Why Do Arteries Have Thick Walls?

Arteries, Veins, and capillaries are the three main types of blood vessels. These blood vessels form an important part of the circulatory system as they carry blood to and from in between the heart and the body. In this process, as it is quite well known, the arteries carry the blood from the heart to the body. During this, the blood is released or pumped from the blood at high pressure so that the blood flows through to different organs and reaches the extreme points of the body. To withstand this high pressure and maintain a significant pressure within themselves, the arteries have thick walls. This is not true for the veins as the pressure in the veins is less as there is no pumping of blood into the veins, unlike the arteries.

The structure of the arterial walls is explained in detail below. This explanation also provides the answer to the question - Why do arteries have thick walls, and what are Veins and Capillaries?

Arteries

Arteries, as it is very well known, carry oxygenated blood from the heart to the different parts of the body. The arteries that actually carry the blood from the heart to different organs is known as the systemic artery. In exception, there are two arteries: the pulmonary artery and the umbilical artery, which carry the blood to the organs that oxygenate the blood. The walls of the arteries are thick and elastic. This is because, as mentioned before, the pressure of the blood that is pumped out from the heart is high, and it is important for the artery to have a strong wall so that it does not burst. The arterial walls also have to be elastic so that they can expand and contract with each heartbeat. A strong wall with an elastic property allows the artery to withstand the high blood pressure that is necessary for the healthy functioning of the body.

Arteries vary greatly in size and thickness, depending on their location in the body. Those arteries that are close to the heart are more dilated than those further away from it. A greater amount of blood is required to be pumped at high pressure when the heart is working hard. For instance, the aorta is the largest artery in the body, and it has to withstand very high blood pressure.

Veins

The veins are different from the arteries as they do not have to withstand high blood pressure. The pressure in the veins is less as there is no pumping of blood into the veins, unlike the arteries. The walls of the veins are thin and less elastic than those of the arteries. This is because the veins do not need to be as strong as the arteries as they do not have to withstand high pressure. Also, the veins do not need to be elastic so that they can expand and contract with each heartbeat as the blood flows through them at a much slower rate than it does through the arteries.

Capillaries

The capillaries are the smallest type of blood vessel, and they are found in all tissues of the body. The walls of the capillaries are very thin and delicate so that they can allow nutrients and oxygen to diffuse from the blood into the tissues and vice versa. The capillaries also have a low pressure so that the nutrients and oxygen can easily flow in both directions.

Wall of Arteries

The walls of the arteries can be divided into three layers. Those are from outside to inside: 

1. Tunica externa, 

2. Tunica media and, 

3. Tunica intima. 

The first layer Tunica externa or also known as tunica adventitia is composed of collagen fibers and elastic tissue. This layer does not have a well-defined boundary as we go towards the lumen of the artery that carries the blood. Normally, it is considered when it reaches or touches the connective tissue layer or the Tunica media. Tunica media or only media is made up of smooth muscle cells, elastic connective tissue and collagen fibres. Following, the Tunica media is the Tunica intima or commonly known as intima is the last layer and the layer which is directly in contact with the blood. This layer is completely made up of endothelial cells. The sectioning of the arteries is shown below in the given schematic diagram.

The elastic layer aids the arteries in providing flexibility for bending and fitting through the places. It is this portion of the Tunica media that creates the difference of the thickness between arteries and veins, arteries and other blood vessels. The presence of more elastic cells and smooth muscle cells in the media of arteries than the veins or any other blood vessels. The main reason is that arteries have a higher blood pressure than observed by other parts of the circulatory system. So, the answer to the question as to why are arteries thicker than veins is that because arteries have higher blood pressure.

One of the important reasons for the question - why do arteries have thick elastic walls, is that the pressure varies in the arteries during the cardiac cycle. The elasticity and the muscular nature provide the arteries with the flexibility to withstand the extreme pressure and pressure changes during the blood flow. The pressure is the highest when the heart contracts and the lowest when the heart relaxes. The variation in the blood pressure produces a pulse that can be felt in different regions of the body. This pulse is known as the radial pulse. 

Apart from the main arteries, the arterioles, which carry oxygenated blood to the extreme ends of the body, also have thick elastic and muscular media. They have a major influence on the local blood pressure and overall blood pressure. They are kind of 'adjustable nozzles' across which maximum pressure drop takes place. The combination of the cardiac output of blood and systemic vascular resistance, which refers to the collective resistance of blood arterioles, are the major deciding factors of arterial blood pressure at any given moment.

Veins, on the other hand, have a smaller Tunica media, comprising a less elastic and muscular layer than arteries because veins do not work in a contractile manner like the arteries and are not subject to high systolic pressure. Hence, to the question - Why are arteries thicker than veins? The answer is - Veins are not subject to high blood pressure, unlike the arteries. Similarly, for the question of why arteries are thick-walled, it is because the thick elastic and muscular walls of the arteries not only help them sustain the cardiac output pressure but also maintain blood pressure throughout the circulatory system of the body. 

The collapse or change in the blood pressure can be catastrophic for the body and hence fatal to the living organism. It is highly important to maintain a constant and regulated blood pressure that works towards making the blood reach different parts of the body. The arteries work in a contractual manner that allows them to make the blood reach the extreme portions. In that process, the pressure difference between the arterioles and the arteries has a huge role to play. 

Thus, to maintain a significant pressure drop and sustain the cardiac blood pressure, it is important structurally for the arteries to have thicker elastic and muscular walls. The arterioles near the skin of the body have more muscular walls than elastic, and the systemic artery has more elastic walls than muscular, both providing different functionality for bending and fitting in close places as and when required. Thus, it is vital for the arteries to have thick walls.