Lecture 37 Introduction to Circulation BY DR QAZI IMTIAZ RASOOL

OBJECTIVES Identify the two divisions of circulation and track the pathway of the blood through both circulations. Outline the various parts of circulation and identify the differences in structure, function, pressure and velocity of blood in different vessels. Describe the physiological anatomy of the heart and identify its various specialized functional parts.

Functions of the Heart Generating blood pressure Routing blood: separates pulmonary and systemic circulations Ensuring one-way blood flow: valves Regulating blood supply 1.Changes in contraction rate and force match blood delivery to changing metabolic needs

Circulatory System Function Move circulatory fluid (blood) around body Gas Transport Nutrient Transport Excretory Product Transport Cell Signal Transport Distribute secretions of endocrine glands, Production/Synthesis Hydraulic Force Heat Conductance Immunity

Overview of the Cardiovascular System Heart- circulates blood through vessels Vascular System /Blood vessels Arteries- away from heart Veins- towards heart Capillaries- location of internal respiration, are tiny, thin-walled blood vessels that connect arteries to veins and are located in all body tissues. - in diameter that blood cells pass through in a single file. 3. Blood- transport medium

Path of Blood Pulmonary Circuit Blood flow between the lungs and heart Supplied by the Right side of the heart Systemic Circuit Blood flow between the rest of the body and heart Supplied by the Left side of the heart

Pulmonary circulation s the part of the circulatory system that takes the blood from the heart to the lungs, where it is oxygenated, and returns it to the heart.

Systemic circulation /greater circulation / peripheral circulation. s the part of the circulatory system that takes the blood from the heart to the lungs, where it is oxygenated, and returns it to the heart.

Less in capillaries

Venous return is aided by both structural modifications and functional adaptations. 1. Structural -Large lumen -Valves - present mostly in extremities, none in ventral body cavity 2. Functional -Respiratory Pump -Muscular Pump -Smooth muscle layer under sympathetic control Venous blood pressure changes very little during the cardiac cycle, and is low, reflecting cumulative effects of peripheral resistance (17 mm Hg in venules dropping to almost 0 mm Hg at the termini of the venae cavae).

Physiological- anatomy Structure of Blood Vessel Walls except the smallest consist of three layers: 1.Tunica intima reduces friction between the vessel walls and blood; 2. Tunica media controls vasoconstriction and vasodilation of the vessel; 3. Tunica externa protects, reinforces, and anchors the vessel to surrounding structures Capillaries are tiny, thin-walled blood vessels that connect arteries to veins and are located in all body tissues. Capillaries are so small in diameter that blood cells pass through in a single file.

A cuff of smooth muscle, called a precapillary sphincter, surrounds each capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary

1.Blood flow is the volume of blood flowing through a vessel, organ, or the entire circulation ml/min (controlled in relation to the tissue need) 2. Blood pressure is the force per unit area exerted by the blood against a vessel wall (mm Hg). (by the tension at the end of the arterioles) 3. Resistance is friction between blood and the vessel wall,. Blood flow is the volume of blood flowing through a vessel, organ, or the entire circulation in a given period and may be expressed as ml/min (blood flow of the entire circulation is equal to cardiac output). Blood pressure is the force per unit area exerted by the blood against a vessel wall and is expressed in millimeters of mercury (mm Hg). Resistance is a measure of the friction between blood and the vessel wall, and arises from three sources: blood viscosity, blood vessel length, and blood vessel diameter.  The variable with the greatest effect on resistance is the diameter (or radius, 1/2 the diameter) of a particular vessel - resistance drops exponentially as the radius increases. TPR is total peripheral resistance - resistance throughout the entire systemic circulation. Relationship Between Flow, Pressure, and Resistance If blood pressure increases, blood flow increases; if peripheral resistance increases, blood flow decreases. Peripheral resistance is the most important factor influencing local blood flow, because vasoconstriction or vasodilation can dramatically alter local resistance while systemic blood pressure remains unchanged (due to homeostatic mechanisms that maintain systemic BP at a constant value Thus, under resting conditions, the velocity averages about 33 cm/sec in the aorta but only 1/1000 as rapidly in the capillaries, about 0.3 mm/sec. However, because the capillaries have a typical length of only 0.3 to 1 millimeter, the blood remains in the capillaries for only 1 to 3 seconds. This short time is surprising because all diffusion of nutrient food substances and electrolytes that occurs through the capillary walls must do so in this short time.

