Circulatory system

III. Circulatory System

A. Overview

1. Central role [Fig. 39.2]

- Internal bulk transport system; carries oxygen, nutrients to body cells, carries secretions, metabolic wastes away. Interacts with digestive, respiratory and urinary systems.

2. Three components

- Blood = fluid (connective tissue) composed of water, solutes, cells, "formed elements".
- Heart = muscular organ that pumps blood through animal's body (muscle contraction converted to pressure)
- Vessels = tubes for directing transport of blood

B. Two basic types of circulatory systems [Fig. 39.3]

1. Open circulatory systems

- blood is pumped from a heart (or heart-like organ) into large cavity where organs are "bathed" in hemolymph
- blood and tissue fluid not distinguishable
- Arthropods, most mollusks have open systems

2. Closed circulatory systems

- blood is pumped from the heart and throughout the body via a network of vessels: from arteries to capillaries to veins (Note: flow in capillaries much slower than in large-diameter vessels; allows time for diffusion into and out of tissue fluid.)
- All vertebrates, some mollusks and annelid worms (ex: earthworms) have closed systems

C. Link with lymphatic system

- pressure in closed system continually forces out through capillary walls water, salts and other dissolved substances, and small proteins
- to avoid build-up of fluid in tissue (and loss of blood volume!) lymphatic system picks up excess interstitial fluid and returns it to circulatory system (approx. 3 liters/day)

D. Characteristics of Blood

1. Functions of blood

- carries oxygen and nutrients to body cells
- carries secretions and metabolic wastes away from cells
- contains phagocytic cells that fight infection
- contains chemicals that buffer internal pH
- helps maintain normal body temperature. (birds and mammals; distributes metabolic heat within the body and helps rid body of excess heat)

2. Blood constituents (vol = 5-6 liters in average person)

- Plasma (50% - 60% of total volume)
(composed mostly of water (92%), but also contains proteins, some of which function in clotting. Plasma also contains ions, glucose, lipids, amino acids, vitamins, globulins, albumin, hormones and dissolved gases.)
- Red blood cells (RBCs; erythrocytes) [Fig. 39.6]
(In lower vertebrates, the RBCs are nucleated. In mammals, the RBCs are anucleate. In either case, RBCs contain hemoglobin (an iron-containing protein) that binds with oxygen. RBCs form from stem cells in the bone marrow.)
- White blood cells (leukocytes)
(nucleated, phagocytic cells that remove worn-out RBCs and unwanted biota from the bloodstream. Leukocytes are also formed from stem cells in the bone marrow. There are five types: Lymphocytes (B and T cells of immune system), monocytes (differentiate into macrophages), neutrophils (60% of WBCs), eosinophils, and basophils (allergic responses))
- Platelets
(anucleate fragments of megakayocytes (very large cells located in the bone marrow). Platelets function in blood clotting)

E. Blood circulation in humans (also other mammals and birds)

1. Double Circuit [Fig. 39.7]

- Heart is divided into right and left halves
- Pulmonary circuit: blood is transported from the right side of the heart to the lungs where it is charged with oxygen and gives up carbon dioxide.
- Systemic circuit: blood is transported from the left side of the heart to the body where it releases oxygen and takes up carbon dioxide.

2. Direction of blood flow in either circuit

heart --> arteries --> arterioles --> capillaries --> venules --> veins --> heart

F. Human heart

1. Structure

- consists of two atria (receiving chambers) and two ventricles (pumping chambers). Atria are separated from the ventricles by atrioventricular valves (= AV valves). Semilunar valves found between ventricles and arteries leading away from heart.
- heart has its own "coronary circulation" branching off from aorta and re entering right atrium

2. Cardiac cycle [Fig. 39.10]

- consists of a phase of vetricular contraction (systole) followed by a phase of relaxation (diastole)
- Diastole - as the atria begin to fill, the ventricles relax. Pressure of returning blood forces the AV valves to open and blood fills the ventricles. At the end of diastole the atria contract to "top off" the ventricles.
- Systole - the ventricles contract (the AV valves close with a thump) and the pressure in the ventricles opens the semilunar valves leading into the aorta and pulmonary arteriy. When the semilunar valves close, another "thump" is sounded. Thus, the heart beat: