The sheep heart is a muscular, cone-shaped organ located in the thoracic cavity between the lungs. Its long axis is oblique, with the apex directed downward, ventrally, and to the left. On average, an adult sheep’s heart weighs 150–250 grams, depending on breed and body size. Beyond its physiological role, studying the ovine heart is critical for understanding cardiovascular function, diagnosing heart disease, and developing clinical and surgical skills in veterinary education.

What is the structure of the sheep’s heart?

Structurally, the sheep heart consists of four chambers:

• The right atrium and left atrium at the upper part of the heart, which receive blood.
• The right ventricle and left ventricle at the lower part, which pump blood out of the heart.

Each side operates in parallel: the right side manages deoxygenated blood returning from the body and sends it to the lungs, while the left side handles oxygenated blood returning from the lungs and pumps it throughout the body. The chambers are separated by muscular walls (septa) and connected by valves that regulate blood flow and prevent backflow.

The outer covering of the heart is the pericardium, a double-layered membrane that protects and lubricates the organ. Beneath it lies the myocardium, the thick muscular layer responsible for contraction, and the endocardium, the smooth internal lining that covers the chambers and valves.

What are the facts about a sheep’s heart?

Some key facts about the sheep heart include:

• It functions in two main circuits: the pulmonary circuit (heart–lungs–heart) and the systemic circuit (heart–body–heart). 
• The left ventricle is thicker and stronger than the right because it pumps blood throughout the body, not just to the lungs. 
• The sheep heart closely resembles the human heart, making it a common specimen in anatomy labs. 
• It beats roughly 70 to 90 times per minute in a healthy adult sheep. 
• The coronary arteries supply the heart muscle itself with oxygenated blood.

Internal and External Anatomy

Externally, the sheep heart displays grooves and ridges marking the internal divisions between chambers.

The interventricular sulci (anterior and posterior) separate the right and left ventricles, while the coronary sulcus circles the heart’s base. The external surface also reveals the entrance and exit points of major vessels.

Internally, wall thickness varies by function: the left ventricle has the thickest myocardium (up to 1 cm) because it must generate higher pressure to pump blood through the systemic circuit. The right ventricle, in contrast, has thinner walls suited for the lower pressure of pulmonary circulation.

Specific Structures & Vessels

The sheep heart contains several key structural and vascular components that reflect its dual circulatory role.

• Right atrium: receives blood from the cranial and caudal venae cavae.
• Left atrium: receives oxygenated blood from the pulmonary veins.
• Right ventricle: sends deoxygenated blood through the pulmonary trunk to the lungs.
• Left ventricle: pumps oxygenated blood into the aorta.
• Tricuspid valve: between right atrium and ventricle.
• Mitral (bicuspid) valve: between left atrium and ventricle.
• Semilunar valves: at the exits of the aorta and pulmonary trunk, preventing backflow.

The heart muscle receives its own blood supply through the coronary arteries, branching from the base of the aorta. Venous drainage occurs through the coronary veins, which empty into the coronary sinus of the right atrium.

Does a sheep’s heart have an epicardium?

Yes, it does. The epicardium is the outermost layer of the sheep heart wall, also known as the visceral layer of the pericardium. It consists of a thin membrane of connective tissue and fat that protects the myocardium and supports the coronary vessels. Beneath it lies the thick myocardium, and internally the endocardium, completing the three main layers of the heart wall.

Cardiac Chambers and Regions

Each chamber of the sheep heart performs a specific role in blood circulation:

• The right atrium collects blood returning from the body.
• The right ventricle pumps that blood to the lungs.
• The left atrium receives oxygenated blood from the lungs.
• The left ventricle pumps oxygenated blood throughout the body.

The interventricular septum divides the two ventricles, while the fibrous cardiac skeleton supports the valves and coordinates muscle contraction.

The sheep heart is anatomically and functionally similar to the human heart, both have four chambers, identical valve types, and a comparable circulatory pattern. The main differences are scale (the human heart is slightly larger) and subtle variations in vessel branching. Because of these similarities, sheep hearts are often used in comparative anatomy and medical training to model human cardiac physiology.

Circulatory Context

The sheep heart operates within a closed double circulatory system, like that of all mammals.

The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. The systemic circuit then delivers oxygenated blood from the left ventricle to the entire body, returning deoxygenated blood to the right atrium.

This constant circulation maintains oxygen and nutrient transport, removes metabolic waste, and supports homeostasis. In veterinary science, studying the sheep heart provides critical insights into mammalian cardiovascular health and disease mechanisms, bridging animal and human medicine.

A detailed understanding of the sheep heart is essential in modern veterinary medicine. It enables professionals to:

• Diagnose and treat cardiovascular diseases effectively.

• Interpret diagnostic imaging with greater precision.

• Perform surgical and therapeutic procedures safely.

• Train veterinary students through practical methods and realistic anatomical models.

In short, mastering ovine cardiac anatomy not only enhances clinical competence but also contributes to the overall well-being, performance, and health of animals.