Source: Textbook Ch. 44, Pre-Session Assignment 11
Tags: internal transport, circulatory system, blood composition, open circulatory system, closed circulatory system, hemolymph, plasma, serum, red blood cells, white blood cells, anemia, blood types, ABO blood group, Rh factor, platelets, blood clotting, coagulation cascade
Difficulty: Intermediate | Prerequisites: Basic cell biology (Ch. 3–4), tissue types
Big Picture
This material covers how animals move substances around their bodies, from simple diffusion in tiny organisms to the complex closed circulatory system in vertebrates. You will need to know what blood is made of, how different circulatory plans compare, how blood types determine transfusion compatibility, and how the clotting cascade prevents fatal blood loss. This is the foundation for everything else in the chapter: heart structure, blood vessels, and pressure regulation all build on understanding what the blood carries and how it flows.
TL;DR
Blood is a connective tissue with plasma (fluid) and formed elements (RBCs, WBCs, platelets). Animals use no circulatory system, an open system (hemolymph bathes organs directly), or a closed system (blood stays in vessels). ABO and Rh antigens determine blood type compatibility, and clotting is a stepwise cascade ending in a fibrin mesh.
Plasma
The fluid compartment of blood. It is mostly water and carries dissolved proteins, nutrients, gases, electrolytes, and waste products. Think of it as the river in which everything else floats.
Serum
The fluid compartment of blood minus the clotting factors. In simple terms, serum is what remains after blood has been allowed to clot and the clot is removed.
Hemolymph
The fluid that circulates in an open circulatory system. It bathes the organs directly rather than staying confined in vessels. Think of it as blood and interstitial fluid rolled into one.
Colloidal pressure (oncotic pressure)
The osmotic pressure exerted by materials (mainly proteins such as albumin) dissolved in the plasma or interstitial fluid. In simple terms, it is the "pull" that draws water back into capillaries.
Hydrostatic pressure
The pressure exerted by the blood against the vessel wall. This is the "push" that forces fluid out of capillaries and into surrounding tissues.
Open circulatory system
One in which the blood vessels are open-ended and fluid (hemolymph) leaves to enter the interstitium, directly bathing organs. Found in arthropods and most molluscs.
Closed circulatory system
One in which the blood vessels are closed and fluid enters the interstitium only via changes in hydrostatic and colloidal pressures. Blood stays within vessels at all times. Found in vertebrates, annelids, and cephalopods.
Cardiac output
The volume of blood pumped by the left ventricle into the aorta in one minute. Calculated as stroke volume multiplied by heart rate.
Stroke volume
The amount of blood pumped out of a ventricle with one contraction. In simple terms, it is how much blood the heart ejects per beat.
Heart
The pump in a circulatory system. In vertebrates, a muscular organ with chambers that generates the pressure to circulate blood.
Artery
A blood vessel carrying blood away from the heart. Arteries have thick, elastic walls to handle high pressure.
Vein
A blood vessel carrying blood back to the heart. Veins have thinner walls and often contain valves to prevent backflow.
Systole
The period of time when the heart is contracting and ejecting blood.
Diastole
The period of time when the heart is not contracting. The chambers fill with blood during this phase.
Semilunar valve
The endothelial tissue valve located in the aorta and in the pulmonary trunk. These valves prevent blood from flowing back into the ventricles after ejection.
Sphincter muscle
A circular muscle regulating flow into or out of an opening. Precapillary sphincters control blood flow into capillary beds.
The vertebrate circulatory system consists of the heart (pump), blood vessels (arteries, veins, capillaries), and blood (the transport medium). Its job is to deliver oxygen, nutrients, and hormones to tissues and carry away carbon dioxide and metabolic waste.
Components of whole blood
Blood is roughly 55% plasma and 45% formed elements by volume. The percentage of blood volume occupied by red blood cells is called the haematocrit.
