The Human Circulatory System

Learning Objectives

Identify the four chambers of the human heart.

Trace the path of blood through the pulmonary and systemic circuits.

Distinguish between the structure and function of arteries, veins, and capillaries.

Describe the major components of blood.

The Body's Transport Network

The circulatory system, powered by the heart, is responsible for transporting oxygen, nutrients, hormones, and waste products throughout the body.

The Heart and Blood Vessels

The human heart is a four-chambered muscular pump.

Atria (Right and Left): The two upper chambers that receive blood.

Ventricles (Right and Left): The two lower, more muscular chambers that pump blood out of the heart.

There are two main circuits of blood flow:

1.Pulmonary Circuit: Carries deoxygenated blood from the right ventricle to the lungs, and returns oxygenated blood to the left atrium.

2.Systemic Circuit: Carries oxygenated blood from the left ventricle to the rest of the body, and returns deoxygenated blood to the right atrium.

Path of Blood Flow:

Vena Cava → Right Atrium → Right Ventricle → Pulmonary Artery → Lungs → Pulmonary Vein → Left Atrium → Left Ventricle → Aorta → Body

Blood Vessels

Arteries: Carry blood away from the heart. They have thick, muscular walls to withstand high pressure.

Veins: Carry blood towards the heart. They have thinner walls and contain valves to prevent the backflow of blood.

Capillaries: The smallest blood vessels, with walls only one cell thick. They form vast networks where the exchange of gases, nutrients, and waste products between the blood and body tissues occurs.

Components of Blood

Plasma: The liquid matrix of blood (about 55%), mostly water, which carries cells, proteins, hormones, and waste products.

Red Blood Cells (Erythrocytes): The most numerous cells. They contain hemoglobin, an iron-containing protein that binds to and transports oxygen.

White Blood Cells (Leukocytes): Part of the immune system, involved in defending the body against pathogens.

Platelets (Thrombocytes): Cell fragments that are essential for blood clotting.

Key Terms

Pulmonary Circuit: The part of the circulatory system that carries deoxygenated blood from the right ventricle to the lungs, and returns oxygenated blood to the left atrium.
Systemic Circuit: The part of the circulatory system that carries oxygenated blood from the left ventricle to the body, and returns deoxygenated blood to the right atrium.
Artery: A muscular-walled tube forming part of the circulation system by which blood (mainly that which has been oxygenated) is conveyed from the heart to all parts of the body.
Vein: Any of the tubes forming part of the blood circulation system of the body, carrying in most cases oxygen-depleted blood toward the heart.
Capillary: Any of the fine branching blood vessels that form a network between the arterioles and venules, where gas and nutrient exchange occurs.