Understanding the Mammalian Heart and Circulation

1. Chapter 42 Circulation and Gas Exchange

2. The Mammalian Heart: A Closer Look • A closer look at the mammalian heart provides a better understanding of double circulation

3. Pulmonary artery Aorta Anterior vena cava Pulmonary artery Right atrium Left atrium LE 42-6 Pulmonary veins Pulmonary veins Semilunar valve Semilunar valve Atrioventricular valve Atrioventricular valve Posterior vena cava Left ventricle Right ventricle

4. The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle • The contraction, or pumping, phase is called systole • The relaxation, or filling, phase is called diastole

5. Atrial systole; ventricular diastole Semilunar valves closed LE 42-7 0.1 sec Semilunar valves open AV valves open 0.3 sec 0.4 sec Atrial and ventricular diastole AV valves closed Ventricular systole; atrial diastole

6. The heart rate, also called the pulse, is the number of beats per minute • The cardiac output is the volume of blood pumped into the systemic circulation per minute

7. Maintaining the Heart’s Rhythmic Beat • Some cardiac muscle cells are self-excitable, meaning they contract without any signal from the nervous system

8. The sinoatrial (SA) node, or pacemaker, sets the rate and timing at which cardiac muscle cells contract • Impulses from the SA node travel to the atrioventricular (AV) node • At the AV node, the impulses are delayed and then travel to the bundle of His then to the Purkinje fibers that make the ventricles contract

11. Impulses that travel during the cardiac cycle can be recorded as an electrocardiogram (ECG or EKG) Signals pass to heart apex. Signals are delayed at AV node. Pacemaker generates wave of signals to contract. Signals spread throughout ventricles. SA node (pacemaker) AV node Bundle branches Purkinje fibers Heart apex ECG

12. The pacemaker is influenced by nerves, hormones, body temperature, and exercise

13. Concept 42.3: Physical principles govern blood circulation • The physical principles that govern movement of water in plumbing systems also influence the functioning of animal circulatory systems

14. Blood Vessel Structure and Function • The “infrastructure” of the circulatory system is its network of blood vessels • All blood vessels are built of similar tissues and have three similar layers

15. Vein Artery 100 µm Endothelium LE 42-9 Valve Basement membrane Endothelium Endothelium Smooth muscle Smooth muscle Capillary Connective tissue Connective tissue Vein Artery Venule Arteriole

16. Structural differences in arteries, veins, and capillaries correlate with functions • Arteries have thicker walls that accommodate the high pressure of blood pumped from the heart

17. In the thinner-walled veins, blood flows back to the heart mainly as a result of muscle action

18. Direction of blood flow in vein (toward heart) Valve (open) LE 42-10 Skeletal muscle Valve (closed)

19. Blood Flow Velocity • Physical laws governing movement of fluids through pipes affect blood flow and blood pressure • Velocity of blood flow is slowest in the capillary beds, as a result of the high resistance and large total cross-sectional area

20. 5,000 4,000 3,000 Area (cm2) 2,000 1,000 0 50 40 Velocity (cm/sec) 30 LE 42-11 20 10 0 120 Systolic pressure 100 80 Pressure (mm Hg) 60 Diastolic pressure 40 20 0 Venae cavae Aorta Capillaries Venules Veins Arterioles Arteries

21. Blood Pressure • Blood pressure is the hydrostatic pressure that blood exerts against the wall of a vessel • Systolic pressure is the pressure in the arteries during ventricular systole; it is the highest pressure in the arteries • Diastolic pressure is the pressure in the arteries during diastole; it is lower than systolic pressure • Blood pressure is determined by cardiac output and peripheral resistance due to constriction of arterioles

22. Blood pressure reading: 120/70 LE 42-12_4 Pressure in cuff below 120 Pressure in cuff below 70 Pressure in cuff above 120 Rubber cuff inflated with air 120 120 70 Sounds audible in stethoscope Sounds stop Artery closed Artery

23. Capillary Function • Capillaries in major organs are usually filled to capacity • Blood supply varies in many other sites

24. Two mechanisms regulate distribution of blood in capillary beds: • Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel • Precapillary sphincterscontrol flow of blood between arterioles and venules

25. Thoroughfare channel Precapillary sphincters LE 42-13ab Venule Arteriole Capillaries Sphincters relaxed Venule Arteriole Sphincters contracted

26. LE 42-13c Capillaries and larger vessels (SEM) 20 µm

27. The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries • The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end

28. Tissue cell INTERSTITIAL FLUID Net fluid movement out Net fluid movement in Capillary LE 42-14 Capillary Red blood cell 15 µm Direction of blood flow Blood pressure Osmotic pressure Inward flow Pressure Outward flow Arterial end of capillary Venous end

29. Fluid Return by the Lymphatic System • The lymphatic system returns fluid to the body from the capillary beds • This system aids in body defense • Fluid reenters the circulation directly at the venous end of the capillary bed and indirectly through the lymphatic system