Human Circulatory System: Heart, Vessels, and Blood

The Human Circulatory System

The circulatory system transports and distributes nutrients and other substances dissolved in the blood throughout the body, collecting waste products to be expelled.

Blood Vessels

Blood vessels are the conduits where blood flows. They include:

• Arteries: Vessels that carry blood *away* from the heart. They have thick, slightly elastic walls to withstand high pressure.
• Veins: Vessels that carry blood *toward* the heart. They possess thin walls and one-way valves to prevent the backflow of blood.
• Capillaries: Tiny blood vessels with very thin walls, facilitating the exchange of substances between blood and cells.

The Heart

The heart is the organ responsible for pumping blood rhythmically to every cell in the body. It's made of cardiac muscle tissue called myocardium, which does not fatigue throughout life. The heart is divided into four chambers: two atria (upper) and two ventricles (lower). The right and left sides of the heart are separated by a thick muscular septum, preventing the mixing of blood.

The atria communicate with the ventricles through openings that possess valves, preventing the backflow of blood from the ventricles to the atria.

Heart Function

Blood enters the heart from the veins into the atria. The atria contract (atrial systole), and blood passes to the ventricles. Then, the ventricles contract (ventricular systole), and blood is propelled through the arteries.

Blood

Humans have about five liters of blood, which corresponds to 7% of their body weight. Blood is composed of plasma and blood cells.

Plasma

Plasma is composed of approximately 90% water and dissolved substances, including:

• Proteins: Such as albumin (carrier function), globulins (defense against infections), and fibrinogen (blood clotting).
• Inorganic salts: Maintain internal pH, water balance, etc. These can be classified into anions: chlorides, bicarbonates, sulfates, and phosphates.
• Organic substances: Resulting from cellular activity or digestion, circulating in the bloodstream, such as chlorides, urea, and uric acid.

Blood Cells

• Red Blood Cells (Erythrocytes): Responsible for transporting oxygen and carbon dioxide. They contain *hemoglobin*, which carries oxygen.
• White Blood Cells (Leukocytes): Cells with a nucleus of diverse morphology, responsible for producing an immune response to pathogens.
• Platelets (Thrombocytes): Cells involved in blood clotting, responsible for plugging blood vessels when an injury occurs.
• Minerals: Regulate pH.
• Albumin: Transport of substances.

Key Concepts

• Artery, Vein: Types of blood vessels.
• High Pressure: Characteristic of arteries.
• Thin Walls: Characteristic of veins.
• Capillary: Site of substance exchange.
• Heart: Blood-pumping organ.
• Valve: Structure that allows communication between heart chambers and prevents backflow.
• Systole: Contraction phase of the heart.
• Diastole: Relaxation phase of the heart.
• Blood Circulation: Oxygen-poor blood never mixes with oxygen-rich blood.