Goals
- Understanding the structure of the heart.
- Knowing how arteries, veins and capillaries work.
- Knowing the stages of circulation.
- Being able to explain the function and importance of the lymphatic system.
The heart and the circulatory system
The circulatory or cardiovascular system is made up of three main parts - the heart, the blood vessels and the blood that flows through them.
The heart is a hollow muscle located inside the thoracic cavity, behind the sternum. It is protected by the ribs. An adult heart weighs about 300 g and is the size of its owner's fist.
The function of the heart is to pump blood to different parts of the body. The heart is divided into two halves, the right and the left. The septum separates the right and left sides of the heart. Both halves are divided into atria and ventricles. The atria (plural of atrium) are the places where blood is collected when it enters the heart. When the heart contracts, the blood leaves the ventricles through the arteries and is returned to the atria via the veins.
Your heart works continuously. This is only possible because your heart rests briefly between every contraction. This periodicity of contractions and pauses is called the heart rate. The heart rate can be measured from the blood vessels of the wrist or by using a blood pressure monitor.
The blood pumped by the heart circulates into the body via the blood vessels. They include thick-walled arteries. The main artery is called the aorta. It also includes thin-walled and narrow capillaries, as well as veins.
The heart functions independently. However, when necessary, the function of the heart can be influenced by both the nervous system and hormones.
The structure of the heart
The heart’s four chambers function like a double-sided pump. The heart needs to work at just the right rhythm. It is divided into right and left halves. These halves are further divided into atria and ventricles. The right ventricle pumps blood to the lungs whereas the left ventricle pumps blood to everywhere else in the body. For this reason, the left ventricle has the thickest muscle mass of all the heart's chambers.
Veins carry blood both to the right and left atria at the same time. The cycle begins when the two atria contract, which pushes blood into the ventricles. After this, the ventricles contract, which forces the blood out of the heart.
The valves between the atria and the ventricles prevent blood from returning to the ventricles. When the heart rests between its beats, the valves between the ventricles and arteries prevent the blood from flowing back into the heart. However, there are no valves between the atria and the veins.
- The heart is a muscle.
- The heart is located under your ribs.
- The heart is the size of its owner's thumb.
- The heart is a voluntary muscle.
- The heart pumps blood into the body from its atria.
- The heart consists of four chambers.
- The heart pumps blood into the muscles through the aorta.
1. Name the parts of the heart.
2. Name the blood vessels.
The function of the heart
The rhythm of your heartbeat is maintained by your sinoatrial (SA) node. The heartbeat impulse starts in a small bundle of specialised cells located in the right atrium. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.
The heart beats at an average rate of 60 to 75 beats per minute, pumping about five litres of blood during these beats. This means that the total volume of a person's blood is moved in a period of just one minute. Under intense exercise, your heart rate can increase by more than two hundred beats.
Thus, the heart can beat up to 100,000 times each day. During this time, your heart will pump almost 7,000 litres of blood. The heart can do this because it can rest for a combined time of four hours between beats each day.
The image shows the electrical activity of the heart (ECG).
Coronary arteries
Like all other muscles, the heart needs oxygen and nutrients in order to function properly.
Coronary arteries branch from the main artery, the aorta. They supply the heart muscle with oxygen.
The condition of the coronary arteries is crucial for the functioning of the heart. Congestion and calcification of the coronary arteries can lead to a lack of oxygen in the heart. Ultimately, these processes can lead to a heart attack.
Coronary artery diseases are caused by the narrowing of the coronary arteries. This restricts the supply of oxygen to the heart.
The systemic and pulmonary circulatory systems
In order to understand the function of the circulatory system properly, it is important to remember the following three things:
- The heart has two halves. There is no movement of blood between them.
- There are three main types of blood vessels. Arteries carry blood from the heart, whereas veins carry blood to the heart. The capillaries are small blood vessels that are located between cells and larger blood vessels.
- During one heartbeat, all the following things happen:
As the heart rests, the inferior vena cava and the superior vena cava carry carbon dioxide-rich blood from all over the body to the right atrium. At the same time, oxygen-rich blood is carried from the lungs to the left atrium. From the atria, blood flows immediately to the right and left ventricles.
Pulmonary arteries carry the blood into the lungs where carbon dioxide is replaced by oxygen. This takes place in the pulmonary capillaries, where carbon dioxide is removed from the blood and oxygen is bound to red blood cells. This oxidised blood returns to the heart via the pulmonary veins.
From the left ventricle, the blood, which is now under a high pressure, exits the heart via the aorta. The arteries that branch from the aorta carry the blood everywhere in the body. Nutrients and oxygen are released to all cells of the body through the thin walls of capillary blood vessels.