Systemic Blood Pressure The pumping action of the heart generates blood flow; pressure results when blood flow is opposed by resistance. Systemic blood pressure is highest in the aorta, and declines throughout the pathway until it reaches 0 mm Hg in the right atrium. Arterial blood pressure reflects how much the arteries close to the heart can be stretched (compliance, or distensibility), and the volume forced into them at a given time. When the left ventricle contracts, blood is forced into the aorta, producing a peak in pressure called systolic pressure (120 mm Hg). Diastolic pressure occurs when blood is prevented from flowing back into the ventricles by the closed semilunar valve, and the aorta recoils (70–80 mm Hg). The difference between diastolic and systolic pressure is called the pulse presssure. The mean arterial pressure (MAP) represents the pressure that propels blood to the tissues. Capillary blood pressure is low, ranging from 40–20 mm Hg, which protects the capillaries from rupture, but is still adequate to ensure exchange between blood and tissues. Thus, under resting conditions, the velocity averages about 33 cm/sec in the aorta but only 1/1000 as rapidly in the capillaries, about 0.3 mm/sec. However, because the capillaries have a typical length of only 0.3 to 1 millimeter, the blood remains in the capillaries for only 1 to 3 seconds. This short time is surprising because all diffusion of nutrient food substances and electrolytes that occurs through the capillary walls must do so in this short time.

Functional Anatomy of the Heart Chambers 2 Atria 2 Ventricles 2 systems Pulmonary Systemic

Functional Anatomy of the Heart Cardiac Muscle Characteristics Striated Short branched cells Uninucleate Intercalated discs T-tubules larger and over z-discs

Functional Anatomy of the Heart Valves Function is to prevent backflow Atrioventricular Valves Prevent backflow to the atria Prolapse is prevented by the chordae tendinae Tensioned by the papillary muscles Semilunar Valves Prevent backflow into ventricles

The Conduction System of the Heart Conduction pathways Depolarization spreads throughout the heart very rapidly facilitating a coordinated contraction pattern Intercalated disks Form junctions between adjacent cardiac muscle fibers Contain a high concentration of gap junctions for rapid transmission of the action potential

Myocardial Physiology Contractile Cells Plateau phase prevents summation due to the elongated refractory period No summation capacity = no tetanus (Which would be fatal)

Myocardial Physiology Autorhythmic Cells (Pacemaker Cells) Altering Activity of Pacemaker Cells Sympathetic activity NE and E increase If channel activity Binds to β1 adrenergic receptors which activate cAMP and increase If channel open time Causes more rapid pacemaker potential and faster rate of action potentials Sympathetic Activity Summary: increased chronotropic effects heart rate increased dromotropic effects conduction of APs increased inotropic effects contractility

Myocardial Physiology Autorhythmic Cells (Pacemaker Cells) Altering Activity of Pacemaker Cells Parasympathetic activity ACh binds to muscarinic receptors Increases K+ permeability and decreases Ca2+ permeability = hyperpolarizing the membrane Longer time to threshold = slower rate of action potentials Parasympathetic Activity Summary: decreased chronotropic effects heart rate decreased dromotropic effects  conduction of APs decreased inotropic effects  contractility

Aging and the CVS Changes occur in the blood, heart, and BVs Blood changes – HCT; thrombi and emboli form more easily; blood pools in leg Heart changes – efficiency and elasticity; atherosclerosis of coronary vessels; scar tissue forms Blood vessel changes – loss of elasticity; calcium deposits damage vessel walls Gradual changes in heart function, minor under resting condition, more significant during exercise Hypertrophy of L ventricle Maximum heart rate decreases tendency for valves to function abnormally and arrhythmias to occur O2 consumption required to pump same amount of blood