Human Cardiovascular System - Heart, Blood, Blood Vessels, Lymphatic System, Arteries and Arterioles, Capillaries

Human Cardiovascular System

The human Cardiovascular system consists of a network of arteries, veins, and capillaries, with the heart pumping blood through it. Its primary role is to provide essential nutrients, minerals, and hormones to various parts of the body. Alternatively, the circulatory system is also responsible for collecting metabolic waste and toxins from the cells and tissues to be purified or expelled from the body.

Features of Circulatory System

The crucial features of human circulatory are as follows:

• The human circulatory system consists of blood, heart, blood vessels, and lymph.
• The human circulatory system circulates blood through two loops (double circulation) – One for oxygenated blood, another for deoxygenated blood.
• The human heart consists of four chambers – two ventricles and two auricles.
• The human circulatory system possesses a body-wide network of blood vessels. These comprise arteries, veins, and capillaries.
• The primary function of blood vessels is to transport oxygenated blood and nutrients to all parts of the body. It is also tasked with collecting metabolic wastes to be expelled from the body.
• Most circulatory system diagrams do not visually represent is its sheer length. Theoretically, if the veins, arteries, and capillaries of a human were laid out, end to end, it would span a total distance of 1,00,000 kilometers.

Organs of Circulatory System

The human circulatory system comprises 4 main organs that have specific roles and functions. The vital circulatory system organs include:

• Heart
• Blood (technically, blood is considered a tissue and not an organ)
• Blood Vessels
• Arteries and Arterioles
• Capillaries
• Veins & Venules
• Plasma
• Lymphatic system

Heart

The heart is a muscular organ located in the chest cavity, right between the lungs. It is positioned slightly towards the left in the thoracic region and is enveloped by the pericardium. The human heart is separated into four chambers; namely, two upper chambers called atria (singular: atrium), and two lower chambers called ventricles.

Though other animals possess a heart, the way their circulatory system functions is quite different from humans. Moreover, in some cases, the human circulatory system is much more evolved when compared to insects or mollusks.

Double Circulation

The way blood flows in the human body is unique, and it is quite efficient too. The blood circulates through the heart twice; hence, it is called double circulation. Other animals like fish have single circulation, where blood completes a circuit through the entire animal only once.

The main advantage of double circulation is that every tissue in the body has a steady supply of oxygenated blood, and it does not get mixed with the deoxygenated blood.

Blood

Blood is the body’s fluid connective tissue, and it forms a vital part of the human circulatory system. Its main function is to circulate nutrients, hormones, minerals and other essential components to different parts of the body. Blood flows through a specified set of pathways called blood vessels. The organ which is involved in pumping blood to different body parts is the heart.  Blood cells, blood plasma, proteins, and other mineral components (such as sodium, potassium and calcium) constitute human blood.

Blood is composed of:

• Plasma the fluid part of the blood and is composed of  90%  of water.
• Red blood cells, white blood cells and platelets constitute the solid part of blood.

Types of Blood Cells

The human body consists of three types of blood cells, namely:

• Red blood cells (RBC) / Erythrocytes

Red blood cells, also known as erythrocytes, are by far the most common type of blood cell and make up about 45% of blood volume. Erythrocytes are produced inside of red bone marrow from stem cells at the astonishing rate of about 2 million cells every second. The shape of erythrocytes is biconcave disks with a concave curve on both sides of the disk so that the center of an erythrocyte is its thinnest part. 

The unique shape of erythrocytes gives these cells a high surface area to volume ratio and allows them to fold to fit into thin capillaries. Immature erythrocytes have a nucleus that is ejected from the cell when it reaches maturity to provide it with its unique shape and flexibility. The lack of a nucleus means that red blood cells contain no DNA and are not able to repair themselves once damaged.

Erythrocytes transport oxygen in the blood through the red pigment hemoglobin. Hemoglobin contains iron and proteins joined to greatly increase the oxygen carrying capacity of erythrocytes. The high surface area to volume ratio of erythrocytes allows oxygen to be easily transferred into the cell in the lungs and out of the cell in the capillaries of the systemic tissues.

Illness of Red Blood Cells

Most people don't think about their red blood cells unless they have a disease that affects these cells. Problems with red blood cells can be caused by illnesses or a lack of iron or vitamins in your diet. Some diseases of the red blood cells are inherited.

Diseases of the red blood cells include many types of anemia. This is a condition in which there are too few red blood cells to carry enough oxygen all over the body. People with anemia may have red blood cells that have an abnormal shape or that look normal, larger than normal, or smaller than normal.

Symptoms of anemia include tiredness, fast heart rate, pale skin, feeling cold, and, in severe cases, heart failure. Children who don't have enough healthy red blood cells grow and develop more slowly than other children. These symptoms show how important red blood cells are to your daily life.

