The cardiovascular or circulatory system is responsible for the movement of blood throughout the body. It delivers oxygen, nutrients, and hormones to tissues and organs and removes carbon dioxide and waste products. It consists of a pump (the heart) and an extensive network of channels (arteries, capillaries, and veins):

• Heart is a muscular organ with three layers in its walls (endocardium, myocardium, and epicardium), which rhythmically contracts to pump blood to the pulmonary and systemic circulation
• Arteries carry blood away from the heart and are classified as large or elastic, medium or muscular, or small arteries
• Capillaries (microvascular beds) allow for exchange of material between the blood and the tissues
• Veins return the blood to the heart and are classified as large, medium, or small veins

In addition to blood vessels, a second network of channels, the lymphatic vessels, will also be discussed in this module. These vessels carry lymph (interstitial or tissue fluid), which is filtered through lymph nodes and eventually returned back to the blood.

The learning objectives for this session are:

1. Identify the histological characteristics of the heart, including the epicardium, myocardium, and endocardium, and the conduction system.
2. Identify and distinguish between arteries, veins, capillaries, and lymph vessels (macrovasculature versus microvasculature).
   
    

Let's first look at the heart.