Circulatory system

The circulatory system is a collection of structures and processes within the body of many animal species, including humans, that facilitates the transport of oxygen and nutrients to target areas within the body, while also allowing for the removal of waste products, among several other functions.

The circulatory system is responsible for:

• Transporting nutrients and oxygen to tissues;
• Carrying waste away from tissues;
• Acting as a vehicle for the immune system;
• Transporting hormones from the endocrine glands;

The system is composed of (1) blood, the liquid medium within which molecules are transported, (2) the heart, responsible for producing the force required to push the blood around the system, and (3) blood vessels, through which blood may flow.

A simplified diagram of the human circulatory system clearly demonstrates how the structure of the system relates to its function in providing oxygen for tissues; deoxygenated blood returning from the systemic circulation enters the right side of the four-chambered heart, whence it is pumped into the pulmonary circulation that it may be oxygenated in the lungs. The newly oxygenated blood flows through the pulmonary vein into the left side of the heart, where it is pumped to the body.

The sound that is often heard when you put your ear to somebody's chest i.e the sound lub-dub lub-dub......is nothing but the opening and closing of valves in the heart.

The heart, when beating at 65 beats per minute, with 2-3 oz. of blood being pumped with each beat, it could fill a 2,000 gallon tanker in one day.

The blood is composed of oxygen-carrying erythrocytes (red blood cells, or RBCs), immunological white blood cells (of varying types), clot-forming platelets, and plasma, a straw-coloured liquid into which vital substances are dissolved, such as glucose, hormones, and carbon dioxide.

An average person maintains a blood volume of roughly 5 liters, but the exact amount present in any one person's circulatory system is always changing. The constituent parts of blood are constantly both produced and used or excreted through some of the body's many examples of homeostatic mechanisms.

Erythrocytes are small (6-8µm) cells that uniquely lack a nucleus. They are shaped like a disk with an central indent on both sides, which is why they appear pale in the center when viewed under a microscope (central pallor). These cells contain a protein called haemoglobin, which contains iron. The iron molecules of the haemoglobin protein are responsible for binding to oxygen, and one haemoglobin molecule can bind up to four oxygen molecules (O2) each.

The amount of oxygen that a haemoglobin molecule binds to is determined in part by the partial pressure of oxygen in the blood, and by the specific individual's haemoglobin's affinity for oxygen. This can be demonstrated using an oxygen-haemoglobin dissociation curve.

The heart is a four-chambered muscular pump, located within the thoracic cavity, with its apex extending slightly to the left of the midline. On examination, the left side of the heart is larger than the right side of the heart, illustrating an important division of labour within it.

On each side, blood is recieved into an atrium (left or right), and is passed from each atrium to the corresponding ventricle when the atria contract. The atrioventricular valves (or AV valves) sit between each ventricle and it's atrium, preventing backflow of blood even under intense pressure. The right AV valve is the tricuspid valve, and the left AV valve is the mitral valve. Subsequently, contraciton of the ventricles forces blood out of the heart through the semilunar valves - pulmonic on the right, aortic on the left.

The right heart receives deoxygenated blood from the systemic circulation, and is responsible for pumping it to the lungs where excess carbon dioxide will be offloaded into the alveoli of the lungs (to be exhaled) and oxygen will be bound to erythrocytes in the blood. The pulmonic circuit is relatively short, and as such there is little resistance to flow, and the right heart - especially the right ventricle - is not required to be particularly large or strong in it's contractions.

The left heart receives oxygenated blood from the pulmonic circuit and is responsible for pumping it throughout the entire body, hence it encounters far greater resistance than the pulmonic circuit due to the much greater extent of the network of vessels. Additionally, the systemic circuit must provide sufficient pressure to the blood to combat the effect of gravity and return blood from the lower extremities back up to the heart.

The contraction of each of the chambers is coordinated by nerve impulses within the cardiac tissue.

1. The sinoatrial node (SAN), located on the surface of the right atrium, is a cluster of "pacemaker" cells, capable of regularly and spontaneously producing an electrical impulse without external stimulation (though their function can be externally modulated).
2. On sending an impulse, it travels through the walls of the atria. Consisting of cardiac muscle, this impulse triggers contraction of both atria simultaneously.
3. The impulse arrives at a structure called the atrioventricular node (AVN), so called because transmits the impulse from the atria to the ventricles via a specific route. The impulse cannot reach the ventricles via any other path as the tissue is specifically non-conductive to electrical impulses in all places except here.
4. The impulse is transmitted down a structure called the Bundle of His, located within the ventricular septum - the wall separating the left and right ventricles.
5. Once it reaches the heart apex, the impulse is carried back up around the external muscular walls of each ventricle, through the Purkinje Fibres. This triggers ventricular contraction.