These are common types of anemia:

• Iron-deficiency anemia. If you don't have enough iron in your body, your body won't be able to make enough red blood cells. Iron-deficiency anemia is the most common form of anemia. Causes of iron deficiency include:

1. A diet low in iron
2. Sudden blood loss
3. Ongoing (chronic) blood loss, such as from heavy menstrual periods
4. Inability to absorb enough iron from food, such as from weight-loss surgery

• Sickle cell anemia. In this inherited disease, the red blood cells are shaped like half moons rather than the normal indented circles. This change in shape can make the cells "sticky" and unable to flow smoothly through blood vessels. This causes a blockage in blood flow. This may cause sudden (acute) or chronic pain. It can also lead to infection or organ damage. Sickle cells die much more quickly than normal blood cells—in about 10 to 20 days instead of 120 days. This causes a shortage of red blood cells.

• Normocytic anemia. With this type of anemia your red blood cells are a normal shape and size. But you don't have enough of them to meet your body's needs. Diseases that cause this type of anemia are often long-term conditions, such as kidney disease, cancer, or rheumatoid arthritis.

• Hemolytic anemia. This type of anemia happens when red blood cells are destroyed by an abnormal process in your body before their lifespan is over. As a result, your body doesn't have enough red blood cells to function. And your bone marrow can't make enough to keep up with demand.

• Fanconi anemia. This is a rare inherited disorder in which your bone marrow isn't able to make enough of any of the components of blood, including red blood cells. Children born with this disorder often have serious birth defects because of the problems with their blood. They may develop leukemia. 

• White blood cells (WBC) / Leukocytes

Your white blood cells account for only about 1% of your blood, but their impact is big. White blood cells are also called leukocytes. They protect you against illness and disease.

Think of white blood cells as your immunity cells. In a sense, they are always at war. They flow through your bloodstream to fight viruses, bacteria, and other foreign invaders that threaten your health. When your body is in distress and a particular area is under attack, white blood cells rush in to help destroy the harmful substance and prevent illness.

White blood cells are made in the bone marrow. They are stored in your blood and lymph tissues. Because some white blood cells called neutrophils have a short life less than a day, your bone marrow is always making them. There are two major classes of white blood cells: granular leukocytes and agranular leukocytes.

Granular Leukocytes: The three types of granular leukocytes are neutrophils, eosinophils, and basophils. Each type of granular leukocyte is classified by the presence of chemical-filled vesicles in their cytoplasm that give them their function. Neutrophils contain digestive enzymes that neutralize bacteria that invade the body. Eosinophils contain digestive enzymes specialized for digesting viruses that have been bound to by antibodies in the blood. Basophils release histamine to intensify allergic reactions and help protect the body from parasites.

Agranular Leukocytes: The two major classes of agranular leukocytes are lymphocytes and monocytes. Lymphocytes include T cells and natural killer cells that fight off viral infections and B cells that produce antibodies against infections by pathogens. Monocytes develop into cells called macrophages that engulf and ingest pathogens and the dead cells from wounds or infections.

Problems Affecting White Blood Cells

Your white blood cell count can be low for a number of reasons. This includes when something is destroying the cells more quickly than the body can replenish them. Or when the bone marrow stops making enough white blood cells to keep you healthy. When your white blood cell count is low, you are at great risk for any illness or infection, which can spiral into a serious health threat.

Your healthcare provider can do a blood test to see whether your white blood cell count is normal.   If your count is too low or too high, you may have a white blood cell disorder.

A number of diseases and conditions may affect white blood cell levels:

• Weak immune system. This is often caused by illnesses such as HIV/AIDS or by cancer treatment. Cancer treatments such as chemotherapy or radiation therapy can destroy white blood cells and leave you at risk for infection.

• Infection. A higher-than-normal white blood cell count usually means you have some type of infection. White blood cells are multiplying to destroy the bacteria or virus.

• Myelodysplastic syndrome. This condition causes abnormal production of blood cells. This includes white blood cells in the bone marrow. 

• Cancer of the blood. Cancers including leukemia and lymphoma can cause uncontrolled growth of an abnormal type of blood cell in the bone marrow. This results in a greatly increased risk for infection or serious bleeding.

• Myeloproliferative disorder. This disorder refers to various conditions that trigger the excessive  production of immature blood cells. This can result in an unhealthy balance of all types of blood cells in the bone marrow and too many or too few white blood cells in the blood.

• Medicines. Some medicines can raise or lower the body's white blood cell count. 

• Platelets / Thrombocytes

Platelets are cells that help to form clots and stop bleeding. They act on the site of an injury or a wound. Platelets form in the red bone marrow from large megakaryocyte cells that periodically rupture and release thousands of pieces of membrane that become the platelets. 

Blood Vessels

Blood vessels are the body’s highways that allow blood to flow quickly and efficiently from the heart to every region of the body and back again. The size of blood vessels corresponds with the amount of blood that passes through the vessel. All blood vessels contain a hollow area called the lumen through which blood is able to flow. Around the lumen is the wall of the vessel, which may be thin in the case of capillaries or very thick in the case of arteries.

All blood vessels are lined with a thin layer of simple squamous epithelium known as the endothelium that keeps blood cells inside of the blood vessels and prevents clots from forming. The endothelium lines the entire circulatory system, all the way to the interior of the heart, where it is called the endocardium.

