The Human Circulatory System

Human circulatory system consists of essentially three main components namely;

• Heart 
• Blood
• Blood vessels

Human heart

Heart is a muscular pumping organ that provides blood to the body.

External structure

Heart is a conical shaped muscular organ. It is covered with a membrane called pericardium. In between the pericardium and heart wall is the pericardial fluid that also provides protection to subside the pressure directly on the heart wall.

Location

The heart is located between the lungs in the middle of the chest, behind and slightly to the left of breastbone which is called sternum.

Internal structure

Internally, heart consists of four chambers. Upper two chambers are called atria (right and left) while the lower chambers are called ventricles (right and left). Ventricles have thicker walls as compared to atria. The walls of left ventricle are thickest as it has to pump the blood to the whole body under greater pressure.

Important vessels originating from heart

Two main blood vessels arising from heart are

Pulmonary artery that takes deoxygenated blood from the right ventricle to the lungs for oxygenation

Aorta takes oxygenated blood from the left ventricle to the whole body parts except lungs.

Important vessels bringing blood back to heart

Superior and inferior venacava bring deoxygenated blood from the upper and lower body parts and drain it to right atrium.

Pulmonary veins bring oxygenated blood from the lungs to the left atrium.

Important valves

Valves prevent back flow of blood.

Bicuspid/Mitral valve is present between left atrium and left ventricle.

Tricuspid valve is found between right atrium and right ventricle. It is more muscular than bicuspid valve.

Pulmonary valve controls blood flow from right ventricle to the pulmonary arteries.

Aortic valve opens the way for blood to flow from left ventricle to the aorta.

Septum

It is a wall of muscles that separate right and left atria from each other and is called inter atrial septum while inter ventricular septum separates both the ventricles from each other.

Mechanism of transportation

The path that the blood takes from heart to the whole body and then back from the body to the heart is a kind of circuit. This can be explained in the following way.

Pulmonary circuit

It is the path way in which blood is pumped by the heart from its right atrium to both the lungs through pulmonary artery for oxygenation and then the oxygenated blood is poured into the left atrium via pulmonary veins.

When the right atrium is filled with the deoxygenated blood, itcontracts and passes blood to right atrium via the tricuspid valve. Once the right atrium is filled with the deoxygenated blood it contracts and pulmonary valve opens hence the blood goes to lungs via pulmonary artery to lungs. Lungs are rich in oxygen the exchange of gases takes place and blood after getting oxygen goes back to left atrium of the heart via pulmonary veins both right and left that are coming from right and left lungs respectively.

Systemic circuit

This is the pathway that is responsible for the pumping of blood from the left ventricle through aorta which descends down called dorsal aorta that further divides into different arteries thatsupply blood to the various organs of lower body parts. However, the upper parts i.e. brain, shoulders and arms also take the blood supply from various arteries originating from aorta. Coronary artery supplies blood to heart muscles (myocardium).

When the left atrium is filled with the oxygenated blood mitral valve opens and pushes the blood to left ventricle. Upon the left ventricle’s contraction, the blood rushes to aorta as the aortic valve opens. The aorta descends down formally and gives off further branches like hepatic artery, renal artery etc that supply oxygenated blood to liver and kidneys respectively. The deoxygenated blood form various organs is given to the blood and veins bring the blood back to heart. For example, hepatic vein collects the deoxygenated blood from liver while renal vein collects the deoxygenated blood from the kidneys. During their way to heart, all the veins from the lower body parts join to form one bigger vein called inferior venacava while the veins from the upper part of the body i.e. brain, arms and shoulders unite to form superior venacava. Both superior and inferior venacava drain the deoxygenated blood in the right atrium and this cycle continues.

                                   Blood

Blood is a connective tissue that flows in the fluid form within the blood vessels.

Components of blood

It consists of

• Plasma
• Blood corpuscles
• Platelets

Plasma

It is the yellowish fluid that constitutes 55% of the blood. About 95% of plasma is water. It has got dissolved nutrients, proteins like fibrinogen, prothrombin etc. Beside this, it has also got hormones, enzymes that have got their specific jobs. It also contains various waste substances like urea, carbon dioxide gas.

