What are the three main differences between arteries and veins?
Arteries are like highways that carry oxygen-rich blood away from the heart to the rest of your body. They have thick, muscular walls to handle the high pressure of blood pumping from the heart.
Veins are like smaller roads that bring oxygen-poor blood back to the heart. They have thinner walls and often have valves to prevent blood from flowing backward. Many veins are closer to the skin’s surface.

How are arteries and veins similar?
The arteries and veins have different functions in the circulatory system. They have many similarities in their overall structure and functionality. These similarities facilitate the smooth coordination needed for the human body’s adequate blood circulation.
First, arteries and veins are essential blood vessels for the circulatory system. Veins ensure that deoxygenated blood returns to the heart, while arteries carry oxygenated blood away. Their existence is essential to the complex process of blood flow in the human body. The similarities between arteries and veins are given below in detail.

Why are veins thinner than arteries?
Veins have much thinner walls than do arteries, largely because the pressure in veins is so much lower. Veins can widen (dilate) as the amount of fluid in them increases.

Are veins bigger than arteries?
No, Veins are generally smaller than arteries.
Arteries, start as large vessels, but decrease in size as they branch out. The aorta, the largest artery, is about 10–25 mm in diameter. On the other hand, Veins range in size from 1 mm to 2 cm in diameter. The largest veins are closest to the heart.

What are the two types of veins?
Veins are often categorized based on their location and any unique features or functions.
Pulmonary and systemic veins
Your body circulates blood on two different tracks called the systemic circuit and the pulmonary circuit. Veins are based on the circuit they’re found in:
Pulmonary veins. The pulmonary circuit carries deoxygenated blood from your heart to your lungs. Once your lungs oxygenate the blood, the pulmonary circuit brings it back to your heart. There are four pulmonary veins. They’re unique because they carry oxygenated blood. All other veins carry only deoxygenated blood.
Systemic veins. The systemic circuit carries deoxygenated blood from the rest of the body back to your heart, where it then enters the pulmonary circuit for oxygen. Most veins are systemic veins.

Do veins have valves?
Many veins, particularly those in the arms and legs, have one-way valves. Each valve consists of two flaps (cusps or leaflets) with edges that meet. Blood flowing toward the heart pushes the flaps open like a pair of one-way swinging doors. If gravity or muscle contractions try to pull the blood backward or if blood begins to back up in a vein, the flaps are pushed closed, so the blood does not flow backward. Thus, valves help the return of blood to the heart—by opening when the blood flows toward the heart and closing when blood might flow backward.

Sources:
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Structure and Function of Blood Vessels

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