There are three major types of blood vessels: arteries, capillaries and veins. Blood vessels are often named after either the region of the body through which they carry blood or for nearby structures. For example, the brachiocephalic artery carries blood into the brachial (arm) and cephalic (head) regions. One of its branches, the subclavian artery, runs under the clavicle; hence the name subclavian. The subclavian artery runs into the axillary region where it becomes known as the axillary artery.

Arteries and Arterioles

Arteries are blood vessels that carry blood away from the heart. Blood carried by arteries is usually highly oxygenated, having just left the lungs on its way to the body’s tissues. The pulmonary trunk and arteries of the pulmonary circulation loop provide an exception to this rule — these arteries carry deoxygenated blood from the heart to the lungs to be oxygenated.

Arteries face high levels of blood pressure as they carry blood being pushed from the heart under great force. To withstand this pressure, the walls of the arteries are thicker, more elastic, and more muscular than those of other vessels. The largest arteries of the body contain a high percentage of elastic tissue that allows them to stretch and accommodate the pressure of the heart.

Smaller arteries are more muscular in the structure of their walls. The smooth muscles of the arterial walls of these smaller arteries contract or expand to regulate the flow of blood through their lumen. In this way, the body controls how much blood flows to different parts of the body under varying circumstances. The regulation of blood flow also affects blood pressure, as smaller arteries give blood less area to flow through and therefore increases the pressure of the blood on arterial walls.

Arterioles are narrower arteries that branch off from the ends of arteries and carry blood to capillaries. They face much lower blood pressures than arteries due to their greater number, decreased blood volume, and distance from the direct pressure of the heart. Thus arteriole walls are much thinner than those of arteries. Arterioles, like arteries, are able to use smooth muscle to control their aperture and regulate blood flow and blood pressure.

Capillaries

Capillaries are the smallest and thinnest of the blood vessels in the body and also the most common. They can be found running throughout almost every tissue of the body and border the edges of the body’s avascular tissues. Capillaries connect to arterioles on one end and venules on the other.

Capillaries carry blood very close to the cells of the tissues of the body in order to exchange gases, nutrients, and waste products. The walls of capillaries consist of only a thin layer of endothelium so that there is the minimum amount of structure possible between the blood and the tissues. The endothelium acts as a filter to keep blood cells inside of the vessels while allowing liquids, dissolved gases, and other chemicals to diffuse along their concentration gradients into or out of tissues.

Precapillary sphincters are bands of smooth muscle found at the arteriole ends of capillaries. These sphincters regulate blood flow into the capillaries. Since there is a limited supply of blood, and not all tissues have the same energy and oxygen requirements, the precapillary sphincters reduce blood flow to inactive tissues and allow free flow into active tissues.

Plasma

Plasma is the often forgotten part of blood. White blood cells, red blood cells, and platelets are important to body function. But plasma also plays a key role. This fluid carries the blood components throughout the body. The main role of plasma is to take nutrients, hormones, and proteins to the parts of the body that need it. Cells also put their waste products into the plasma. The plasma then helps remove this waste from the body. Blood plasma also carries all parts of the blood through your circulatory system.

Lymphatic System

The human circulatory system consists of another body fluid called lymph. It is also known as tissue fluid. It is produced by the lymphatic system which comprises a network of interconnected organs, nodes and ducts.

Lymph is a colorless fluid consisting of salts, proteins, water, which transport and circulates digested food and absorbed fat to intercellular spaces in the tissues. Unlike the circulatory system, lymph is not pumped; instead, it passively flows through a network of vessels.

Functions of Circulatory System

The cardiovascular system has three major functions: transportation of materials, protection from pathogens, and regulation of the body’s homeostasis.

• Transportation: The cardiovascular system transports blood to almost all of the body’s tissues. The blood delivers essential nutrients and oxygen and removes wastes and carbon dioxide to be processed or removed from the body. Hormones are transported throughout the body via the blood’s liquid plasma.
• Protection: The cardiovascular system protects the body through its white blood cells. White blood cells clean up cellular debris and fight pathogens that have entered the body. Platelets and red blood cells form scabs to seal wounds and prevent pathogens from entering the body and liquids from leaking out. Blood also carries antibodies that provide specific immunity to pathogens that the body has previously been exposed to or has been vaccinated against.
• Regulation: The cardiovascular system is instrumental in the body’s ability to maintain homeostatic control of several internal conditions. Blood vessels help maintain a stable body temperature by controlling the blood flow to the surface of the skin. Blood vessels near the skin’s surface open during times of overheating to allow hot blood to dump its heat into the body’s surroundings. In the case of hypothermia, these blood vessels constrict to keep blood flowing only to vital organs in the body’s core. Blood also helps balance the body’s pH due to the presence of bicarbonate ions, which act as a buffer solution. Finally, the albumins in blood plasma help to balance the osmotic concentration of the body’s cells by maintaining an isotonic environment.