The **circulatory system** is the body system responsible for the transportation of oxygen and nutrients in the body, and picks-up waste materials such as carbon dioxide for excretion. The human circulatory system is composed of the following structures–**the heart, blood vessels in the form of arteries, veins, capillaries, and lastly, the blood**.

**Blood vessels** are pathways or channels that function to transport blood from the heart to the body’s tissues, and vice versa. There are three types of blood vessels called the **arteries, veins, and capillaries**.

Three types of blood vessels:

1. **Arteries** – transport blood away from the heart. These blood vessels carry oxygen-rich blood, that is why they have relatively thick walls that help them withstand the strong pressure from the contraction of the heart.

2. **Veins** – brings back blood from the body tissues to the heart. It carries deoxygenated blood, and it has thinner walls compared to the arterial walls.

3. **Capillaries** – are the smallest type of blood vessel, that connects the veins and the arteries. The main function of the circulatory system to bring nutrients and absorb carbon dioxide is done in these vessels.

The **pulmonary circulation** transports the deoxygenated blood from the right ventricle of the heart to the lungs and then returns oxygenated blood to the left atrium of the heart. The **systemic circulation**, on the other hand, pumps oxygen-rich blood to all parts of the body through the arteries and picks-up carbon dioxide and waste materials from the body through the veins.

The nervous system works closely with the circulatory system as the circulatory system provides the brain with enough oxygen to function and in return, the nervous system regulates and controls the heart function and blood pressure. The specific division of the nervous system called the **autonomic nervous system** directs and influences our heart rate.

The neurotransmitter produced in the brain and released by the autonomic nervous system can either increase the heart rate or decrease heart rate.

While standing, the blood pressure will be higher in the arms. because the arms are closer to the heart than the legs. Given the blood has to travel a longer distance to and from the legs. the legs have a lower level of blood pressure.