Circulatory System

The Heart 

The walls of the heart consists mostly of cardiac muscle. The atria have thin walls and function in collecting blood returning from the body. The ventricles have thick, powerful walls that pump blood to the organs of the body.





Heart cycle is the sequence of events during each heart beat, lasting about 0.8 seconds. 1). Systole occurs when the heart muscles contract and the chambers pump blood. 2). diastole occurs when the ventricles fill with blood.


Blood pressure is the hydrostatic force that blood exerts against a vessel wall. It is greater in the arteries than the veins. Blood pressure velocity decreases from the aorta to the veins. The area of the capillaries aids in this slow down of velocity.

Atrioventricular valves are found between each atrium an ventricle keeping blood from flowing backwards.

Semilunar valves are found between the ventricle and artery or vein.

Heart Rate or pulse is the number of times the heart beats per minute. There is an inverse relationship between the animals size and its pulse. 

Cardiac muscle is myogenic( self excitable).

S-A Node (sinoatrial node) or pacemaker controls the rate of contraction of the heart. It is located in the right atrium wall. Tissue is both nerve and muscle. 

Heart rate is controlled by hormones, body temperature, and by the opposing actions of two sets of nerves.

Blood Flow

Blood vessels contain 3 layers of tissue. Outer layer connective tissue with elastic fibers, middle layer of smooth muscle and elastic fibers, and an inner layer of endothelium of simple squamous epithelium. Arteries carry blood away from the heart, while veins carry blood toward the heart. Veins contain valves to prevent the back flow of blood on its way to the heart.

Capillaries connect veins to arteries. It is here that any and all exchanges take place between the blood and the body. Materials may leave due to passive diffusion or hydrostatic pressure. Capillaries are one cell thick.

Lymphatic System contains a network of vessels that complement the arteries and veins. The fluid is called lymph and it is mainly composed of fatty materials, large proteins, and ions. The vessels have valves in them and the fluid is pushed along by the skeletal muscles. Lymph nodes collect bacteria and viruses, and try to rid the body of them.


A. Plasma: 90% is water. 10% are electrolytes and proteins. The electrolytes help maintain the osmotic balance of blood while the proteins function in a variety of ways: blood clotting, immunity, buffering agents, increase the viscosity of the blood. Plasma also contains waste materials, hormones, nutrients, and dissolves gases also included in the 10%.

B. Blood cells: Erythrocytes, biconcave discs, that transport oxygen. Lack nuclei, mitochondria; generate ATP exclusively by anaerobic metabolism. Contains hemoglobin, an iron containing protein that reversibly bonds with oxygen. Kidneys secrete a hormone called ERYTHROPOIETIN, which stimulated the production of red cells by the bone marrow. Leucocytes: function in body defense and immunity. 5 types (basophils, eosinophils, neutrophils, lymphocytes, and monocytes).

C. Platelets: noncellular structures that function in blood clotting.

Blood Clotting: Platelets break upon hitting the rough area of a wound. It releases a chemical called THROMBOPLASTIN which changes prothrombin (floating in the blood) to THROMBIN. Thrombin now will react with several other chemicals and eventually cause FIBRINOGEN (found floating in the blood ). into an insoluble material called FIBRIN.

To develop the concept of circulation click here and complete the AP Lab 10: Physiology of the Circulatory System.