Blood

The cardiovascular system moves blood through the heart and blood vessels. It is the body’s transportation system for gases, nutrients, and waste products.

The heart is the central pump in this system. It has two atria, which receive blood, and two ventricles, which pump blood out of the heart. Its rhythmic contractions, called heartbeats, keep blood moving continuously.

Blood vessels carry blood to the body’s tissues and organs. Arteries are muscular, elastic vessels that move oxygenated...

Blood is a specialized connective tissue that helps the body transport materials, regulate conditions, and protect against harm. It makes up about 8% of body mass. Its extracellular matrix is thick and liquid. It contains cells, dissolved proteins, and electrolytes, which makes blood about five times more viscous than water.

Blood is warm, with a temperature of about 38°C, and it has an alkaline pH between 7.35 and 7.45. Its main job is to carry oxygen and carbon dioxide between tissues and...

Blood plasma and formed elements make up whole blood. Plasma is the yellowish fluid part and accounts for about 55% of blood volume. It is mostly water, with electrolytes and proteins mixed in. Plasma also carries blood cells and transports nutrients, enzymes, hormones, antibodies, and gases.

The formed elements make up the remaining 45% of blood volume. These include red blood cells, white blood cells, and platelets. Red blood cells, also called erythrocytes, are the most common formed...

Blood plasma contains water, proteins, and other dissolved solutes that help blood do its job. It is about 92% water and 8% solutes. The solutes include proteins, which make up about 7% of the total plasma solutes.

The major plasma proteins are albumins, globulins, and fibrinogen. These high-molecular-weight proteins are essential to plasma function. Albumins make up about 60% of the plasma proteins. They help maintain osmotic balance in blood vessels by limiting water loss. Albumins also act...

Hematopoiesis is the process that makes blood cells throughout life. It produces red blood cells, white blood cells, and platelets from hematopoietic stem cells, or HSCs.

The first HSCs form in the embryonic yolk sac, which is an early site of blood cell production. These stem cells then move to fetal tissues such as the liver, spleen, thymus, and lymph nodes. During this stage, the liver becomes the main site of hematopoiesis and greatly increases the number and variety of blood cells made.

Blood cell formation begins with hemangioblasts and hematopoietic stem cells, or HSCs. Hemangioblasts are multipotent stem cells that come from the mesoderm, the middle embryonic layer. They give rise to HSCs, which start hematopoiesis, the process that makes the formed elements of blood.

Hematopoiesis is controlled by a network of hematopoietic growth factors. These include transcription factors, growth factors, and cytokines. They signal HSCs to divide and specialize, but some HSCs stay...

Hematopoietic growth factors guide blood cell growth by controlling how hematopoietic stem cells, or HSCs, differentiate. These signaling molecules help direct the formation of red blood cells, platelets, and other blood cells in the bone marrow.

Erythropoietin, or EPO, is mainly produced by the kidneys. When blood oxygen levels are low, EPO enters the bloodstream and reaches the bone marrow. There, it stimulates HSCs to mature into erythrocytes, the red blood cells that carry oxygen.

Erythropoiesis is the process that makes red blood cells. Red blood cells carry oxygen throughout the body, so this process is essential for normal blood function.

The process begins in the bone marrow. There, blood-forming stem cells develop through several stages until they become mature red blood cells. As they mature, these cells change shape and prepare for their role in oxygen transport.

Erythropoiesis helps maintain a steady supply of red blood cells. The body must replace these cells...

The body controls red blood cell levels through erythropoiesis, the process of making new erythrocytes. This helps keep oxygen transport efficient. It also prevents too many erythrocytes from building up, which would raise blood viscosity and slow flow.

Low tissue oxygen is one of the strongest signals for this process. Intense exercise and high altitudes can both cause tissue hypoxia, which means too little oxygen reaches the tissues. In response, the kidneys release more erythropoietin, or...

Red blood cells, or erythrocytes, carry oxygen through the blood. There are about 4.2 to 6 million erythrocytes in every microliter of blood. Their small, flattened biconcave shape helps them move through the body efficiently.