The blood flow from the left ventricle of the heart to the right atrium is called systemic circulation. The circulation of blood in the lungs (right ventricle → lungs → left atrium) is called pulmonary circulation.
Cell respiration uses the oxygen transported by the blood and produces toxic carbon dioxide. The inferior and superior vena cava transport blood containing carbon dioxide into the right atrium.
Blood pressure
Arterial blood pressure is expressed as a measurement with two numbers. In this measurement, one number is found on the top (systolic) and another is located on the bottom (diastolic), like a fraction. The top number refers to the amount of pressure in your arteries when your heart contracts, whereas the bottom number refers to the pressure in your arteries when your heart is resting between contractions. The average reading for a young adult is 120/80 mmHg.
In hypertension (high blood pressure), the blood pressure readings are significantly higher than normal. Symptoms of high blood pressure may include headache, dizziness and visual disturbances. Maintaining a healthy diet and exercising are good methods of preventing cardiovascular disease.
If the venous valves in the lower limbs are not working properly, blood can flow back into the veins and the pressure inside the veins can increase. This results in varicose veins, as the fluid is drained from the veins to the surrounding tissues. Typically, varicose veins can be found just under the skin.
Electrocardiography (ECG)
An electrocardiogram (electro refers to electricity, cardio to the heart and gram to a curve) is used to determine the stages of heart contraction.
Look at the image and the coloured middle part of the curve with numbers 1-6. There is electrical activity in the yellow-coloured parts of the heart.
In steps 1 and 2, the electrical impulses occur in the atria. As a result, the atria contract and the blood flows to the ventricles.
The electrical impulse is transmitted to the muscles of the ventricular walls. The ventricles contract (step 3), and the blood flows into the aorta and pulmonary arteries.
In step 5, the ventricles relax. In step 6, all muscle cells inside the heart are at rest.
Blood vessels
Arteries are the blood vessels that transport blood away from the heart. When the ventricles of the heart contract, blood is transported into the arteries. Arteries have thick muscular walls, which help them withstand high pressures caused by blood leaving the heart.
The largest artery is the aorta, which begins from the heart's left ventricle. The aorta branches above the heart into several arteries, which continue to branch into thousands of smaller arterioles throughout the body.
Arterioles connect with even smaller blood vessels called capillaries. You can see these capillaries, for example, in the white of your eyes. Capillaries distribute oxygen and nutrients into the surrounding cells.
All harmful substances produced in the body are transported from the intercellular space back to the capillaries for further transport.
Veins are the blood vessels that transport blood to the heart. They carry oxygen-poor blood, and therefore appear as blue through the skin. Little pressure remains by the time blood leaves the capillaries and enters the venules. The flow of blood back to the heart is referred to as venous return. It is dependent on muscle action.
Muscle contractions that occur near the veins help to squeeze the blood through venous valves. The purpose of these valves is to prevent blood from flowing in the wrong direction.
- The aorta
- Thick walls
- Contain valves
- Can sometimes be seen as blue lines under your skin.
- Strong pressure
- Distribute oxygen and nutrients to the cells.
The lymphatic system
The lymphatic system consists of lymphatic vessels and lymph nodes. Blood plasma leaks into the body's tissues through the thin walls of capillary blood vessels. The purpose of the lymphatic system is to remove these fluids and materials from tissues, returning them to the bloodstream via lymphatic vessels. The lymphatic system also plays a key role in transporting fats into your cells from the intestines.
The structure of the lymphatic vessels resembles that of the blood vessels, as the lymphatic vessels also have valves that cause lymph to eventually flow forward instead of travelling backwards. The contraction of your muscles and the pressure inside the arteries acts as a kind of pump that helps the fluid move around your body. Exercise can help the lymphatic system to function more effectively.
The lymph passes through lymph nodes before returning to the bloodstream via veins located near the heart. Lymph nodes can be located, for example, in the armpits and the groin. They are rich in white blood cells that fight microbes that may have entered your lymphatic system. This explains why your doctor may test your neck to see if your lymph nodes are swollen when you are sick.
Summary
- The heart is a tireless muscle with four chambers. It pumps blood and heat into the body by contracting.
- Arteries are blood vessels that carry blood from the heart to various parts of the body, transporting oxygen and heat.
- Veins are blood vessels that carry blood to the heart. The blood flows in the right direction because of muscle contractions and valves.
- The capillaries exchange substances through their thin walls. The exchange of gases (oxygen and carbon dioxide) in the lungs is handled by the pulmonary circulation.
- The circulatory system carries oxygen and nutrients throughout our body. Blood transferred to the tissues is transported back to the bloodstream through the lymphatic system and the lymph nodes.
- Key words: heart, arteries, veins, capillaries, aorta, coronary artery, systemic circulation, pulmonary circulation, lymphatic system.
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