READ: Cardiovascular and Respiratory Systems
4. Structures
Parts of the Cardiovascular System
Your heart pushes the blood around your body through the blood vessels. The heart, shown in Figure below, is made of cardiac muscle. The heart is connected to many blood vessels that bring blood all around the body. The cardiac muscle contracts and pumps blood through the blood vessels.
Blood is collected in the heart and pumped out to the lungs, where it releases carbon dioxide and picks up oxygen before it is pumped to the rest of the body.
Blood Vessels
The job of the blood vessels is to move the blood around the body. There are three main types of blood vessels in the body.
- Arteries are blood vessels that carry blood away from the heart. Arteries have thick walls that have a layer of smooth muscle, as shown in Figure below. Arteries usually carry oxygen-rich blood around the body. The blood that is in arteries is under pressure. The contractions of the heart muscle causes blood to push against the walls of the arteries. This "push" is referred to as blood pressure. Blood pressure is highest in the arteries and decreases as the blood moves into smaller blood vessels. Thick walls help prevent arteries from bursting under the pressure of blood.
- Veins are blood vessels that carry blood back to the heart. Veins have thinner walls than arteries do, as you can see in Figure below. The blood in veins is not under pressure. Veins have valves that stop blood from moving backward. Blood is moved forward in veins when the skeletal muscles squeeze the veins. Blood that is carried by veins is usually low in oxygen. The only veins that carry oxygen-rich blood are called the pulmonary veins, which carry blood to the heart from the lungs.
- Capillaries these are the tiniest blood vessels in the body. Every cell in the body needs oxygen, but arteries are too large to bring oxygen and nutrients to single cells. Further from the heart, arteries form capillaries. The walls of capillaries are only as thick as a single layer of cells. Capillaries connect arteries and veins together, as shown in Figure below. Capillaries also send water, oxygen and other substances to body cells, while they collect carbon dioxide and other wastes from cells and tissues. Capillaries are so narrow that blood cells must move in single file through them. A capillary bed is the network of capillaries that supply an organ with blood. The more active a tissue or organ is, the more capillaries it needs to get nutrients and oxygen.
Arteries are thick-walled vessels with many layers, including a layer of smooth muscle.
The walls of veins are not as thick as artery walls; veins have valves that stop blood from flowing backward.
Capillaries connect arteries and veins.
Blood
Blood is a body fluid that is a type of connective tissue. Blood is made of blood cells, and a liquid called plasma. The main types of cells found in blood are red blood cells and white blood cells.
- Red blood cells carry oxygen. Oxygen-rich blood is bright red and oxygen-poor blood is dark red.
- White blood cells fight against infection and disease.
The cardiovascular system of humans is "closed." That means the blood never leaves the blood vessels inside of the body Other organisms have blood vessels that interact with the environment.