Combined with the cardiovascular system, the circulatory system helps fight off disease, helps the body maintain a normal body temperature, and provides the right chemical balance for the body to achieve homeostasis, or a state of stability among all its systems.
The circulatory system consists of four major components:
- heart
- arteries
- veins
- blood
About the size of two adult hands held together, theheartrests near the center of the chest. Thanks to consistent pumping, the heart keeps the circulatory system working at all times.
心wall
There are three layers of the heart wall. The epicardium is the heart wall’s outer layer, the myocardium is the middle — and muscular — layer, and the endocardium is the heart’s innermost layer.
Chambers
The heart has four chambers: therightandleftatria, and therightandleftventricles. Together, they make up the heart’s internal cavity.
The four chambers play an important role in circulation. The atria receive blood from the veins, while the ventricles push blood out of the heart. Because the ventricles have to be much stronger to perform this pumping activity, their myocardial layers are thicker than those of the atria.
Arteriescarry blood away from the heart.
The artery walls have three layers: tunica intima (inner), tunica media (middle), and tunica externa (outer).
The middle layer is usually the thickest. It’s made up of smooth muscle that changes the size of the artery to regulate blood flow.
There are three main types of arteries. They get smaller and smaller the further they are from the heart.
Elastic arteries
The aorta and pulmonary arteries are the elastic arteries. They receive blood directly from the heart and need to be elastic to accommodate the surge and contraction as blood pushes through with each heartbeat.
The aorta is the body’s most important artery.
Pulmonary arteries take deoxygenated blood from the right ventricle to thelungs. They’re the only arteries that carry deoxygenated blood.
Muscular arteries
The muscular arteries move blood from the elastic arteries through the body. They’re made of smooth muscle, which can expand and contract as blood flows. Thefemoralandcoronaryarteries are two examples of muscular arteries.
Arterioles
The smallest arteries are the arterioles, which move blood from the muscular arteries to thecapillaries. The capillaries connect the arteries, which take bloodfromthe heart, and the veins, which take bloodtothe heart.
The number of capillaries in a body system depends on the amount of material exchange. Skeletal muscle, theliver, and the肾脏all have a large number of capillaries because their body systems need a lot of oxygen and nutrients. Thecornea of the eyeis one area that has no capillaries.
The blood moves back to the heart throughveins.
The blood travels from the capillaries into the venules, which are the smallest veins. As the blood moves closer to the heart, the veins get larger and larger.
Like the arteries, veins have walls made up of layers called the tunica intima, tunica media, and tunica externa. There are someimportant differences between the arteries and veins:
- In veins, the walls have less smooth muscle and connective tissue.
- The walls of veins are thinner than artery walls.
- Veins have less pressure and can hold more blood than arteries.
At any time,
Valves
The veins include valves, small pieces of tissue which keep blood flowing in the right direction.
中型和大型的阀门静脉保持blood flowing towards the heart. In the arms and legs, these valves make sure gravity doesn’t pull blood in the wrong direction.
有四个阀门的心。
Thetricuspid valveseparates the right atrium from the right ventricle. Themitral valve, or bicuspid valve, separates the left atrium from the left ventricle.
The remaining two valves are the semilunar valves. Thepulmonic valve, or pulmonary valve, separates the pulmonary artery from the right ventricle. Theaortic valveseparates the aorta and the left ventricle.
Blood is the transport medium of nearly everything within the body. It moveshormones, nutrients, oxygen, antibodies, and other important things needed to keep the body healthy.
The blood has four main components.
Plasma
About 55 percentof the blood isplasma. Plasma is what makes blood liquid.
Plasma moves blood cells through the body by way of the circulatory system. It also carries hormones, nutrients, antibodies, and waste products.
Plasma is made up of:
- water
- salts
- sugar
- fat
- protein
Red blood cells
Red blood cells, also called erythrocytes, make upabout 40 to 45 percentof the blood’s volume. These cells have no nucleus, which means they can easily change shape as they move through the body’s arteries and veins.