Blood corpuscles

Blood corpuscles are

         i.            Red blood cells (Erythrocytes)

       ii.            White blood cells (Leukocytes)

i.                    Red blood cells

They are biconcave disc shaped cells without nucleus.Their size ranges from 6 to 8 micrometre in diameter. Blood contains around 5,000,000 RBCs in 1 cubic millimetre of blood. They appear red because of an important pigment, called haemoglobin. It carries oxygen to all body parts and then also attaches carbon dioxide.

 

 

ii.                  White blood cells

They are of irregular shape and bigger in size as compared to red blood cells. They are nucleated cells and provide defence to the body. White blood cells can be divided broadly into two categories namely

a.       Granulocytes: They are the types of white blood cells that contain small granules of proteins. These include neutrophils, eosinophils, basophils

Neutrophils are twice the size of RBCs and constitute about 62% of WBCs, their nucleus is 2-5 lobed. They destroy small particles by phagocytosis.

Eosinophilsconstitute about 2% of the total WBCs and possess a nucleus which is bilobed. They attack parasites and inactivate inflammation producing substances.

Basophils constitute less than 1% of the total WBCs in the blood. Their nucleus is bilobed. They release heparin which is an anticoagulant that prevents blood clotting. They also have histamine which causes inflammation.

b.      Agranulocytes: They are the types of white blood cells that do not contain granules. These include monocytes and lymphocytes.

Monocytes are 3 to 4 times larger than RBCs with a nucleus having a shape from round to lobes. There are around 3% of monocytes of total WBCs. They are macrophages and destroy large particles by phagocytosis.

Lymphocytes are slightly larger than RBCs. Their nucleus nearly occupies the cell. Around 32% of WBCs are lymphocytes. They produce immune response by releasing antibodies.

iii.                Platelets

                They are the fragments of cells which are formed from large precursor cells in the bone marrow. They help in clotting. If somebody gets an injury that cause rupturing of blood vessels, platelets rush to that area and produce an enzyme that causes the soluble fibrinogen to form insoluble fibrin that forms a network of fibres (plug/clot) around wound.

          

Functions of blood

Blood is responsible for the transportation of various substances within the human body.

• It transports essential nutrients to every single cell.
• It also takes the hormones from the endocrine glands towards the target site in order to stimulate that particular organ.
• It carries enzymes that are released from the glands and responsible for specific functions.
• It helps in the gaseous exchange.
• It also collects waste substances like urea and takes it to the liver from where it is removed.
• It helps in clotting and thereby heals the wounds.
• It takes the particular medicine to its receptors where it binds and produces the effect.
• It provides defence to the body by its powerful cells called WBCs.

Blood vessels

Blood vessels are the tube like structure that carry blood from the heart and bring back to it. There are three main types of blood vessels.

Arteries: These are the blood vessels that carry blood away from the heart and contain oxygenated blood except pulmonary artery. Their wall is composed of three layers, the inner most is called tunica intima and consists of epithelial tissues, middle layer called tunica media consists of smooth muscles and elastic fibres while the outer wall is made up of flexible connective tissues and called tunica adventitia. They lack valves. They are flexible and strong in order to sustain the blood pressure.

Veins: The blood vessels that bring the blood back to heart are called veins. They carry deoxygenated blood with the exception of pulmonary vein that brings oxygenated blood from the lungs. The composition of the walls of veins is similar to that of arteries but they have got thinner walls as compared to arteries. They have large lumen. They are supplied with semilunar valves in order to prevent the back flow of blood. Blood flows in the vein with the help of skeletal muscles.

Blood capillaries: They are the microscopic blood vessels that supply each cell in the tissue with the nutrients. They have the thinnest walls that are made up of endothelium. The blood capillaries allow the substances to diffuse through their thinner walls. Blood capillaries originate from arteries and form venules when they leave the organs.