Plasma: water (~92%), dissolved proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, gases, hormones, and waste
Formed elements: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes)
Red blood cells (erythrocytes)
Biconcave disc shape, which increases surface area for gas exchange
Contain haemoglobin, the iron-containing protein that binds oxygen
Mature mammalian RBCs are anucleate (no nucleus), giving more room for haemoglobin
Produced in red bone marrow (erythropoiesis), stimulated by erythropoietin (EPO) from the kidneys
Lifespan: roughly 120 days; old RBCs are broken down in the spleen and liver
Types of anaemia
Iron-deficiency anaemia: insufficient iron for haemoglobin synthesis; RBCs are smaller and paler (microcytic, hypochromic)
Sickle cell anaemia: a genetic mutation produces abnormal haemoglobin (HbS) that causes RBCs to deform into a sickle shape under low-oxygen conditions
Pernicious anaemia: vitamin B12 deficiency impairs RBC production, resulting in fewer but larger RBCs (megaloblastic)
Haemorrhagic anaemia: excessive blood loss reduces total RBC count
Aplastic anaemia: bone marrow failure leads to reduced production of all blood cell types
White blood cells (leukocytes)
WBCs are nucleated cells involved in immune defence. They are divided into two groups based on whether they contain visible cytoplasmic granules when stained.
Granulocytes (granular WBCs):
Neutrophils: the most abundant WBC; first responders to bacterial infection; phagocytic
Eosinophils: target parasitic infections and modulate allergic responses
Basophils: release histamine and heparin; involved in inflammatory and allergic reactions
Agranulocytes (agranular WBCs):
Lymphocytes: include B cells (antibody production), T cells (cell-mediated immunity), and natural killer cells
Monocytes: leave the blood and differentiate into macrophages in tissues; phagocytic
Platelets (thrombocytes)
Cell fragments derived from megakaryocytes in the bone marrow. They are essential for haemostasis (stopping bleeding) and initiate the clotting cascade when a vessel is damaged.
No circulatory system
Found in very small or thin organisms (e.g. cnidarians such as hydra, flatworms such as planaria)
Every cell is close enough to the external environment for diffusion alone to handle gas and nutrient exchange
No heart, no vessels, no specialised transport fluid
Limitation: only works if the body is thin enough (typically two cell layers) so that diffusion distances remain short
Open circulatory system
Found in most arthropods (insects, crustaceans) and most molluscs (snails, clams)
The transport fluid is called hemolymph
A heart pumps hemolymph through vessels that empty into open spaces (sinuses) called the haemocoel
Hemolymph directly bathes the organs, then re-enters the heart through openings called ostia
Lower pressure compared to closed systems; adequate for smaller, less metabolically demanding animals
Components: heart, open-ended vessels, sinuses (haemocoel), hemolymph
Closed circulatory system
Found in vertebrates, annelids (earthworms), and cephalopods (octopus, squid)
Blood is always contained within vessels and is distinct from interstitial fluid
A heart generates higher pressure, allowing faster and more efficient delivery of oxygen and nutrients
Fluid exchange between blood and tissues occurs at capillaries via hydrostatic and colloidal pressure gradients
Components: heart, arteries, veins, capillaries, blood
Allows precise regulation of blood flow to specific organs via vasoconstriction and vasodilation
Blood type is determined by the presence or absence of specific antigens on the surface of red blood cells and corresponding antibodies in the plasma.
ABO blood group system
Type A: A antigens on RBCs, anti-B antibodies in plasma
Type B: B antigens on RBCs, anti-A antibodies in plasma
Type AB: both A and B antigens on RBCs, no anti-A or anti-B antibodies (universal recipient for RBCs)
Type O: no A or B antigens on RBCs, both anti-A and anti-B antibodies in plasma (universal donor for RBCs)
Rh factor
Rh-positive (Rh+): Rh antigen (D antigen) is present on RBCs
Rh-negative (Rh-): Rh antigen is absent
Rh incompatibility matters especially in pregnancy: an Rh- mother carrying an Rh+ foetus can develop anti-Rh antibodies that attack foetal RBCs in subsequent pregnancies (haemolytic disease of the newborn)
Blood type compatibility table
Blood Type | Antigens on RBCs | Antibodies in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
O- | None | Anti-A, Anti-B | All types (universal donor) | O- only |
O+ | Rh | Anti-A, Anti-B | O+, A+, B+, AB+ | O-, O+ |
A- | A | Anti-B | A-, A+, AB-, AB+ | A-, O- |
A+ | A, Rh | Anti-B | A+, AB+ | A-, A+, O-, O+ |
B- | B | Anti-A | B-, B+, AB-, AB+ | B-, O- |
B+ | B, Rh | Anti-A | B+, AB+ | B-, B+, O-, O+ |
AB- | A, B | None | AB-, AB+ | A-, B-, AB-, O- |
AB+ | A, B, Rh | None | AB+ only (universal recipient) | All types |
The key rule: a recipient must not receive blood carrying antigens they have antibodies against, or agglutination (clumping) and haemolysis (RBC destruction) will occur.