Red blood cells contain a protein called hemoglobin. It carries oxygen from the lungs to the rest of the body and returns carbon dioxide to the lungs, where it’s exhaled.
White blood cells
White blood cells, also called leukocytes, make up just1塞ntof the blood. They protect the body from infection.
There are five major types of white blood cells.
Most white blood cells areneutrophils, which live for less than 1 day. Neutrophils are the body’s immediate response team.
Types oflymphocytesinclude B lymphocytes (B cells) and T lymphocytes (T cells). B lymphocytes make antibodies, while T lymphocytes regulate other immune cells and target infected cells and tumors.
The other major types arebasophils, eosinophils, and monocytes.
Platelets
Platelets, also called thrombocytes, are cell fragments.
Platelets are essential for blood clotting. They stick to an injured blood vessel lining to provide the basis for a clot. This stops bleeding and promotes healing.
Oxygen enters the bloodstream through tiny membranes in the lungs that absorb oxygen as it’s inhaled. As the body uses oxygen and processes nutrients, it creates carbon dioxide, which your lungs expel as you exhale.
The circulatory system works thanks to constant pressure from the heart and valves throughout the body. This pressure ensures that veins carry blood to the heart and arteries transport it away from the heart. (Hint: To remember which one does which, remember that that “artery” and “away” both begin with the letter A.)
There are three different types of circulation that occur regularly in the body:
- Pulmonary circulation.This part of the cycle carries oxygen-depleted blood away from the heart, to the lungs, and back to the heart.
- Systemic circulation.This is the part that carries oxygenated blood away from the heart and to other parts of the body.
- Coronary circulation.This type of circulation provides the heart with oxygenated blood so it can function properly.
Did you know?Arteries and veins may be classified as pulmonary, systemic, or coronary.
- Pulmonary arteriestake blood with low levels of oxygen from the right ventricle to the lungs.
- Pulmonary veinsmove oxygen-rich blood from the lungs to the heart’s left atrium.
- Systemic arteriestake oxygen-rich blood from the left ventricle to the body’s tissues.
- Systemic veinsmove blood with low levels of oxygen from the body’s tissues to the heart’s right atrium.
- Coronary arteriestake oxygen-rich blood from the aorta to the heart muscle.
- Coronary veinsmove blood with low levels of oxygen from the heart’s myocardium (middle muscular layer) to its right atrium.
There are manysymptoms of poor circulation, including:
- chest pain
- dizzinessorfeeling faint
- shortness of breath
- pain, weakness, ornumbnessin the limbs
- 肢体肿胀
- slowor rapid heartbeat, orpalpitations
- fatigue
The symptoms depend on the type ofcirculatory condition. As an example,peripheral arterial diseasecan cause leg and foot problems like:
There are several conditions that can affect the heart and circulatory system, including:
- Peripheral arterial disease.In peripheral arterial disease, blood flow in the arteries of the legs is restricted. This is usually due tobuildup of plaque in the arteries.
- Arteriosclerosis.In arteriosclerosis, plaque buildup in the blood vessels becomes calcified and hard. The arteries are less flexible, leading tohigher blood pressure,stroke, heart damage, and肾脏damage.
- 心attack.During aheart attack, a blockage of blood flow to the heart muscle leads to death of heart muscle tissue. It’s also known as a myocardial infarction.
- Angina.Inangina, the heart muscle isn’t getting enough blood. This leads to crushing chest pain, fatigue,nausea, and shortness of breath.
- Mitral valve conditions.Inmitral valve prolapse,mitral valve stenosis, or mitral valve regurgitation,problems with the mitral valvecause oxygenated blood in the heart to flow backward, or blood flow to be slowed down or constricted.
- Arrhythmias or dysrhythmias.These terms are both used to describea heart rate that’s abnormal.
- Ischemia.In ischemia, there’s not enough blood flow in the heart, and muscles don’t get enough oxygen.
- Aortic disease.This group of conditions affects the aorta. One example isaortic aneurysm, where the aorta is weak and bulges out.