Anatomy and Physiology of Cardiovascular System
Overview of cardiovascular system
The cardiovascular system is also called as ‘blood vascular system’ or ‘circulatory system’. The main function of the circulatory system is to ‘circulate’ the blood pumped by the heart around a closed system of blood vessels. Cardiovascular system is responsible for transportation of hormones, food, and gases to and from the cells. It consists of heart and closed system of blood vessels.
The cardiovascular system is mainly responsible for maintaining homeostasis. Homeostasis refers to the maintenance of internal environment, which can be affected by factors like temperature, salinity, acidity, and the concentrations of nutrients and wastes.1
Homeostasis depends on constant and controlled movements of the blood through the capillaries that filter through every tissue and reach every cell. The blood capillaries are microscopic in nature. They are mainly responsible for the ‘final transport function’, which involves movement of nutrients into the cells and removal of waste products from the cells.
Regulation and integration of the cardiovascular system is controlled by various mechanisms. These mechanisms make sure that a constant internal environment is maintained within the cells with respect to providing nutrition and removal of waste.
Components of the circulatory system include:
- Blood vessels
Blood is a mixture of cells and watery liquid, called ‘plasma’. It addition to this, the blood also consists of hormones, clotting agents, nutrients and waste products. There are three kinds of blood cells.
I. Erythrocytes or Red Blood Cells
These cells are made up of 40–45% of the blood and give its characteristic ‘red color’ to the blood. The red blood cells are tiny and contain a groove in the center, which contains ‘hemoglobin’. The hemoglobin molecule can bind to the oxygen molecules during exchange of gases in the lungs. It can readily shed the oxygen it is carrying and allows the cells to absorb it.
II. Leukocytes or White Blood Cells
These cells protect the body against ‘infection and invasion’. White blood cells are of three types.
They are so called because they contain granules that hold the digestive enzymes. There are three kinds of granulocytes.
Neutrophils—These cells ingest and digest the invading bacteria.
Eosinophils—These cells kill the parasites that are involved in allergic reactions.
Basophils—These cells are also involved in allergic reactions.
Their chief function is to protect the body’s immune system. There are two kinds of lymphocytes.
T-lymphocytes: They direct the activity of the immune system.
B-lymphocytes: They produce antibodies to destroy the foreign body.
They are the largest white blood cells. Once they are inside the body tissues, they convert themselves into macrophages and consume and destroy bacteria, old and dead cells of the body.
Platelets are also called as ‘thrombocytes’. They are small, colorless, enucleated bodies, which are 2–3 microns in size. The platelets help the blood to form the clot by clumping and releasing a protein called ‘fibrinogen’, which turns into fibrin. This fibrin forms long threads, which entangle the red blood cells to form the clot.
Plasma consists of 90% water and the other 10% includes enzymes, hormones, clotting agents, immunoglubulins, proteins that carry hormones, vitamins, cholesterol etc. Plasma also contains glucose and electrolytes like sodium, potassium and calcium.
Blood vessels comprise the vascular system. All blood vessels consist of a similar basic structure, which includes:
- Tunica Intima—This is the inner thinnest layer.
- Tunica Media—This is the middle thickest layer.
- Tunica Adventitia—This is outer most layer and contains nerves.
Types of Blood Vessels
Blood is carried to various parts of the body through muscular blood vessels called ateries. All arteries carry oxygenated blood except for pulmonary and umbilical arteries. Blood is carried out of the heart through the largest artery called ‘aorta’.
The various branches of the aorta are.
- The carotid artery
- The subclavian artery
- The celiac trunk
- The mesenteric artery
- The renal artery
- The iliac artery
These are the smaller divisions of the arteries, called arterioles and are located between the arteries pertaining to muscles and the capillaries.
Arterioles divide further into capillaries. Capillaries are tiny blood vessels measuring 10 micrometers in diameter. They pass blood from the arteries into the veins. Their walls are thin and allow materials to pass into the capillaries. Different types of capillaries exist and perform different functions in the body.
These are the blood vessels that collect blood from the capillary plexuses and join together to form veins.
