Difficulty: Intermediate | Prerequisites: Basic cell biology, tissue types, general anatomy orientation.
The cardiovascular system is the body's transport network: the heart pumps blood through a closed loop of vessels so that every cell receives oxygen, nutrients and signalling molecules, and ships out carbon dioxide and metabolic waste. This unit covers the composition and function of blood itself (plasma, red and white blood cells, platelets), then moves into how the heart and vessels keep that blood moving, and how the body stops bleeding when a vessel is damaged. You need a working sense of basic cell structure (nucleus, organelles, membrane proteins) and diffusion before this material will click. If you are coming in cold, understand that nearly every organ system you study later (renal, respiratory, immune) plugs directly into circulation, so this is foundational.
Blood is roughly 55% plasma and 45% cells (mostly red blood cells). The heart pumps it through arteries, capillaries and veins in two loops: pulmonary (to the lungs for gas exchange) and systemic (to every other tissue). When a vessel is damaged, the body stops bleeding through a three-step process: vascular spasm, platelet plug formation and coagulation (clotting), collectively called hemostasis.
Cardiovascular system
The organ system comprising the heart, blood vessels (arteries, capillaries, veins) and blood, responsible for transporting gases, nutrients, waste, hormones and heat throughout the body. In simple terms, it is the body's delivery and waste-removal network.
Hematocrit
The percentage of whole blood volume occupied by red blood cells. Normal range is roughly 47% ± 5 for males and 42% ± 5 for females. Think of it as the "red cell fraction" when you spin a tube of blood in a centrifuge.
Formed elements
The cellular components of blood: erythrocytes (red blood cells), leukocytes (white blood cells) and platelets (thrombocytes). In simple terms, everything in blood that is not plasma.
Plasma
The liquid portion of blood (about 55% by volume), composed of water, proteins (albumins, globulins, fibrinogen), electrolytes, nutrients, gases and waste products. Think of it as the "broth" that carries the cells and dissolved substances.
Erythrocyte (red blood cell / RBC)
A biconcave, anucleate disc packed with haemoglobin. Its sole job is gas transport: carrying O2 from the lungs to tissues and returning CO2 to the lungs. In simple terms, the oxygen taxi of the blood.
Hemoglobin (Hb)
An iron-containing protein inside red blood cells. Each molecule has four haem groups, so one Hb molecule can bind up to four O2 molecules. Each RBC contains roughly 250 million Hb molecules. Think of it as the oxygen-binding pigment that makes blood red.
Oxyhemoglobin / Deoxyhemoglobin
Oxyhemoglobin (HbO2) is haemoglobin loaded with oxygen (ruby red). Deoxyhemoglobin is haemoglobin that has released its oxygen (dark red). In simple terms, "loaded" versus "unloaded" haemoglobin.
Carbaminohemoglobin
Hemoglobin bound to CO2 (about 20% of blood CO2 is carried this way). CO2 binds to the globin portion, not to the haem iron. Think of it as the return-trip version of haemoglobin, ferrying waste CO2 back to the lungs.
Erythropoiesis
The production of red blood cells, occurring in red bone marrow. Stimulated by the hormone erythropoietin (EPO), which the kidneys release in response to hypoxia (low O2). In simple terms, the manufacturing line for new red blood cells.
Erythropoietin (EPO)
A hormone produced mainly by the kidneys when oxygen levels drop. It signals the bone marrow to ramp up red blood cell production. Think of it as the "low oxygen alarm" that orders more RBCs.
Reticulocyte
An immature red blood cell that still contains some residual RNA. Reticulocytes enter the bloodstream and mature into full erythrocytes within 1 to 2 days. In simple terms, the "rookie" RBC that just left the marrow.
Hemopoiesis (haematopoiesis)
The process by which all blood cells are produced from pluripotent stem cells in the red bone marrow. Think of it as the umbrella term for making any blood cell, whether red, white or platelet.
Colony stimulating factors (CSFs)
Secreted glycoproteins that bind receptors on haematopoietic stem cells, triggering intracellular signalling pathways that drive proliferation and differentiation into specific blood cell types. In simple terms, chemical signals that tell the marrow which type of blood cell to make.
Leukocyte (white blood cell / WBC)
Nucleated blood cells responsible for immune defence. Classified as granular (neutrophils, eosinophils, basophils) or agranular (lymphocytes, monocytes). Think of them as the body's security force.
Diapedesis (emigration)
The process by which white blood cells squeeze through capillary walls to reach infected or inflamed tissues. In simple terms, WBCs "crawling through the cracks" to get to where they are needed.
