49 Circulatory Systems: Pumps, Vessels, and Blood In open circulatory systems, the blood or circulating fluid is not kept separate from the tissue fluid. The most simple systems squeeze tissue fluid through and around intercellular spaces. Arthropods, mollusks, and other invertebrates utilize this type of circulatory system.

49 Circulatory Systems: Pumps, Vessels, and Blood A closed circulatory system keeps the blood and tissue fluid separate. One or more muscular hearts and a branching network of vessels (the vascular system) move the blood. There are different types.

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49 Vertebrate Circulatory Systems The four-chambered hearts of birds and mammals completely separate the pulmonary and systemic circuits. The advantages of separate circuits are:  Oxygenated and deoxygenated blood cannot mix.  Gas exchange is maximized because the lungs receive only blood with low O 2 and high CO 2 content.

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Figure 49.3 The Human Heart and Circulation (Part 2)

49 The Human Heart: Two Pumps in One The right atrium receives blood from the superior and inferior vena cavas. From the right atrium, blood goes to the right ventricle. The right ventricle sends blood through the pulmonary artery to the lung. Pulmonary veins return oxygenated blood to the left atrium. From the left atrium, blood goes to the left ventricle. The left ventricle sends blood through the aorta to the body and the capillary beds. Blood returns to the right atrium via veins.

Figure 49.7 The Heartbeat

Figure Anatomy of Blood Vessels (Part 1)

Figure One-Way Flow

49 Blood: A Fluid Tissue Blood is connective tissue: it consists of living cells within an extracellular matrix. The fluid matrix is called plasma. The cellular components of blood are the red blood cells (erythrocytes), the white blood cells (leukocytes), and the platelets (cell fragments).

Figure The Composition of Blood

49 Blood: A Fluid Tissue Most of the cells in blood are erythrocytes. At maturity they are biconcave, flexible discs packed with hemoglobin. The hemoglobin carries O 2, and the flexible shape of the cell lets them squeeze through narrow capillaries.

49 Blood: A Fluid Tissue Bone marrow makes about 2 million red blood cells per second. Each red blood cell lives about 120 days and then breaks down. The spleen serves as a reservoir for old blood cells that have been squeezed and ruptured. The cell remnants are then broken down by macrophages.

49 Blood: A Fluid Tissue Cell damage leads to conversion of an inactive enzyme in the blood, prothrombin, to its active form, thrombin. Thrombin causes a plasma protein, fibrinogen, to polymerize, forming fibrin threads. These threads form a meshwork to seal the damaged vessel and provide a base for scar tissue.

Figure Blood Clotting (Part 2)

49 Blood: A Fluid Tissue Plasma contains gases, ions, nutrients, proteins, hormones, and other chemicals. Nutrient molecules in plasma include glucose, amino acids, lipids, lactic acid, and cholesterol. Circulating proteins include albumin, antibodies, hormones, and carrier molecules.