As opposed to arteries, the veins transport low oxygen blood back to the lungs and heart. Valves are present in the veins to help the blood to flow in the proper direction and prevent backward flow. These valves are like gates and allow the blood to flow in one direction. The subclavian vein, the jugular vein, the renal vein and the iliac vein are the large collecting blood vessels. The superior vena cavae and inferior venacavae are the largest veins that carry low oxygen blood to the heart.
Apart from transporting oxygenated blood from the lungs to various organs of the body and collecting deoxygenated blood from these organs and taking it back to the lungs, the blood and blood vessels also maintains the body temperature.
Maintenance of Body Temperature Through Thermoregulation
Thermoregulation is the ability of the body to maintain normal temperature and is brought by the methods of vasodilation (dilation of the vessels by releasing the vascular smooth muscle in the vessel walls) and vasoconstriction (narrowng of the blood vessels by contraction of the vascular smooth muscle of the vessel walls).
What is Blood Pressure?
Blood pressure is the pressure of the blood exerted on the walls of the blood vessels. Blood pressure is created when the heart pumps the blood into the arteries and the force exerted by the arteries to resist the flow of blood.
Systole is the contraction of the heart chambers. Systolic blood pressure is the highest pressure in the arteries, which occurs during contraction of heart at the beginning of the cardiac cycle. (cardiac cycle is the term referred to the events related to the flow of blood that occur from the beginning of one heartbeat to the beginning of the next).2
Diastole is the relaxation of the heart chambers. Diastolic pressure is the lowest pressure in the arteries, which occurs when heart relaxes at the resting phase of the cardiac cycle. The blood pressure is expressed in millimetres of mercury (1 mmHg=133 Pa). The systolic blood pressure in the arteries is 120 mmHg s and diastolic pressure is 80 mmHg. It is written as 120/80 and said as ‘120 over 80’.
The heart is cone-shaped, four-chambered muscular organ and sized about a man’s fist. It weighs about 250–300 g. The heart is located in the chest area, between the lungs with two-thirds of the mass towards the left midline and its apex pointing towards the left. The walls of the heart are composed of cardiac muscles. The heart is enclosed in a pericardial sac and is lined by serous membrane. The serous membrane consists of parietal and visceral layer. The visceral layer forms the epicardium.
The heart consists of three layers.
- Epicardium—Outer layer
- Myocardium—Middle layer
- Endocardium—Inner layer
Internally, the heart is divided into four chambers
- Right atrium
- Right ventricle
- Left atrium
- Left ventricle
The right atrium receives deoxygenated blood from the superior and inferior vena cava. The left atrium receives oxygenated blood from the pulmonary veins. The walls of the atria are thin. The walls of the ventricle are thick and pump the blood forcefully out of the heart.
The heart has two types of valves to maintain the flow of blood in the correct direction.
Atrioventricular valves: They are present between the atria and the ventricles.
- Semilunar valves: They are the valves present at the bases of large vessels leaving the ventricles.
Tricuspid valve is the right atrioventricular valve. Mitral or bicuspid valve is the left atrioventricular valve. The pulmonary semilunar valve is the one present between the right ventricle and pulmonary trunk. The aortic semilunar valve is the valve between the left ventricle and the aorta.
When the ventricles contract, atrioventricular valves close to prevent blood flow back into the atria. When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles.3
The circulatory system circulates approximately five liters of blood constantly throughout the body. Depending on the course of blood, circulation can be divided into:
- Systemic circulation
- Pulmonary circulation
- Coronary circulation
Systemic circulation starts from aorta, where the oxygenated blood is carried to all parts of the body and back to the heart. The heart pumps the oxygen rich blood out through the arteries. Then the blood moves through the smaller blood vessels called capillaries and releases the oxygen and nutrients into the body’s cells. At the same time it carries, waste products like carbon dioxide. The capillaries unite to form venules and they ultimately to form two large venous trunks called inferior and superior vena cava. Inferior and superior vena cavae open into the right atrium of the heart. The blood now low in oxygen, flows into these veins, which carry it back to the heart.