Thrombocyte (platelet)
Cell fragments (no nucleus) produced when megakaryocytes in the red bone marrow splinter. Each megakaryocyte yields 2,000 to 3,000 platelets. Platelets survive 5 to 9 days and are essential for clotting. Think of them as the body's "patch kit" for damaged vessels.
Hemostasis
The sequence of responses that stops bleeding: vascular spasm, platelet plug formation and blood clotting (coagulation). In simple terms, the body's emergency protocol for sealing a leak.
Coagulation (blood clotting)
A cascade of reactions involving clotting factors that converts soluble fibrinogen into insoluble fibrin threads, forming a mesh that traps blood cells and seals the wound. Activated through an extrinsic pathway (tissue factor from damaged tissue) or intrinsic pathway (factors already in the blood), both converging on a common pathway. Think of it as the reinforced concrete that strengthens the initial platelet patch.
Fibrinolysis
The process of dissolving a clot after tissue repair is complete. Plasminogen (trapped in the clot) is converted to plasmin, which digests the fibrin mesh. In simple terms, the clean-up crew that removes the clot once the vessel is healed.
Thrombus / Embolus / Embolism
A thrombus is a clot that forms and stays in an unbroken vessel. An embolus is a thrombus (or other material) that breaks free and floats in the bloodstream. An embolism is the blockage of a vessel by an embolus. Think of a thrombus as a roadblock, an embolus as a drifting roadblock, and an embolism as the traffic jam it causes.
Anemia
A condition in which the blood's oxygen-carrying capacity is too low to support normal metabolism. It is a sign of an underlying problem, not a disease in itself. Symptoms include fatigue, pallor, dyspnoea and chills. In simple terms, not enough working red blood cells or haemoglobin to deliver adequate oxygen.
Polycythemia
An abnormally high red blood cell count, increasing blood viscosity and the risk of clotting, stroke or heart failure. Think of it as the opposite of anaemia: too many red blood cells thicken the blood.
The cardiovascular system transports gases (O2, CO2), nutrients, waste, hormones and heat. It also defends against pathogens and helps regulate body temperature.
The heart is a muscular pump divided by a septum into left and right halves. Atria receive returning blood; ventricles pump blood out.
Blood vessels form two circuits: pulmonary circulation (heart to lungs and back) and systemic circulation (heart to body tissues and back).
A portal system connects two capillary beds in series (e.g. hepatic portal system between the digestive tract and the liver).
When a tube of blood is centrifuged, it separates into three visible layers:
Erythrocytes (bottom layer, ~45%): The densest component. The percentage of blood volume they occupy is the hematocrit.
Buffy coat (< 1%): A thin, whitish layer between the erythrocyte and plasma layers, containing leukocytes and platelets.
Plasma (top layer, ~55%): The least dense component, consisting of water, proteins (albumins, globulins, fibrinogen) and other solutes (electrolytes, nutrients, dissolved gases, regulatory substances, waste products).
Transport: Carries O2, CO2, nutrients, hormones, heat and waste products.
Regulation: Maintains homeostasis of body fluids, pH, body temperature and water content of cells.
Protection: Prevents excessive blood loss through clotting; deploys white blood cells against infection.
Biconcave discs with no nucleus or organelles, maximising surface area for gas exchange.
Contain haemoglobin (Hb), the iron-bearing protein that binds O2. Each Hb molecule binds up to four O2. Each RBC carries about 250 million Hb molecules.
The membrane protein spectrin gives them flexibility to squeeze through capillaries narrower than their own diameter.
O2 loading in lungs: Produces oxyhemoglobin (bright ruby red).
O2 unloading in tissues: Produces deoxyhemoglobin (dark red).
CO2 loading in tissues: About 20% of blood CO2 binds to Hb as carbaminohemoglobin.
Haemoglobin also releases nitric oxide (NO), which causes vasodilation and improves blood flow.
RBCs contain carbonic anhydrase, the enzyme that catalyses the conversion of CO2 + H2O to carbonic acid (H2CO3).
Lifespan: About 120 days. Old cells are removed by the spleen and liver; breakdown products are recycled.
Erythropoiesis: Red blood cell production in red bone marrow, stimulated by erythropoietin (EPO) from the kidneys in response to hypoxia.
Reticulocytes (immature RBCs) enter the bloodstream and mature in 1 to 2 days.
EPO abuse in athletes: Artificial EPO increases hematocrit (potentially from 45% to 65%), thickening the blood, raising the risk of clotting, stroke and heart failure.
Anemia: Blood's O2-carrying capacity is too low. Three causal groups: blood loss, insufficient RBC production, excessive RBC destruction.
