Blood transports oxygen, nutrients, carbon dioxide and metabolic waste products, supports immune defense and coagulation, regulates body temperature around 37°C, and buffers pH fluctuations within 7.35–7.45.
Blood consists of 40–45% blood cells and 55–60% plasma; hematocrit is the volume fraction of blood cells in total blood volume and averages 46% in men and 42% in women.
Blood plasma is about 90% water, 8% proteins and 2% other substances, including ions, glucose, vitamins, hormones, enzymes, urea, uric acid and creatinine.
Plasma proteins perform these functions:
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The stem-cell lineages produce:
Erythrocytes are anucleate, biconcave cells about 7.5 μm in diameter; they number about 5 million per μl and account for 99% of blood cells.
Hemoglobin is a red blood pigment consisting of four polypeptide chains and iron-containing heme groups, and it transports oxygen and carbon dioxide while buffering blood.
Oxygen deficiency triggers erythropoietin release from the kidney, erythrocytes live for 120 days, and they are broken down mainly in the spleen as well as in the liver and bone marrow.
Further detail
🔄 Erythrocyte development proceeds through:
Iron released from heme is carried by transferrin back to the bone marrow for erythropoiesis or stored as ferritin or hemosiderin.
Proerythroblast → erythroblast → normoblast → reticulocyte → erythrocyte
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Leukocytes are nucleated, mobile cells involved in defense against foreign substances and pathogens and in inflammatory processes; their total count is about 5,000 per μl.
Neutrophils make up 50–70% of leukocytes, circulate for at most 10 hours, migrate especially to mucous membranes and phagocytose bacteria and dead cell debris as an early bacterial defense.
Monocytes make up 2–8% of leukocytes, are the largest leukocytes at 12–20 μm, circulate for at most three days, then enter tissues and differentiate into macrophages that phagocytose microorganisms and present antigens.
T lymphocytes mediate specific defense, B lymphocytes form specific antibodies and present antigens, and natural killer cells defend against viruses; only about 1% of lymphocytes are in blood and 99% are in tissues.
Further detail
Eosinophils make up 2–4% of leukocytes and defend against parasites, especially worms, by releasing granules and phagocytosis; they are also associated with allergies and occur especially in digestive and respiratory mucosa.
Basophils make up about 2% of leukocytes, and their granules contain histamine involved in immediate-type allergic reactions; basophils resemble mast cells.
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🔄 Primary hemostasis proceeds through: constriction of the injured vessel by smooth muscle, platelet adhesion to the wound, platelet aggregation with small projections, forming a white thrombus
In secondary hemostasis, mutual activation of coagulation factors triggers a cascade that cleaves fibrinogen into fibrin; fibrin forms a firm mesh containing platelets and erythrocytes, producing a red thrombus.
Further detail
Vessel constriction → platelet plug → fibrin-reinforced clot
Erythrocyte concentrates are stored at 4°C for 42 days and are used for anemia, including after major blood loss from accidents or operations and in cancer patients.
Platelet concentrates are stored at 20–24°C for five days and are used for bleeding tendencies caused by thrombocytopenia.
Fresh frozen plasma (FFP) is stored at −30 to −40°C for two years and replaces proteins and coagulation factors, including in massive blood loss or consumptive coagulopathy.
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In the ABO system, erythrocyte surface structures react with IgM antibodies and can cause agglutination, the clumping of cells.
Rhesus-positive erythrocytes have the D antigen and rhesus-negative erythrocytes lack it; anti-D antibodies are not present from birth and can develop in a rhesus-negative person after direct contact with D-positive blood.
When a rhesus-negative mother encounters erythrocytes from a rhesus-positive child at birth, she may form IgG anti-D antibodies; in a later pregnancy with a rhesus-positive child, these placentally transferable antibodies can destroy fetal erythrocytes, causing anemia and potentially hemolytic disease of the newborn.
Further detail
ABO antibodies are already present; anti-D develops after contact
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The lymphatic system provides drainage, immune defense and fat transport.
Lymphatic organs are classified as:
🔄 Lymph flows through:
Further detail
Lymph is about 10% of the fluid filtered from blood capillaries and resembles blood plasma but contains fewer proteins.
The spleen lies in the left upper abdomen beneath the diaphragm and contains red pulp with blood sinuses and reticular tissue and white pulp with lymphatic tissue, periarterial lymphatic sheaths and follicles; it removes old blood cells and coagulation products and forms blood before birth.
Lymph capillaries → lymph vessels → nodes → large ducts → thoracic duct
★ Must-know
The thymus lies in the anterior mediastinum above the pericardium and supports T-lymphocyte maturation and selection; its cortex contains T cells and precursors, while its medulla contains few lymphocytes.
In the thymus, T cells that recognize self MHC with low affinity continue developing through positive selection in the cortex, whereas cells that bind self-antigen–MHC complexes strongly undergo apoptosis through negative selection in the medulla.
In bone marrow, stromal cells present self-antigens to B cells; weak B-cell receptor binding permits further maturation, whereas strong autoreactive binding usually causes apoptosis through clonal deletion.
The bone marrow forms all immunocompetent cells from a pluripotent stem cell and matures and selects B lymphocytes.
Further detail
Weak self-binding permits maturation; strong self-binding triggers deletion
Bacteria contain a cell membrane, cell wall, flagellum, 70S ribosomes, nucleoid, plasmids and cytosol.
Bacterial pathogenicity factors include:
Take a 33-question practice quiz on Blood and Lymphatic System, with every answer explained.
1. A person's blood helps maintain a stable internal environment; which set of functions best describes how it does so?
2. A laboratory measures the volume fraction of blood cells in a blood sample; what is this measurement called?
Memorize the key concepts of Blood and Lymphatic System with 68 flashcards.
What functions does blood perform in transport, defense, and coagulation?
It transports oxygen, nutrients, carbon dioxide, and metabolic waste, and supports immune defense and coagulation.
What body temperature does blood help regulate?
Around 37°C.
What pH range does blood buffer?
7.35–7.45.
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