The center of each erythrocyte is depressed, which gives the cell its disc-like form. The plasma membrane is linked to proteins such as spectrin. Spectrin forms a flexible meshwork that lets the cell twist, turn, become cup-shaped, and then return to its biconcave shape...

Hemoglobin is a globular protein that helps blood carry oxygen and other gases. It has four subunits. Two subunits are alpha chains, and two are beta chains. Each subunit contains a heme group with an iron atom that can bind oxygen.

Oxygen binding changes hemoglobin’s shape. When one oxygen molecule binds to a heme group, the other heme groups bind oxygen more easily. When all four heme groups are bound, the molecule is called oxyhemoglobin. This form gives arterial blood its bright red color.

Red blood cells, or erythrocytes, are constantly moving through tiny blood capillaries. This movement creates friction that wears down the plasma membrane. Over about 100 to 120 days, the cells become rigid and fragile.

Old erythrocytes often get trapped as they pass through small vessels in the spleen and liver. There, they can break into fragments. Resident phagocytic macrophages then engulf the damaged cells and separate the globin and heme parts.

The globin chains are broken down into...

Red blood cell disorders change the number, shape, or function of erythrocytes. Erythrocytes are red blood cells, and they help carry oxygen in the blood. These disorders are often grouped into two main types: anemic conditions and polycythemic conditions.

Anemia happens when blood has a low oxygen-carrying capacity because erythrocytes are lost, made too slowly, or destroyed too early. Hemorrhagic anemia can develop after bleeding from an external wound or an internal ulcer. Iron deficiency...

White blood cells, also called leukocytes, help protect the body from toxins, pathogens, and abnormal cells. In a healthy adult, blood usually contains about 4,500 to 11,000 leukocytes per microliter. That is about 1% of total blood volume.

Leukocytes differ from red blood cells in an important way. They have a nucleus and other cell organelles, but they do not contain hemoglobin. Most white blood cells are found in connective tissues, especially in lymphatic organs such as the lymph nodes,...

Leukocytes are grouped by whether they have cytoplasmic granules. Granular leukocytes contain granules and belong to the myeloid lineage. They include neutrophils, eosinophils, and basophils, which are all roughly spherical cells with visible granules in the cytoplasm.

Neutrophils are the most common granular leukocytes and make up about 50-70% of all leukocytes. They have small, evenly spread granules and a polymorphonuclear shape with two to five lobes in the nucleus. Their main role is...

Leukocyte disorders can change the number of white blood cells in the blood. A low leukocyte count is called leukopenia, and a high leukocyte count is called leukocytosis. These changes can help show whether the body is struggling to make leukocytes or responding to a problem.

Leukopenia can result from bone marrow disorders, autoimmune disease, and infectious disease. Multiple myeloma and aplastic anemia can damage the bone marrow’s ability to produce enough leukocytes. Lupus and HIV can also...

Platelets help blood clot and stop bleeding after a blood vessel is damaged. These cell fragments are disc-shaped and are about 3 bcm wide and 1 bcm thick. They work with plasma proteins, blood cells, and blood vessel tissues in the bodys vascular clotting system.

Platelets are constantly replaced in the bloodstream. They usually circulate for 9 to 12 days before phagocytes, mainly in the spleen, remove them. A microliter of blood contains about 150,000 to 450,000 platelets, with an average...

Hemostasis is the body’s way of stopping bleeding after a blood vessel is injured. It keeps blood fluid inside the circulatory system, but it also allows blood to clot when a vessel is damaged. This fast response depends on clotting factors in plasma, platelets, and substances released by injured tissue cells.

The first stage is the vascular phase. Blood vessels narrow in a process called vasoconstriction, which reduces blood flow to the injury site and limits blood loss. This early step helps...

Blood vessel constriction is the first step of hemostasis, the process that helps stop bleeding after an injury. Also called vascular spasm, this response begins when a vessel is cut and nerves in the damaged area send pain and other sensory signals. At the same time, the smooth muscle in the vessel wall contracts and narrows the opening at the injury site. This slowing of blood flow helps reduce blood loss through the damaged vessel wall.

The spasm usually lasts about 30 minutes. It becomes...

