Circulation and Gas Exchange Chapter 42 Circulation and Gas Exchange

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

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

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

LE 42-7 Atrial systole; ventricular Semilunar diastole valves closed 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 sphincters control flow of blood between arterioles and venules

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

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

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

LE 42-14 Tissue cell INTERSTITIAL FLUID Net fluid movement out Net fluid movement in Capillary 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

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