Circulatory System


The cardiovascular system comprises the heart, blood vessels, and blood, functioning alongside accessory filtering organs such as the kidneys (filtration) and the liver (detoxification).

1. Major Functions of the Cardiovascular System

  • Transport: Carries oxygen (O2\text{O}_2), carbon dioxide (CO2\text{CO}_2), nutrients, metabolic wastes, and hormones throughout the body.

  • Protection: Distributes white blood cells and immune factors to sites of infection and injury.

  • Regulation: Maintains homeostasis by balancing fluid volume, solute concentrations, and osmotic pressure.

2. Blood Vessel Structure and Hierarchy

Blood flows sequentially through systemic pathways: Heart →\rightarrow Arteries →\rightarrow Arterioles →\rightarrow Capillaries →\rightarrow Venules →\rightarrow Veins →\rightarrow Heart.

  1. Arteries and Arterioles

    • Carry blood away from the heart under high hydrostatic pressure.

    • Wall architecture consists of three distinct layers: an outer connective tissue sheath, a thick middle layer of smooth muscle and elastic fibers, and an inner endothelium.

    • The elastic and muscular composition accommodates fluctuations in systolic pressure and maintains vessel integrity.

  2. Capillaries

    • Microscopic vessels composed of a single endothelial cell layer to enable efficient diffusion of gases, nutrients, and waste products.

    • Blood delivery across capillary networks is regulated by precapillary sphincters, which constrict or dilate based on regional tissue demands.

  3. Veins and Venules

    • Return blood toward the heart under low pressure.

    • Feature thinner smooth muscle layers and wider lumens compared to arteries, serving as a blood reservoir holding approximately 70%70\% of total circulating blood.

    • Contain one-way internal valves to prevent retrograde flow against gravity, supported mechanically by skeletal muscle contractions and respiratory pressure changes.

3. Heart Anatomy and Pathway of Blood Flow

The heart contains four chambers: two superior atria and two inferior ventricles, separated into left and right sides by a muscular septum.

  • Pulmonary Circuit (Deoxygenated Blood to Lungs):

    1. Deoxygenated blood returns from systemic tissues via the superior and inferior venae cavae into the right atrium.

    2. Blood passes through the right atrioventricular (AV) valve into the right ventricle.

    3. Contraction of the right ventricle propels blood through a semilunar valve into the pulmonary trunk and pulmonary arteries toward the lungs.

  • Systemic Circuit (Oxygenated Blood to Body):

    1. Oxygenated blood from pulmonary gas exchange returns via the pulmonary veins into the left atrium.

    2. Blood moves through the left atrioventricular (AV) valve into the left ventricle.

    3. The left ventricle contracts, forcing blood through the aortic semilunar valve into the aorta for systemic distribution.

4. Cardiac Muscle and Conduction System

  • Histology: Cardiac muscle tissue consists of branched uninucleated cells connected by intercalated discs containing gap junctions, permitting synchronous bioelectrical impulse transmission.

  • Electrical Pathway:

    1. Sinoatrial (SA) Node: Located in the right atrium; serves as the physiological pacemaker initiating the electrical signal for atrial contraction.

    2. Atrioventricular (AV) Node: Delays and routes the electrical impulse down through the bundle branches in the ventricular septum.

    3. Purkinje Fibers: Distribute the depolarizing impulse across the ventricular myocardium, prompting coordinated ventricular systole.

5. Cardiac Cycle and Blood Pressure

  • Cycle Phases:

    • Systole: The active contraction and pumping phase of the chambers.

    • Diastole: The relaxation and chamber-filling phase.

    • Heart Sounds: The characteristic "lub-dub" sound arises from the sequential closure of the heart valves (AV valves closing, followed by semilunar valves closing).

  • Hemodynamics and Measurement:

    • Measured with a sphygmomanometer, standard resting blood pressure is approximately 120/80 mmHg120/80\,\text{mmHg}.

    • Systolic Pressure (upper value): The peak arterial pressure exerted during ventricular contraction.

    • **Diastolic Pressure (lower