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The Circulatory System


MIT Self Work
Made by:
Vaisakh Murali
Group 4B 2018

Introduction

The circulatory system is centred on the Heart, a muscular organ that rhythmically pumps blood around a complex network of blood vessels extending to every part of the body. Blood carries the oxygen and nutrients needed to fuel the activities of the body’s tissues and organs, and it plays a vital role in removing the body’s waste products. An average-sized adult carries about 5 litres of blood.


Parts of the System


All the output of blood from the left side of the heart goes into the aorta, the body’s largest artery. Other arteries branch from the aorta to supply blood to the head, limbs, and internal organs. The blood is drained from all these parts by veins into two large vessels, the inferior and superior venae cava, which deliver the blood back to the right side of the heart.

Heart

The heart contracts tirelessly – more than 2.5 billion times over an average lifetime – to pump blood around the body. These contractions are triggered by electrical impulses that originate in a specialized area of heart tissue. The signals spread through the muscle in the wall of the heart via a network of conducting fibres.

The heart has two upper chambers, called atria, and two lower chambers, called ventricles. Blood from the body arrives in the right atrium. This blood is low in oxygen, and is shown here in blue. The blood passes to the right ventricle, which pumps it to the lungs to pick up more oxygen. The left atrium receives oxygen-rich blood (red) back from the lungs. This passes to the left ventricle, which pumps it by way of the aorta to the body.

Heart Valves

At the exit of each heart chamber lies a valve, which ensures the one-way flow of blood through the heart and into the circulation. These valves are made of flaps that open to allow blood to pass through but snap tightly shut to prevent backflow. The valves have three flaps, except for the valve between the left atrium and left ventricle, which has two.



Blood Vessels

If an adult’s blood vessels were laid end to end, they would stretch out over 100,000 km (62,500 miles). There are three main types of vessel. Arteries carry blood from the heart to the body’s tissues, while veins carry blood back from the tissues to the heart. Small arteries are called arterioles and small veins are referred to as venules. The third and smallest type of vessel, capillaries, form a network connecting the smallest arterioles with the smallest venules.


Blood

Blood is composed of a straw-coloured fluid, plasma, and huge numbers of blood cells that float in the plasma. Of the two main types of blood cell, red blood cells carry oxygen to the body’s tissues, and white blood cells help defend the body against infection. Blood also transports nutrients, proteins needed for blood clotting, and waste products.



Blood Cells


A drop of blood contains millions of red cells, and each cell contains 250 million molecules of a substance called haemoglobin. In the lungs, oxygen binds to haemoglobin, but in the tissues the oxygen is released again. Several types of white blood cell exist, and all are important to the body’s immune system. Platelets are tiny cells that are needed for blood clotting.

Blood Clotting

If a blood vessel is damaged, a clot forms to stop blood leaking. First, platelets stick together to form a plug that stops the leak. At the same time, a complex sequence of chemical events in the blood leads to the production of long strands of a protein called fibrin. These bind the blood cells and debris together to form a gel-like clot that gradually solidifies. The solid clot remains until the blood vessel has been repaired.


How does the Circulatory System Work?

The blood circulatory system (cardiovascular system) delivers nutrients and oxygen to all cells in the body. It consists of the heart and the blood vessels running through the entire body. The arteries carry blood away from the heart; the veins carry it back to the heart. The system of blood vessels resembles a tree: The “trunk,” the main artery (aorta), branches into large arteries, which lead to smaller and smaller vessels. The smallest arteries end in a network of tiny vessels, the capillary network.
There is not only one blood circulatory system in the human body, but two, which are connected: The systemic circulation provides organs, tissues and cells with blood so that they get oxygen and other vital substances. The pulmonary circulation is where the fresh oxygen we breathe in enters the blood. At the same time, carbon dioxide is released from the blood.
Blood circulation starts when the heart relaxes between two heartbeats: blood flows from both atria (the upper two chambers of the heart) into the ventricles (the lower two chambers) which then expand. The following phase is called ejection period, which is when both ventricles pump the blood into the large arteries.
In the systemic circulation, the left ventricle pumps oxygen-rich blood into the main artery (aorta). The blood travels from the main artery to larger and smaller arteries into the capillary network. There the blood releases oxygen, nutrients and other important substances and takes on carbon dioxide and waste substances. The blood, which is now low in oxygen, is now collected in veins and travels to the right atrium and into the right ventricle.
Now pulmonary circulation starts: The right ventricle pumps blood that carries little oxygen into the pulmonary artery, which branches off into smaller and smaller arteries and capillaries. The capillaries form a fine network around the pulmonary vesicles, grape-like air sacs at the end of the airways. This is where carbon dioxide is released from the blood into the air contained in the pulmonary vesicles and fresh oxygen enters the bloodstream. When we breathe out, carbon dioxide leaves our body. Oxygen-rich blood travels through the pulmonary vein and the left atrium into the left ventricle. The next heart beat starts a new cycle of systemic circulation.
References:
3. NIH

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