Sickle cell disease: genetic anaemia caused by hemoglobin-S, which makes RBCs sickle-shaped when they release O2.
Iron-deficiency anaemia: caused by low iron intake, impaired absorption or chronic blood loss.
Polycythemia: Abnormally high RBC count, increasing viscosity and clotting risk.
Nucleated cells with organelles but no haemoglobin. Main function: combat invading microbes.
Granular leukocytes (contain stainable vesicles): neutrophils, eosinophils, basophils.
Agranular leukocytes (no granules): lymphocytes, monocytes.
WBCs can leave the bloodstream through diapedesis (emigration) to reach infection sites.
An elevated WBC count generally signals infection or inflammation; a low count (leukopenia) may indicate drug effects, radiation exposure or bone marrow suppression.
Differential WBC count highlights:
Neutrophils: high in bacterial infection, burns, stress; low in radiation exposure, drug toxicity, B12 deficiency, SLE.
Lymphocytes: high in viral infections, some leukaemias, infectious mononucleosis; low in prolonged illness, HIV, immunosuppression.
Monocytes: high in viral/fungal infections, TB, some leukaemias; low in bone marrow suppression.
Eosinophils: high in allergic reactions, parasitic infections, autoimmune disease; low in drug toxicity, stress.
Basophils: high in allergic reactions, leukaemias, cancers, hypothyroidism; low in pregnancy, ovulation, stress.
Leukemia: Cancerous overproduction of abnormal WBCs.
Leukopenia: Abnormally low WBC count, often drug-induced (anticancer drugs, glucocorticoids).
Infectious mononucleosis ("kissing disease"): Highly contagious viral illness caused by the Epstein-Barr virus, common in young adults.
Megakaryocytes in red bone marrow fragment into 2,000 to 3,000 platelets each. Platelets have no nucleus but contain many vesicles.
Lifespan: 5 to 9 days.
Differentiation is driven by the hormone thrombopoietin.
Maturation pathway: haematopoietic stem cell, megakaryoblast, megakaryocyte stage II/III, megakaryocyte stage IV, platelets.
Bone marrow transplants replace cancerous marrow with healthy marrow, typically harvested from the iliac crest.
Umbilical cord blood stem cells are an alternative: they can differentiate into many cell types (not only blood cells) and may have advantages over traditional marrow transplants.
Hemostasis is a three-step emergency response that stops bleeding from a damaged vessel.
Smooth muscle in the vessel wall contracts immediately after injury, narrowing the vessel and reducing blood flow.
Activated platelets swell, become spiky and sticky, and release chemical messengers.
ADP causes more platelets to adhere. Serotonin and thromboxane A2 enhance vascular spasm and platelet aggregation.
This is a positive feedback cycle: more platelets sticking means more chemicals released, which recruits still more platelets.
A platelet plug alone can stop bleeding for minor injuries.
Coagulation involves a cascade of clotting factors and can be initiated through two pathways:
Extrinsic pathway: Triggered by tissue factor (TF) released from damaged tissue outside the blood.
Intrinsic pathway: Triggered by contact with negatively charged surfaces (activated platelets, collagen, even glass). All required factors are already present in the blood.
Both pathways converge to form prothrombinase, which enters the common pathway:
Phase 2 (common pathway, part 1): Prothrombinase converts prothrombin (factor II) to the active enzyme thrombin.
Phase 3 (common pathway, part 2): Thrombin converts soluble fibrinogen to insoluble fibrin threads, which form the structural mesh of the clot. Fibrin traps formed elements and plasma, turning it into a gel.
After the clot forms:
The clot retracts (tightens), pulling the edges of the damaged vessel together.
Vitamin K is essential for synthesising four of the clotting factors.
Small, unwanted clots are dissolved by plasmin (part of the fibrinolytic system).
Fibrinolysis: plasminogen, trapped in the clot, is converted to plasmin, a fibrin-digesting enzyme that removes the clot once repair is complete.
Thrombus: A clot that develops and persists in an unbroken vessel, potentially blocking circulation and causing tissue death.
Embolus: A thrombus that breaks free and floats in the bloodstream.
Embolism: An embolus that obstructs a vessel (e.g. pulmonary embolism, cerebral embolism).
Risk factors: atherosclerosis, inflammation, slow blood flow, blood stasis from immobility.
Students often think veins carry deoxygenated blood and arteries carry oxygenated blood, full stop. That is true only for systemic circulation. In pulmonary circulation, the pulmonary arteries carry deoxygenated blood to the lungs and the pulmonary veins return oxygenated blood to the heart. The defining feature is direction: arteries carry blood away from the heart, veins carry blood toward it.
