CST - Introduction to the Circulatory System Lesson

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Introduction to the Circulatory System

Most of the cells in the human body are not in direct contact with the external environment.  he circulatory system acts as a transport service for these cells. Two fluids move through the circulatory system: blood and lymph. The blood, heart, and blood vessels form the circulatory system. The lymph, lymph nodes, and lymph vessels form the lymphatic system.  

What is Blood?

Blood is the body's internal transportation medium. Pumped by the heart, blood travels through a network of vessels carrying materials such as oxygen, nutrients, hormones, and waste products to and from each of the hundred trillion cells in the human body.

The average adult has approximately 5 L of blood in their body. Blood is a type of connective tissue that contains both dissolved substances and specialized cells. This tissue performs many important functions within the body, including:

  • Supplying oxygen to tissues
  • Supplying nutrients such as glucose, amino acids, and fatty acid
  • Removing waste such as carbon dioxide, urea, and lactic acid
  • Immunological functions, including circulating white blood cells and detecting foreign material by antibodies
  • Coagulation, which is one part of the body's self-repair mechanism
  • Messenger functions, including the transport of hormones and the signaling of tissue damage
  • Regulation of body pH
  • Regulation of core body temperature
  • Hydraulic functions (penis and clitoris)

Blood Components

The components of blood are:
Plasma: 55% of total blood
Buffy Coat (leukocytes and platelets): <1% of total blood
Erythrocytes: 45% of total blood

 

Whole blood is made of four major components: plasma, erythrocytes, leukocytes, and thrombocytes. These components can be separated using a centrifuge, where a tube of blood is spun and the heaviest components will move towards the bottom of the tube (see the picture above). 

Plasma

About 55% of blood is blood plasma.About 55% of blood is blood plasma. Plasma is blood's liquid medium, made of 90% water. The other 10% is composed of various dissolved substances such as gases, salts, nutrients, enzymes, hormones, waste products, and proteins.

The largest group of solutes in plasma consists of three proteins:

  1. Albumin - produced by the liver; functions to maintain the osmotic balance between the blood and tissue fluids and to assist in the transport of various materials such as bilirubin and fatty acids
  2. Globulins - function to transport various substances in the blood; gamma globulins assist the body's immune system
  3. Clotting proteins - produced mainly in the liver; functions to create blood clots to slow the loss of blood in response to tissue damage

 

Erythrocytes

Around 45% of whole blood consists of erythrocytes. Erythrocytes, also known as red blood cells or RBCs, function to transport oxygen in the bloodstream.

Sickle cell disorder is an autosomal recessive disorder that affects the structure and function of hemoglobin. The change in the structure of hemoglobin causes a sickling effect to red blood cells. Sickle cell disorder can cause anemia, shortness of breath, fatigue, repeated infections, and periodic episodes of pain due to the stiff and inflexible sickle cells getting stuck in small vessels.  Some people have mild symptoms, while others have more serious complications.

In normal red blood cells, RBCs flow freely within the blood vessel. With abnormal, sickled, RBCs (sickle cells), the sticky sickle cells block the blood flow. An abnormal hemoglobin form causes strands that sickle the RBC shape.

 

Leukocytes

Less than 1% of whole blood consists of leukocytes. Leukocytes, also known as white blood cells or WBCs, function to attack foreign substances or organisms.

Leukemia is a cancer of the white blood cells that causes bone marrow to produce abnormal white blood cells. These abnormal cells crowd out healthy blood cells, preventing their normal functions. Leukemia can develop quickly or slowly. Children most often develop the acute type that grows rapidly. Treatments include chemotherapy, radiation, and stem cell transplantation.

Human immunodeficiency virus (HIV) is a virus that attacks the body's immune system (specifically T cells). Over time, the virus can destroy so many of these cells that the body is unable to fight off infections and diseases causing acquired immunodeficiency syndrome (AIDS). No effective cure for HIV currently exists, but the virus can be controlled with proper medical care.

Main symptoms of acute HIV infection include:

Systemic: fever, weight loss
Central: malaise, headache, neuropathy
Pharyngitis
Mouth: sores, thrush
Lymph nodes: lymphadenopathy
Esophagus: sores
Skin: rash
Muscles: myalgia
Liver and spleen: enlargement
Gastric: nausea, vomiting

 

Platelets

Platelets

Less than 1% of whole blood consists of thrombocytes. Thrombocytes, also known as platelets, are membrane-bound cell fragments. Platelets are produced from stem cells in the bone marrow. Around 100,000,000,000 platelets are produced each day in the average healthy adult, with an average lifespan of just 5 to 10 days.How platelents work:

1. Injury. A blood vessel is severed. Blood and blood components (e.g., erythrocytes, white blood cells, etc.) are leaking out of the breaks.

2. Vascular spasm. The smooth muscle in the vessel wall contracts near the injury point, reducing blood loss.

3. Platelet plug formation. Platelets are activated by chemicals released from the injury site and by contact with underlying collagen. The platelets become spiked and stick to each other and the wound site. 

4. Coagulation. In coagulation, fibrinogen is converted to fibrin, which forms a mesh that traps more platelets and erythrocytes, producing a clot. Fibrin strands secure platelets and erythrocytes, effectively plugging the break.

Platelets function in the formation of blood clots (also known as coagulation) when a vessel is damaged to prevent further loss of blood. The sticky surface of platelets allows them to accumulate at the damage site to form a clot.

If the platelet numbers are too high, blood clots (thrombosis) can form. Blood clots can obstruct blood vessels and cause strokes, heart attacks, pulmonary embolisms, or the blockage of blood vessels to other parts of the body. If the platelet levels are too low, excessive bleeding can occur.

Platelets also secrete clotting factors to facilitate the coagulation process by attracting more platelets. Hemophilia is a sex-linked, recessive disorder that is caused by a lack of certain clotting factors. This rare bleeding disorder causes you to bleed excessively even after a minor injury or even in the absence of injury.  

Blood Groups

If a person loses an excessive amount of blood due to injury or surgery, a transfusion may be needed.   Blood transfusion is the process of receiving blood intravenously. When the procedure was first performed, whole blood was used; however, now only components of blood such as RBC, WBC plasma, clotting factors, and platelets, are transfused.

Doctors quickly discovered that people had different types of blood or blood groups due to the presence of glycoproteins called antigens on the surface of RBCs.

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