Deoxygenated blood from all parts of the body enters the right atrium of the heart and moves into the right ventricle. From here, the blood is pumped through the pulmonary artery to the lungs. The oxygen-rich blood from the lungs is carried to the heart by the pulmonary veins. It enters the left atrium before flowing into the left ventricle. The oxygen-rich blood now, from the left ventricle is pumped out through the aorta to the rest of the body.
The heart needs to be supplied with blood and this is accomplished by coronary circulation; two coronary arteries and their branches do this. Coronary arteries are the vessels that supply oxygenated blood to the heart muscle itself. They are so named because they encircle the heart in the form of a crown. The right coronary artery supplies blood to the right atrium and right ventricle. The left coronary artery supplies blood to ventricles, inter-ventricular septum and left atrium. Most of the blood supplied by the coronary arteries is returned to the right atrium by coronary sinus.
Conduction System of Heart
The nerve impulse for cardiac contraction is transmitted through conduction system of heart and is directed by the nervous system. This system consists of following:
- Sinoatrial node (SAN)
- Atrioventricular node (AVN)
- Bundle of His
- Purkinje fibers
The whole sequence is summarized through the following steps.
- The sinoatrial node (SAN) is stimulated by the vagus nerve. The SAN is also called as ‘the pacemaker of the heart’. It is a tiny area of the heart muscle near the upper wall of the right atrium. The rhythmic contraction of the SAN (approximately 70 times per minute) helps the impulse to get dispersed across the atrial cardiac muscle.
- This leads to simultaneous contraction of both left and right atria, which in turn pushes the blood from atria into the ventricles through the tricuspid and bicuspid valves.
- The contractions in the atria send impulses, which in turn stimulate the atrioventricular node (AVN). The AVN is a mass of cardiac muscle present at the lower and central part of the right atrium of the heart.
- The impulses are transmitted from the AVN through the left and right branches of the Purkinje fibers into the Bundle of His. The Purkinje fibers and the Bundle of His are bundles of modified cardiac muscle fibers that transmit impulses from the atria through the AVN, to the ventricles.
- This causes the ventricles to contract, which in turn pushes the blood upwards into the arteries that take the blood away from the heart that is, the pulmonary artery taking blood to the lungs for purification and aorta distributing the blood to different parts of the body.
The heart produces four sounds, which can be heard through the instrument called ‘stethoscope’. The first sound ‘Lub’ and the second sound ‘Dub’ can be heard. The third and fourth sound cannot be heard. The first sound is produced by the closure of the atrioventricular valves. This is loud and heard for a long duration. The second sound is produced by the closure of the semilunar valves. It is of a short duration and has shrill sound.
Electrocardiography (ECG, EKG)
An electrocardiogram records the electrical current generated by the heart and is generally used to evaluate the cardiac status. P, R and T are the three slow, negative changes. The P wave is caused by the contraction of the atria, is also known as the depolarization of the atria. The waves Q, R, S are produced by the contraction of the ventricles, and also known as depolarization of the ventricles. T wave is produced by the relaxation of the ventricles is also called repolarization of the ventricles. The ECG is recorded at a speed of 25 mm/sec, and the voltages are calibrated so that 1 mV=10 mm in the vertical direction. Each small 1-mm square represents 0.04 sec (40 msec) in time and 0.1 mV in voltage.
The cardiovascular system is essential for the supply of oxygen and other vital nutrients to the various organs and organ systems of our body. It also is essential for maintaining the optimum body temperature, which is essential for the proper functioning of all the organ systems. Hence, the importance of maintaining proper cardiac health by following correct diet and healthy lifestyle cannot be overemphasized.
1. Available at: http://www.easternct.edu/personal/faculty/saull/sect9/resous9/cartos9/homeos9.html. Accessed on: 19th April 2008.
2. Guyton AC, Hal, JE. Textbook of Medical Physiology, 11th edn. Philadelphia: Elsevier. 2006.
3. Anthony CP, Thibodeau GA. Textbook of Anatomy & Physiology. St. Louis: Mosby, 1983. Anatomy Clipart (Designs4Free); Dennis Kunkel.
Written by: healthplus24.com team
Date last updated: February 05, 2015