Students sometimes assume anemia is a disease. It is not. Anemia is a sign of an underlying problem (blood loss, insufficient production, excessive destruction of RBCs). The exam may test whether you recognise it as a symptom rather than a diagnosis.
Students frequently confuse the extrinsic and intrinsic clotting pathways. The extrinsic pathway is triggered by tissue factor from outside the blood (tissue damage). The intrinsic pathway is triggered by factors already within the blood contacting negatively charged surfaces. Both converge on the same common pathway.
Students sometimes think red blood cells have a nucleus. Mature human RBCs are anucleate (no nucleus, no organelles). This is what makes them efficient oxygen carriers and gives them their biconcave shape.
⚠️ Know the three layers of a centrifuged blood tube (erythrocytes, buffy coat, plasma) and their approximate percentages.
⚠️ Be able to trace the erythropoiesis feedback loop: low O2, kidneys release EPO, bone marrow produces more RBCs.
⚠️ Distinguish the extrinsic vs. intrinsic clotting pathways and where they converge (prothrombinase, common pathway).
⚠️ Understand the three steps of hemostasis in order: vascular spasm, platelet plug, coagulation.
⚠️ Know that vitamin K is needed for synthesis of four clotting factors.
⚠️ Be able to list the five WBC types and associate each with high/low count clinical indicators.
⚠️ Understand why EPO abuse is dangerous (increased hematocrit, blood viscosity, clotting risk).
⚠️ Sickle cell disease: hemoglobin-S causes sickling when O2 is released. Know this is a genetic anaemia.
True or False: Plasma makes up approximately 45% of whole blood. (False, plasma is ~55%; erythrocytes are ~45%.)
Fill in the blank: The hormone ______ is released by the kidneys in response to low oxygen and stimulates red blood cell production. (Erythropoietin / EPO)
True or False: The intrinsic clotting pathway is triggered by tissue factor released from damaged tissue. (False, that is the extrinsic pathway. The intrinsic pathway is triggered by contact with negatively charged surfaces.)
Fill in the blank: Mature red blood cells live for approximately ______ days before being removed by the spleen and liver. (120)
True or False: Fibrinolysis is the process of forming a blood clot. (False, fibrinolysis is the process of dissolving a clot after repair is complete.)
Q: What are the three layers visible after centrifuging a tube of blood, and what does each contain?
A: Bottom layer: erythrocytes (~45%, densest). Middle layer: buffy coat (< 1%, leukocytes and platelets). Top layer: plasma (~55%, least dense, containing water, proteins and solutes).
Q: Describe the erythropoiesis feedback loop that maintains adequate red blood cell levels.
A: When tissue oxygen levels drop (hypoxia), the kidneys release erythropoietin (EPO). EPO travels to the red bone marrow and stimulates haematopoietic stem cells to differentiate into erythrocytes, restoring oxygen-carrying capacity.
Q: Compare the extrinsic and intrinsic clotting pathways. Where do they converge?
A: The extrinsic pathway is triggered by tissue factor (TF) from damaged tissue outside the blood. The intrinsic pathway is triggered by negatively charged surfaces (activated platelets, collagen, glass) with all factors already in the blood. Both converge at the formation of prothrombinase, which enters the common pathway: prothrombinase converts prothrombin to thrombin, and thrombin converts fibrinogen to fibrin.
Q: Why is EPO abuse dangerous for athletes?
A: Artificial EPO raises hematocrit from a normal ~45% to potentially 65%. The blood becomes highly viscous ("sludge-like"), increasing the risk of clotting, stroke and heart failure. Dehydration during exercise concentrates the blood further.
Q: Distinguish between a thrombus, an embolus and an embolism.
A: A thrombus is a clot that forms and remains in an intact blood vessel. An embolus is a thrombus (or other material) that breaks free and travels through the bloodstream. An embolism is the obstruction of a vessel by an embolus (e.g. pulmonary embolism, cerebral embolism).
Q: Name the five types of white blood cells and classify each as granular or agranular.
A: Granular: neutrophils, eosinophils, basophils. Agranular: lymphocytes, monocytes.
This material connects directly to ventilation and gas exchange: the oxygen and CO2 carried by haemoglobin are loaded and unloaded in the lungs and tissues, which is the focus of the respiratory unit. Understanding blood composition is also prerequisite for the immune system unit, since leukocytes and their differential counts are central to immune responses. The hemostasis content ties into pharmacology (anticoagulants such as warfarin target vitamin-K-dependent clotting factors) and cardiovascular pathology (thromboembolic events in atherosclerosis).
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