Platelet adhesion and aggregation are key steps in hemostasis after a blood vessel is injured. This platelet phase is the second stage of hemostasis and begins about 15-20 seconds after the injury. It follows the vascular phase, when the blood vessel constricts to reduce blood loss, and the two phases overlap.

As the injured vessel contracts, endothelial cells also contract. This exposes collagen fibers in the basement membrane and the tissue below it. The endothelial cell surface also becomes...

Blood clot formation helps stop blood loss after a blood vessel is injured. It is the final stage of hemostasis, the body’s process for stopping bleeding. This stage uses a series of chemical reactions to seal the damaged area with a clot.

Clotting starts soon after injury. In severe damage, it can begin within 15 to 20 seconds. With minor injuries, it often begins within 1 to 2 minutes. Clotting factors, also called procoagulants, start and carry forward the coagulation cascade. This cascade...

Blood clotting, also called coagulation, happens through the extrinsic and intrinsic pathways. These two pathways end by joining the common pathway, which forms a fibrin clot. Both pathways are activated after a blood vessel is injured, but they begin in different ways.

The extrinsic pathway starts when tissue damage exposes tissue factor, or TF. TF is released by damaged cells outside the blood vessels. When TF interacts with activated Factor VII, the TF-Factor VIIa complex forms and...

Blood clots shrink and break down through clot retraction and fibrinolysis. After a fibrin clot forms, platelet contractile proteins such as actin and myosin pull the fibrin strands closer together. This condenses the clot into a smaller, denser mass that helps seal the damaged vessel.

Clot retraction also supports wound healing. As the clot tightens, it brings the edges of the injured blood vessel closer together. Platelets also release growth factors that help repair the injury.

Disorders of hemostasis can cause blood to clot too much or clot too little. Hemostasis is the process that stops bleeding after a blood vessel is injured. When this balance fails, the result may be a thromboembolic disorder or a bleeding disorder.

Thromboembolic disorders involve a clot, or thrombus, forming inside a blood vessel. They can be linked to irregular vessel texture, such as from arteriosclerosis, infection, or injury. Slow blood flow through vessels can also promote thrombin...

The ABO blood group system explains how blood types are matched for transfusions and organ transplants. It depends on specific antigens, also called agglutinogens, on the surface of red blood cells, or RBCs. It also depends on antibodies, called agglutinins, in the blood plasma.

Antigens are substances that can trigger an immune response and lead to the production of antibodies. In the ABO system, antigen A and antigen B are the two main antigens. These protein molecules are genetically...

The Rh factor is a protein on the surface of red blood cells and is an important part of blood typing. If the Rh antigen is present, a person is Rh-positive. If it is absent, the person is Rh-negative. For example, someone with type A blood can be A positive or A negative depending on whether the Rh antigen is found on their red blood cells.

Rh status matters in blood transfusions because donor and recipient blood must be compatible. If Rh factors do not match, the immune system can react...

Blood grouping with the ABO and Rh systems identifies the antigens on red blood cells. These surface antigens help determine a person's blood type. They are also important for transfusion medicine, organ transplants, and pregnancy, where compatibility matters.

The main antigens used in this process are A, B, and D, which is the Rh antigen. Antigens are protein molecules on the surface of red blood cells. Blood groups are identified by testing these antigens with specific anti-sera.

Blood transfusion depends on matching blood types to keep the procedure safe. A transfusion is a medical treatment that moves blood from a donor to a recipient. It can replace blood lost from injury or surgery, and it can also help treat conditions such as anemia or cancer.

The ABO blood group system is the key part of this matching process. It classifies blood by the antigens, which are surface markers, on red blood cells. The four ABO blood types are A, B, AB, and O.

Compatibility matters...

Bone marrow transplants can treat serious diseases by replacing damaged red bone marrow with healthy stem cells. Doctors use this procedure for patients with aplastic anemia, certain leukemias, severe combined immunodeficiency disease (SCID), Hodgkin's disease, non-Hodgkin's lymphoma, multiple myeloma, thalassemia, sickle-cell disease, and some cancers.

The process begins with high doses of chemotherapy and radiation. These treatments destroy the cancerous or defective red bone marrow and help...