Chapter 42: Circulation and Gas Exchange

Transcription

1 Chapter 42: Circulation and Gas Exchange Open vs. Closed Circulatory Systems Types of Vertebrate Circulatory Systems Fig Generalized circulatory schemes of vertebrates. Red symbolizes oxygen rich blood, and blue represents oxygen poor blood. (a) Fishes have a twochambered heart and a single circuit of blood flow. (b) Amphibians have a three chambered heart and two circuits of blood flow: pulmocutaneous and systemic. This double circulation delivers blood to systemic organs under high pressure. In the single ventricle, there is some mixing of oxygen rich blood with oxygen poor blood. (It is customary to illustrate cardiovascular systems with the right side of the heart on the left and the left side on the right, as if the body is facing you from the page.) (c) Mammals have a four chambered heart and double circulation. Within the heart, oxygen rich blood is kept completely segregated from oxygen poor blood. A more detailed diagram of blood flow through the mammalian cardiovascular system is shown in FIGURE 42.4, which has numbers keyed to the circled numbers in this text discussion. Beginning our tour with the pulmonary (lung) circuit, the right ventricle pumps blood to the lungs via the pulmonary arteries. As the blood flows through capillary beds in the right and left lungs, it loads oxygen and unloads carbon dioxide. Oxygen rich blood returns from the lungs via the pulmonary veins to the left atrium of the heart. Next, the oxygen rich blood flows into the left ventricle, as the ventricle opens and the atrium contracts. The left ventricle pumps the oxygen rich blood out to body tissues through the systemic circuit. Blood leaves the left ventricle via the aorta, which conveys blood to arteries leading throughout the body. The first branches from the aorta are the coronary arteries (not shown), which supply blood to the heart muscle itself. Then come branches leading to capillary beds in the head and arms (or forelimbs). The aorta continues in a posterior direction, supplying oxygen rich blood to arteries leading to arterioles and capillary beds in the abdominal organs and legs. Within the capillaries, blood gives up much of its oxygen and picks up the carbon dioxide produced by cellular respiration. Capillaries rejoin to form venules, which convey blood to veins. Oxygen poor blood from the head, neck, and forelimbs is channeled into a large vein called the anterior (or superior) vena cava. Another large vein called the posterior (or inferior) vena cava drains blood from the trunk and hind limbs. The two venae cavae empty their blood into the right atrium, from which the oxygen poor blood flows into the right ventricle Mammalian Cardiovascular System

2 42.05 Heart Parts Fig The mammalian heart: a closer look. In this detailed view of the structure of the heart, notice the valves, which prevent backflow of blood within the heart, and the relative thickness of the walls of the heart chambers. The atria, which pump blood only into the ventricles, have thinner walls than the ventricles, which pump blood to the pulmonary and systemic circuits Cardiac Cycle Fig The cardiac cycle. The heart contracts (systole) and relaxes (diastole) in a rhythmic cycle. For an adult human at rest with a pulse of about 75 beats per minute, one complete cardiac cycle takes about 0.8 sec. During a relaxation phase (atria and ventricles in diastole) lasting about 0.4 sec, blood returning from the large veins flows into the atria and ventricles. A brief period (about 0.1 sec) of atrial systole then forces all remaining blood out of the atria into the ventricles. During the remaining 0.3 sec of the cycle, ventricular systole pumps blood into the large arteries. Note that seven eighths of the time all but 0.1 sec of the cardiac cycle the atria are relaxed and filling with blood returning in the veins.

3 42.07 Control of Heart Rhythm Fig The control of heart rhythm. The SA node, or pacemaker, sets the tempo of the heartbeat by generating electrical signals (gold) that spread through both atria, making them contract simultaneously. The signals to contract are delayed at the AV node for about 0.1 sec, during which blood in the atria empties into the ventricles. Specialized muscle fibers called bundle branches and Purkinje fibers then conduct the signals to the apex of the heart and throughout the ventricular walls. The signals trigger powerful contractions of both ventricles from the apex toward the atria, driving blood into the large arteries. Gold color in the graphs at the bottom indicates the components of an electrocardiogram (ECG) corresponding to the sequence of electrical events in the heart. In step 4, the black portion of the ECG to the right of the gold "spike" represents electrical activity after the ventricles contract; during this phase of the ECG, the ventricles become re primed electrically and thus are able to conduct the next round of contraction signals Blood Vessel Structures

4 42.10 Blood Flow Dynamics Fig The interrelationship of blood flow velocity, cross sectional area of blood vessels, and blood pressure. Blood flow velocity decreases markedly in the arterioles and is slowest in the capillaries, owing to an increase in total cross sectional area. Blood pressure, the main force driving blood from the heart to the capillaries, is highest in the arteries. Peaks in blood pressure corresponding to ventricular systole alternate with lower blood pressures corresponding to diastole (elasticity in arterial walls keeps diastolic pressure from falling to zero). Resistance to flow through the arterioles and capillaries, due to contact of the blood with a greater surface area of endothelium, reduces blood pressure and eliminates pressure peaks How to Measure Blood Pressure

5 42.12 Blood Flow thru Capillary Bed Capillary Exchange of Fluids Components of Mammalian Blood

6 42.16 Blood Clotting Countercurrent Exchange Fig Countercurrent exchange. The arrangement of blood vessels in fish gills is an adaptation that maximizes oxygen transfer from the water to the blood. The countercurrent flow of water and blood maintains a concentration gradient down which O2 diffuses from the water into the blood over the whole length of a capillary. As the blood flows through a lamellar capillary, it becomes more and more loaded with oxygen because it continuously passes by water that is more and more concentrated in O2.

7 42.23ab Mammalian Respiratory System Fig The mammalian respiratory system. (a) From the nasal cavity and pharynx, inhaled air passes through the larynx and down the trachea and bronchi to the tiniest bronchioles, which end in lobed, microscopic air sacs called alveoli. (b) A thin, moist epithelium lining the inner surfaces of the alveoli is the respiratory surface. Branches of the pulmonary artery (see FIGURE 42.4) convey oxygen poor blood to the alveoli, while branches of the pulmonary vein transport oxygen rich blood from the alveoli back to the heart Negative Pressure Breathing Fig Negative pressure breathing. A mammal breathes by changing the air pressure within its lungs relative to the pressure of the outside atmosphere. During inhalation, the rib muscles and diaphragm contract. The volume of the thoracic cavity and lungs increases as the diaphragm moves down and the rib cage expands. Air pressure in the lungs falls below that of the atmosphere, and air rushes into the lungs. Exhalation occurs when the rib muscles and diaphragm relax, restoring the thoracic cavity to its smaller volume.

8 42.26 Automatic Breathing Control Respiratory Gas Exchange Fig Loading and unloading of respiratory gases. The colored bars indicate the partial pressures of O 2 (P O2 ) and CO 2 (P CO2 ) (in mm Hg) in different locations. At each location, the respiratory gases diffuse down their partial pressure gradients.

9 42.28 Hemoglobin Dissociation Curve Blood and CO2 Transport