Cardiovascular System: Heart and Blood Vessels

• The cardiovascular system is a closed system comprised of the heart and blood vessels.
• Blood circulates to all body parts via blood vessels.
• Oxygen, nutrients and waste products are transported by the cardiovascular system.

The Heart

• Functions as a dual pump.
• The right heart pumps blood through the lungs.
• The left heart pumps blood through the peripheral organs.
• The heart is divided into chambers: right and left atria (receiving), and right and left ventricles (discharging).
• Each atrium delivers blood into a ventricle.
• Each ventricle discharges blood.

Functions of the Heart

• Serves as a muscular pump to optimize blood flow.
• Interconnects pulmonary and systemic circulation.
• Separates oxygenated and deoxygenated blood.
• Maintains one-way blood flow.
• Produces ANF (atrial natriuretic factor) to regulates blood pressure.

Valves of the Heart

• Atrioventricular valves exist between the atria and ventricles.
• Atrioventricular valves prevent backflow during systole.
  • Bicuspid valve is on the left.
  • Tricuspid valve is on the right.
• Semilunar valves exist between ventricle and artery.
• Semilunar valves prevent backflow during diastole.
  • Pulmonary semilunar valve.
  • Aortic semilunar valve.

Blood Vessels

• Transports blood to the tissues and back.
• Arteries, arterioles, capillaries, venules, and veins classify as blood vessels

Arteries vs Veins

• Arteries carry blood away from the heart, while veins carry it towards the heart.
• Arteries have thick walls and absent valves, while veins have thin walls and present valves.
• Arteries have high blood pressure (100mm of Hg), while veins have low pressure (7 mm of Hg).
• Arteries carry oxygenated blood in systemic circulation while veins carry deoxygenated blood.

Blood Circulation Types

• Systemic circulation.
• Pulmonary circulation.
• Systemic circulation supplies blood flow to all the tissues of the body except the lungs (greater or peripheral circulation).

Pulmonary Circulation

• Oxygen-poor blood enters the right side of the heart from the body through the vena cava to the atrium.
• The pulmonary artery transports blood that has low oxygen content from the ventricle to the lungs.
• Oxygen is absorbed by the blood and carbon dioxide is lost in lung capillaries.
• Oxygen-rich blood is carried by the veins back to the left atrium of the heart.

Systemic Circulation

• Oxygen rich blood goes back to the heart via pulmonary veins.
• The blood is pushed out from left ventricle through the aorta.
• The aorta separates into numerous branches called arteries-arterioles-tissue capillaries.
• Blood releases O2 and acquires CO2 in tissue capillaries near cells.
• The blood (O2 poor) goes back to the heart through venules and veins in the right side

Pulmonary vs Systemic Circulation

• Pulmonary circulation is smaller, supplies the lungs, has less pressure, is equal at 5L/min flow, is faster circuits, has thin walls, and less velocity.
• Systemic circulation is larger, supplies the entire body, has higher pressure, is equal at 5L/min flow, is slower circuits, has thick walls, and more velocity.

Coronary Circulation

• The myocardium is not nourished by blood in the heart chambers.
• The heart contains its own circulatory system: coronary arteries, cardiac veins.
• The coronary sinus facilitates blood emptying into the right atrium.

Properties of Cardiac Muscle Fibers

• Many individual cells connected in series and parallel make up cardiac muscle fibers.
• Intercalated discs connect cells.
• Cell membranes fuse at intercalated discs, which forms "communicating" junctions (gap junctions) to allow diffusion of ions.
• This allows the action potential to travel easily from one cardiac muscle cell to the next.

The Conduction System

• An intrinsic conduction system enables heart muscle cells to contract regularly, continuously, and without nerve impulses.
• Brain-initiated nerve impulses, circulating chemicals, and hormones can stimulate or depress the intrinsic system.
• Specialized neuromuscular cells in the myocardium initiate and conduct impulses causing synchronized contraction.

Sinoatrial (SA) Node

• The SA node is located in the wall of the right atrium near the superior vena cava.
• The SA node acts as the 'pace-maker' as it initiates impulses more rapidly than other groups of neuromuscular cells.

Atrioventricular (AV) Node

• It is situated in the wall of the atrial septum near the atrioventricular valves.
• Impulses that sweep over the atrial myocardium normally stimulate the AV node.
• The AV node delays the signal from the SA node, providing sufficient time for complete atrial contraction before the ventricles contract.

Atrioventricular Bundle (Bundle of His)

• Specialized fibers originating from the AV node form a mass.
• The AV bundle crosses the fibrous ring separating atria and ventricles, then divides into right and left bundle branches at the upper end of the ventricular septum.
• Branches in the ventricular myocardium break up into fine Purkinje fibers.
• The AV bundle, bundle branches, and Purkinje fibers convey electrical impulses from the AV node to the apex of the myocardium.
• Ventricular contraction rises upwards and outwards, allowing blood to be pumped into the pulmonary artery and aorta.

The Cardiac Cycle

• The heart completely relaxes and contracts in each beat.
• Diastole: Ventricular and atrial relation.
• Systole: Ventricular and atrial contraction.

