Cardiovascular Physiology

Cardiovascular Physiology

# I. Circulation

• Pulmonary Circulation: Originates in the right ventricle, pumps blood to the lungs and in the pulmonary capillaries, then returns to the heart in the left atrium.

• Systemic Circulation: Originates in the left ventricle, pumps blood through the aorta to the organs. The aorta branches into arteries into capillaries, where exchange takes place between blood and tissues, then the blood returns through veins, emptying into the right atrium.

# II. Structure of the Heart

• Structure: The heart is a hollow organ with 4 chambers: 2 atria and 2 ventricles, separated by a septum.

• Sinoatrial Node: Located in the right atrium, it has the ability to generate its own heart rhythm due to the nature of the sinoatrial node cell membrane to certain ions (Na+, Ca2+).

• Path of Electrical Potential: The impulse spreads from the sinoatrial node to the atrioventricular node, to the Bundle of His, splits into the right and left branches, and spreads throughout the myocardium.

• Nervous System Distribution to the Heart: Sympathetic and parasympathetic nervous systems.

• What artery supplies the heart? Right and left coronary arteries.

• What veins drain into the coronary sinus? Great, small, middle, posterior cardiac veins, posterior left ventricular vein and oblique vein of left atrium.

• What type of muscle is the heart muscle? Striated muscle.

# III. Physiology of the Heart Muscle

• Propagation of action potential in the heart:

• Atrial depolarization

• Depolarization of the interventricular septum from left to right

• Depolarization of the anterior wall of the septum toward the apex

• Depolarization of the majority of the ventricular muscle from endocardium to epicardium

• Depolarization of the base of the ventricle

• The entire heart is depolarized

• Physiological properties of the heart:

• Excitability: Ability to respond to stimuli.

• Refractory period: Inability to respond to stimuli for a certain period of time.

• Automaticity: Ability of the nodal system to spontaneously and rhythmically generate impulses.

• Conductivity: Ability to transmit impulses from one cell to another.

# IV. Cardiac Cycle

• Cardiac cycle: Consists of systole and diastole.

• Systole:

• Atrial systole: 0.1 seconds, creates the fourth heart sound.

• Ventricular systole: 0.3 seconds, creates the first heart sound.

• Diastole: 0.4 seconds, creates the third heart sound.

• Blood volume to the ventricle:

• Atrial systole: 20%

• Diastolic period: 80%

# V. Ventricular Systole

• Ventricular systole has 2 phases:

• Isovolumetric contraction: The ventricle contracts, ventricular pressure increases rapidly. The atrioventricular valves and semilunar valves close, the muscle fibers do not shorten or very little, no blood is ejected into the arteries.

• Isobaric contraction: The pressure in the ventricle opens the aortic valve, blood is pushed into the aorta. The first 1/3 is the phase of rapid ejection/pumping, the next 2/3 is the phase of slow ejection/pumping.

# VI. Diastole

• Diastole has 2 phases:

• Isovolumetric relaxation: The ventricle relaxes rapidly ? pressure in the two ventricles decreases rapidly. The aortic valve and pulmonary valve close, creating the second heart sound.

• Blood return to the heart: 80% of the blood flows back to the ventricle during this phase.

# VII. Cardiac Output

• Stroke volume: The volume of blood the heart pumps out during one contraction.

• Stroke volume = End-diastolic volume – End-systolic volume.

• Cardiac output: The amount of blood pumped by the heart in one minute.

• Cardiac output = Volume of blood pumped by the heart in one beat x Number of heart beats in one minute.

# VIII. Regulation of Cardiac Activity

• Intrinsic regulation of the heart muscle:

• Frank-Starling Law: The contractility of the heart muscle depends on the length of the muscle fiber before contraction.

• Regulation of heart rate and contractility by the autonomic nervous system and humoral mechanisms:

• Autonomic nervous system:

• Sympathetic nerve stimulation:

• Increases heart rate (positive chronotrope)

• Increases heart muscle contractility (positive inotrope)

• Increases the speed of impulse conduction in the heart (positive dromotrope)

• Increases the speed of relaxation of the heart muscle (positive lusitrope).

• Parasympathetic nerve stimulation:

• Decreases heart rate

• Decreases heart contractility

• Decreases the speed of impulse conduction in the heart

• Decreases the speed of relaxation of the heart muscle.

• Humoral mechanisms:

• Hormonal:

• Adrenaline and noradrenaline from the adrenal medulla: Increase heart rate and heart muscle contractility.

• Thyroid hormone (Thyroxine): Increases cellular metabolism, increases energy metabolism, increases oxygen demand ? increases heart rate.

• Oxygen and carbon dioxide levels in the blood: Increased carbon dioxide levels and decreased oxygen levels cause the heart to beat faster.

• Ions:

• Calcium ion level: Excessive increase ? heart muscle spasm, decrease ? heart muscle weakness.

• Potassium ion level: High level ? heart muscle relaxes, slows heart rate. Level of 8-12 mEq/L ? severe heart muscle weakness, arrhythmia and death.

• Body temperature: Increased temperature: faster heartbeat, decreased temperature: slower heartbeat.

# IX. Reflexes Affecting Cardiac Activity

• Baroreceptor reflex: Increased pressure affects receptors in the aortic arch ? Cyon nerve ? brainstem ? vagus nerve ? heart slows down, blood pressure decreases. Increased pressure affects receptors in the carotid sinus ? Hering nerve ? brainstem ? vagus nerve ? heart slows down, blood pressure decreases.

• Reflex that increases heart rate: When oxygen levels decrease and carbon dioxide levels increase in the blood ? chemoreceptors in the aortic arch and carotid sinus.

• Oculocardiac reflex: Strong pressure on both eyeballs ? stimulation of the terminal ends of the V nerve ? stimulation of the vagus nerve ? slows down the heart.

• Goltz reflex: Strong blow to the epigastrium ? vagus nerve ? can cause the heart to stop.

# X. Influence of the Cerebral Cortex and other Brain Centers

• Stimulation of the nucleus in the midbrain, anterior and medial in the thalamus causes an increase in heart rate.

• Stimulation of the posterior hypothalamus causes an increase in heart rate and climbing movements.

• When inhaling, the heart beats faster, when exhaling, the heart beats slower.

# XI. Specialized Terms

• Preload: The volume or pressure in the ventricle at the end of diastole. Represents the stretch of the heart muscle fiber at the beginning of contraction.

• Afterload: The pressure in the aorta. Peripheral vasoconstriction causes increased peripheral resistance, increasing afterload.

• Stroke volume: The volume of blood the heart pumps out during one contraction.

• Cardiac output: The amount of blood pumped by the heart in one minute.

• Purkinje network: The system of fibers that conduct impulses in the heart, which has the fastest conduction velocity.

• Average QRS vector: Determined on a horizontal plane through the chest.

• Dihydropyridine receptor: Related to Ca2+ channels.

Note: This article is just a summary of basic knowledge about cardiovascular physiology. For a deeper understanding of this topic, you should consult additional specialized literature.