaker, a special area of heart muscle.The SA (sinoatrial) node is the pacemaker, located in the wall of the right atrium. When the SA node fibers fire, an electrical signal is spread across both atria simultaneously, via the gap junctions in the intercalated discs between cells. The signals last longer in cardiac muscle than in skeletal muscle cells. The AV (atrioventricular) node, located in the lower right atrium, acts as a delay and relay node. After a slight delay, transmission continues into the Purkinje fibers, which make up the atrioventricular (AV) bundle. The AV bundle splits, sending branches upwards over both ventricles, resulting in ventricular contraction. The cardiac cycle lasts about 0.8 sec; consisting of atrial systole(pump) and ventricular diastole(relax)- then atrial diastole(relax) and ventricular systole(pump). Two main heart sounds can be distinguished. The "lub-dup" heart sounds mark the beginning of ventricular systole, then ventricular diastole. A heart murmur is caused by valves which do not close properly and results in hissing heart sounds

7. Cardiac output varies with the body's need. The amount of blood pumped by one ventricle in one beat is the stroke volume. The stroke volume x the number of ventricular contractions per minute = the cardiac output (about 5.25 L per minute.) Changes in cardiac output are based on changes in either stroke volume or heart rate (or both). Heart rate is regulated by the nervous system.

Blood pressure depends on blood flow and resistance to blood flow. When cardiac output increases, blood pressure increases. Peripheral resistance is the resistance to blood flow by the viscosity of the blood itself, and the friction between the blood and the walls of the vessels. Blood pressure varies during ventricular systole and diastole, varying from 120 to 80 mm Hg. Blood pressure is expressed as a fraction, with systolic as the numerator, and diastolic as the denominator. When diastolic pressure is typically over 95 mm Hg, hypertension may be problematic.

Blood pressure is highest in arteries due to ventricular systole and smaller diameters than veins. Because pressure is lower in veins, most large veins have valves to prevent backflow. Blood pressure is carefully regulated

8. Blood is pumped through pulmonary and systemic circuits. The pulmonary circulation oxygenates the blood. Blood is pumped to the lungs by the right side of the heart. The pulmonary trunk carries blood from the right ventricle, and then splits inot two arteries which lead to each lung. Respiratory gases diffuse into capillaries in the alveoli of the lungs. The pulmonary veins carry blood back to the left atria, carrying oxygenated blood.

The systemic circulation delivers blood to the tissues. Blood leaves the left ventricle and flows into the aorta. Most of the blood supplies the tissues of the body. The carotid arteries supply the brain. The subclavian arteries supply the upper appendages. The mesenteric arteries supply the intestines. The renal arteries supply the kidneys. The iliac arteries supply the lower appendages

Veins return blood to the right side of the heart. Jugular veins return blood from the brain. The subclavian veins return blood from the upper appendages. Renal veins return blood from the kidneys. Iliac veins return blood from the lower appendages. Hepatic veins return blood from the liver. Renal, iliac, and hepatic veins empty into the inferior vena cava. Jugular and subclavian veins empty into the superior vena cava.

The coronary circulation delivers blood to the heart. A branch of the aorta supplies blood to the wall of the heart. Coronary arteries branch into coronary capillaries, where nutrient exchange takes place. Coronary veins form the coronary sinus, which empties into the right atrium. Blockage of the coronary arteries leads to a myocardial infarction, colloquially known as a heart attack

9. The lymphatic system is an accessory circulatory system. The functions of the lymphatic system include return of interstitial fluid to the circulatory system, immunity, and absorption of lipids from the gastrointestinal tract. The lymphatic system consists of lymphatic vessels and lymph tissue. Lymph vessels conduct lymph, derived from interstitial fluid. Lymph tissue is an organ composed of connective tissue with many lymphocytes. Lymph nodes and nodules are small masses of lymph tissue. Lymph nodes function to filter the lymph

The spleen, tonsils, and thymus are larger organs. Tonsils are masses of lymph tissue in the pharyngeal region which filter out pathogens. Lymph capillaries are one-way vessels, which join and merge to form larger lymphatics (= lymph veins) into the subclavian veins. The lymphatic system plays an important role in fluid homeostasis.

10. Vertebrate blood consists of plasma, blood cells, and platelets. The human circulating blood volume is about 5.6 L. Approximately 55% of blood volume is plasma. Approximately 45%of blood volume is cells and platelets.

Plasma is the fluid component of blood. Plasma is mostly water, with dissolved proteins, salts, gases, nutrients, wastes, and hormones. Composition of the plasma varies with location of the body, as material fluxes change. Plasma proteins have a variety of functions. Fibrinogen is involved in clotting. When clotting proteins have been removed from plasma, it is called serum

There are three types of globulins- alpha, beta, and gamma. Alpha globulins include some hormones and proteins that transport hormones. Beta globulins include some lipoproteins, as well as proteins that transport some vitamins and minerals. High density lipoproteins transport triglycerides and cholesterol. The gamma globulins include many antibodies. Albumins are important in maintaining the appropriate osmotic balance of the blood.

Red blood cells transport oxygen. Erythrocytes are the most abundant of the cellular elements. Erythrocytes are flexible, biconcave discs which are packed with hemoglobin. As they lack most cellular organelles, their lifespan is only about 120 days. Erythrocytes are produced in the red bone marrow in vertebrae, the sternum, ribs, and some skull and long bones. Old erythrocytes are removed by the spleen and liver, and various components are recycled. Anemia is a deficiency of hemoglobin, or the number of RBCS, or both. Anemia may be due to bleeding, decreased production of hemoglobin or RBCs due to lack of iron, or destruction of RBCs (hemolytic anemias such as sickle cell anemia)

White blood cells defend the body against disease organisms. Leukocytes (WBCS) function in immunity, and may be classified by the presence or absence of conspicuous cytoplasmic granules. Granular leukocytes are manufactured in bone marrow, have lobed nuclei and granules visible under the light microscope. Neutrophils are the most abundant WBCS, and are primarily phagocytic. The cytoplasmic granules contain digestive enzymes. Eosinophils stain red in an eosin dye and function during allergic reactions and parasitic worm infections. Basophils stain in basic dyes, and are also involved in allergic reactions and produce histamines and heparin. The agranular leukocytes do not have conspicuous granules, and their nuclei are round or kidney-shaped. Lymphocytes produce antibodies. Monocytes are the largest WBCS, but typically stay within the circulatory system for a very short time. Monocytes leave the circulatory system and complete development within connective tissue, turning into a macrophage, a type of phagocyte.

Platelets function in blood clotting. Thrombocytes in most vertebrates have nuclei; in mammals, the thrombocytes are small cell fragments lacking nuclei. Platelets form a temporary clot, or platelet plug at the site of a cut. Next, a complex process involving a series of reactions forms a more permanent clot of thrombin and fibrin fibers.