Phases of the Cardiac Cycle

• Ventricular filling occurs mid-to-late during ventricular diastole.
  • Blood flows into the atria via open AV valves and into ventricles (pressure is lower).
  • The aortic and pulmonary valves are closed.
  • The atria contracts, forcing the rest of the blood (20%) into ventricles.
• Ventricular systole:
  • Isovolumetric contraction occurs as ventricles contract and intraventricular pressure rises, which closes the AV valves.
  • Ventricles are briefly closed chambers.
  • Ventricular ejection happens due to rising ventricular pressure.
  • Semilunar valves are forced open, and blood ejected from the heart.
• Isovolumetric relaxation:
  • Ventricles relax and ventricular pressure drops.
  • Semilunar valves shut with blood backflow.
  • Ventricles are closed off.
  • The atria fill with blood.
  • AV valves open when atrial pressure is higher than that of the ventricles and filling starts again.

Blood Flow and Pressure Changes

• Alternating contraction and relaxation of the heart causes alternating pressure changes that affect blood flow.
• Blood moves along a pressure gradient (from high to low pressure) through openings.
• Blood flows forward as heart valves open and close due to pressure changes.

Cardiac Output (CO)

• The amount of blood that each side of the heart pumps in one minute.
• CO = (heart rate [HR]) x (Stroke volume [SV])
  • Heart rate is the number of beats per minute.
  • Stroke volume is the volume of blood pumped by each ventricle in one contraction.

Factors Affecting Heart Rate

• Autonomic nervous system: Parasympathetic activity decreases rate and force; whereas sympathetic activity increases rate and force.
• Circulating chemicals increase heart rate, such as adrenaline, noradrenaline, and thyroxine (via metabolic effect).
• Position: Upright position can result in a faster heart rate.
• Other factors: exercise, emotional state, gender (faster in women), age (faster in babies/children), temperature, and baroreceptor reflex.

Determine Stroke Volume (SV) factors

• Contractility is indicated by the strength of myocardial contraction.
• Blood volume: Increasing blood volume return (preload and afterload) increases stroke volume.
  • Preload is the force stretching the muscle before contraction and indicates the volume of blood that the ventricle has available to pump.
  • Afterload is the pressure to overcome for the ventrilces to eject blood.

Factors that Determine Stroke Volume (SV)

• The amount of blood that collects in a ventricle during diastole called Diastolic volume (EDV).
  • The volume of blood that is remaining in a ventricle after it has contracted is called systole volume (ESV).
• Stroke volume is amplified due to a increase in EDV.
  • The venous return contributes, through the use of gravity assistance,skeletal muscle pump,and the respiratory pump to contribute towards the stroke volume.

Blood Pressure

• Hydrostatic pressure exerted by blood laterally on the walls of a vessel.
• Measurements are made in large arteries by health professionals.
  • Systolic describes pressure at ventricular contraction peak.
  • Diastolic means pressure when ventricles relax.
• Pressure declines with distance from the heart.

Measuring Arterial Blood Pressure

• Clinically, the brachial artery is used most often.
• The turbulent flow of blood is auscultated in the antecubital area on the brachial artery with a stethoscope.
• A sphygmomanometer's rubber cuff is inflated to a pressure greater than systolic.
• Cuff pressure is released slowly by opening the air valve.
• Artery opens and blood flows again when pressure is greater than the cuffs.

Korotkov Sounds

• Phase I: Systolic pressure can be measured by the appearance of clear tapping sounds.
• Phase II: A brief period may follow during which the sounds soften and acquire a swishing quality.
• Phase III: Sounds regain or exceed the intensity of Phase I.
• Phase IV: The sounds abruptly become muffled.
• Phase V: The point at which all sounds finally disappear completely indicates diastolic pressure.
• The second and third sounds have no known clinical significance.

Results and Interpretations of Blood Pressure

• Normal BP: 120/80 mm of Hg.
• Hypotension: Low BP, with systolic pressure below 100 mm Hg.
• Hypertension: Sustained elevated arterial pressure of 140/90 or higher.
• Transient elevations are normal: fever, physical exertion, and emotional upset.
• Chronic elevation is a major cause of heart failure, vascular disease, renal failure, and stroke.
• Orthostatic hypotension: Temporary low BP and dizziness from sitting or reclining position.
• Chronic hypotension is a sign of poor nutrition and addison's disease.

Factors Affecting Blood Pressure

• Cardiac output is proportional to BP.
• Vascular resistance: Opposition to flow, friction between blood and vessel walls.
  • Radius and length of vessels & blood viscosity impact this.
• Systemic vascular resistance (SVR): All the resistance offered by systemic vessels.
  • The smaller the vessels, the more the resistance.
• Venous return constitutes blood flowing back to the heart through systemic veins.

Blood Pressure Regulation

• Short term (neurological).
• Long term (RAAS).

Short-Term Control

• Proprioceptors monitor limb/muscle position.
  • Nerve impulses go to the cardiovascular center at an increased frequency.
  • Input contributes to a quick heart rate increase during physical activity.
• Baroreceptors monitor stretching major veins/arteries due to blood pressure.
  • These are located in the arch of the aorta and in the carotid arteries.
  • They send info about BP changes to the cardiovascular center.
• Chemoreceptors:
  • Monitor chemical changes in the blood mostly in carotid/aortic bodies.
  • Respond to declines in O2 supply, preventing a pressure decline by exciting the vasomotor center.

Long Term Control

• Renin-angiotensin-aldosterone system regulates blood pressure.

Role of Atrial Natriuretic Factor

• High venous return results in release of atrial natriuretic factor from the right atrium.
• This acts on the kidney, causing loss of Na+ and water.
• Blood volume, venous return, and the cardiac output is decreased.
• Blood pressure is decreased.