Simple organisms like the unicellular bacteria and protozoa, or the multicellular jellyfish do not possess a circulatory system as diffusion through cell surfaces is enough to supply oxygen and nutrients. In arthropods and molluscs, a simple but open circulatory system is present. However, higher animals such as the vertebrates are equipped with a closed circulatory system that comprises blood vessels. The blood vessels ensure a ceaseless supply of oxygen-rich blood from the heart to every corpuscle at every niche of the body. To ensure that the oxygenated and deoxygenated blood don't mix, the blood vessels have been designated to particularly carry either of the two types of blood. Arteries and arterioles are associated with oxygenated blood, while the veins and venules are in charge of deoxygenated blood.
Capillaries, the third kind of blood vessel, are the smallest division and facilitate exchange between the blood and the tissues. Broadly, the blood vascular system is divided into two circuits, namely, the systemic circuit, which supplies functional blood to the entire body and returns impure blood to the heart, and the pulmonary circuit, which operates between the heart and the lungs. Let us explore the types and functions of the blood vessels in detail
Arteries supply blood to the body, as it is pumped from the heart. The route of the systemic arteries begins from the left ventricle, reaching every part of the body through smaller distributions. Hence, arteries are instrumental in supplying blood and nutrients to the organs. Arteries hold up to 10% of the blood in the body at any given time
The pulmonary artery of the pulmonary circuit is the only artery in the body that carries unoxygenated blood from the heart (right ventricle) to the lungs.
Arterial vessels are built on three layers of tissue viz., the tunica interna, tunica media and the tunica externa/ tunica adventitia, composition of each layer is subject to variation in different arteries. Tunica media, the middle layer of the arterial wall, is the thickest layer and regulates blood pressure, by changing the diameter of the vessel. The composition of the tunica media particularly varies with the type of vessel and the functions it performs
The cardiovascular system is assisted by three major types of arteries. These are as follows−
The Elastic Arteries
Also known as conduit arteries, they are the largest arteries and the closest to the heart, such as the aorta and the pulmonary arteries
Given that they receive blood directly from the heart, the tunica media is supported with lots of collagen and elastin, to accommodate the blood surge. This allows easy stretching of the arterial walls as the heart pulses, given that elastin can reach a strain level of 200% before it collapses
The elastin also helps in maintaining a constant pressure in the arteries
The elastic arteries buffer the cyclic changes that occur in the arterial blood pressure.
The collagen fibres of the arterial wall bear the mechanical load at higher stresses, while the elastin fibres handle the low-pressure wall stretches
The Muscular Arteries
These are medium-sized blood vessels that arise from the larger elastic arteries
As the name suggests, the tunica media of muscular arteries is predominated by smooth muscles and a lesser amount of elastin fibres
These arteries are distributing arteries; they collect blood from the elastic arteries and distribute it to the various organs and parts of the body
The smooth muscles in the tunica media contract or relax as needed via actomyosin-mediated contractions and relaxations, decreasing or increasing the lumen of the artery to regulate the blood pressure through the arteries
Examples− radial artery, brachial artery, femoral artery, etc.
Arterioles
These are the smallest of the arterial blood vessels, with an average internal diameter of only 30 𝝁pm
The walls usually comprise only 1-2 layers of smooth muscles
Their main function is to lead the blood from the larger arteries into the capillaries
Arterioles function as vascular resistors, and regulate the blood flow and the blood pressure, owing to their smaller diameter which opposes blood flow. Hence, they are also known as resistance vessels
Veins are those blood vessels that collect the deoxygenated blood from the tissues back to the heart for re-oxygenation. The blood flow is opposed to that of the arteries- in the venous system, the blood enters the venules from the capillaries and progresses through the larger veins and ultimately reaches the heart. At any given time, the veins hold up to 70% of the blood in the body
While the systemic veins carry deoxygenated blood back to the right atrium, the pulmonary veins supply oxygen-rich blood from the lungs to the heart
The veins are built with the same three layers of tissue as the arteries. However, the diameter and the number of smooth muscles are lesser, as per the decreased pressure of blood flowing through them. Additionally, veins are also equipped with valves that ensure the flow of blood in the forward direction
Systemic veins
These veins are part of the systemic circuit, hence they carry blood from the body tissues to the right atrium
Vena cava, the largest vein of the body, is a systemic vein that empties blood from the upper and lower regions of the body to the right atrium
Pulmonary veins
These veins are associated with the pulmonary circuit that operates between the lungs and the heart. They carry oxygenated blood from the lungs into the heart’s left atrium
There are 4 pulmonary veins; two emerge from each lung
Systemic veins
These veins lie near the skin surface, and are present in large numbers
Examples− Saphenous veins, cephalic veins, basilic veins, etc
Deep Veins
These veins are deep-seated within the muscle tissues and often are accompanied by a systemic artery of the same name. For example, the femoral vein occurs beside the femoral artery, the brachial vein beside the brachial artery, etc
Venules
These are the smallest veins of the body, their function is to collect the deoxygenated blood from the capillaries and supply it to the larger veins
| Arteries | Veins |
|---|---|
| Arteries (except pulmonary artery) supply blood to the body from the heart | Veins carry blood from the body to the heart (except the pulmonary vein) |
| Carry oxygenated blood to the heart | Return deoxygenated blood to the heart |
| Arteries have thicker walls | Venular walls are thinner |
| Arteries are built with more musculature in the walls | Lesser amount of smooth muscles are present in the tunica media |
| Tunica media is the thickest layer. | Tunica adventitia is the thickest layer |
| Arterial walls have higher elastin content | Veins have little to no elastin |
| Arteries are more deep-seated | Veins are usually closer to the body surface |
| Arteries don't have valves | Veins are equipped with valves |
| Blood flowing through arteries is under high pressure | Veins carry blood with lesser pressure |
| Blood is oxygenated and hence has a reddish colour | Veins carry blood that is deprived of haemoglobin, hence has a bluish hue |
| Blood CO2 levels are lower | Blood CO2 levels are higher |
The cardiovascular system comprises an efficient network of blood vessels of three types, each with their specific functions
The arteries supply oxygen-laden blood from the heart to the other body parts
Arteries are of three kinds- elastic arteries, muscular arteries, and the arterioles
Veins return oxygen-depleted blood from tissues to the heart
Veins may be deep-seated or superficial
For every artery that supplies blood to an area, there is a corresponding vein that carries blood away
Q1. What is the vasa vasorum?
Ans. Vasa vasorum are the “vessels in vessels”, i.e., the vessels that supply blood to the walls of the arteries and the veins..
Q2. Are bronchial arteries the same as pulmonary arteries?
Ans. No. Bronchial arteries are a part of the systemic circulation and supply oxygenated blood to the lungs. Pulmonary arteries make up the pulmonary circuit, carrying deoxygenated blood from the right ventricle to the lungs for oxygenation
Q3. What are some of the diseases related to the blood vessels?
Ans. Atherosclerosis, Vasculitis, Aneurysms, Coronary Artery Diseases, etc are related to the blood vessels
Q4. Why don't arteries have valves?
Ans. Arteries receive blood from the heart which is already under high pressure, and this naturally prevents any backflow
Q5. Which blood vessels are associated with the brain?
Ans. Two arteries, namely the carotid artery and the vertebral artery supply oxygen to the brain, while the jugular vein carries oxygen-depleted blood away from the brain
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