Role of platelets
Platelets are cell fragments that circulate in the blood and are the first responders when a vessel is damaged. They adhere to exposed collagen at the injury site, become activated, change shape, and release chemical signals that recruit more platelets. This forms the initial platelet plug (primary haemostasis).
Sequence of events in blood clotting (secondary haemostasis)
Vascular spasm: smooth muscle in the damaged vessel wall contracts, narrowing the vessel and reducing blood flow to the area
Platelet plug formation: platelets adhere to exposed collagen, become sticky, aggregate together, and release ADP and thromboxane A2 to recruit additional platelets
Coagulation cascade: a series of clotting factor activations occurs in two converging pathways (intrinsic and extrinsic), both leading to the common pathway
Prothrombin activator converts prothrombin (inactive) into thrombin (active enzyme)
Thrombin converts soluble fibrinogen into insoluble fibrin threads
Fibrin threads form a mesh that stabilises the platelet plug into a firm clot
Clot retraction: the clot tightens and pulls the wound edges together
Fibrinolysis: once the vessel is repaired, plasmin breaks down the fibrin mesh and the clot is dissolved
Real-world application
Anticoagulant drugs such as warfarin (which inhibits vitamin K-dependent clotting factors) and heparin (which enhances antithrombin activity) are used clinically to prevent dangerous clot formation in conditions like deep vein thrombosis and atrial fibrillation.
Students often confuse plasma and serum. Plasma contains clotting factors (including fibrinogen); serum does not. If the blood has clotted, the remaining liquid is serum.
"Open circulatory system" does not mean the animal has no heart. Insects and crustaceans have hearts that pump hemolymph; the vessels just empty into open sinuses rather than forming a closed loop.
Type O blood is called the "universal donor" for red blood cell transfusions, but it does contain anti-A and anti-B antibodies in its plasma. In large-volume plasma transfusions, this matters.
Students sometimes think clotting is a single event. It is a multi-step cascade, and a deficiency in any one clotting factor (e.g. factor VIII in haemophilia A) can prevent normal clot formation.
⚠️ Be able to match every vocabulary term to its definition (the PSA matching section tests this directly).
⚠️ Know the difference between plasma and serum, and between hydrostatic and colloidal pressure. These are favourite short-answer traps.
⚠️ The blood type compatibility table is high-yield. Expect a question asking which types a given patient can safely receive.
⚠️ Be able to list the steps of clotting in order: vascular spasm, platelet plug, coagulation cascade (prothrombin to thrombin, fibrinogen to fibrin), clot retraction, fibrinolysis.
⚠️ Comparing open vs closed circulatory systems with specific animal examples is a common exam question.
True or false: Serum contains fibrinogen.
False. Serum is plasma minus the clotting factors, including fibrinogen.
Fill in the blank: The fluid in an open circulatory system is called ________.
Hemolymph.
True or false: Arteries carry oxygenated blood and veins carry deoxygenated blood.
False. Arteries carry blood away from the heart and veins carry blood toward the heart. The pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood.
Fill in the blank: A person with type AB+ blood has ________ antibodies in their plasma.
No anti-A or anti-B antibodies (and no anti-Rh, since they are Rh+).
True or false: Platelets are complete cells with a nucleus.
False. Platelets are cell fragments derived from megakaryocytes and lack a nucleus.
Q: Name three components of plasma and state one function of each.
A: Albumin (maintains colloidal/oncotic pressure), fibrinogen (clotting factor, converted to fibrin during coagulation), and immunoglobulins/globulins (antibodies involved in immune defence).
Q: How does an open circulatory system differ from a closed circulatory system? Give one animal example of each.
A: In an open system, hemolymph is pumped into sinuses where it directly bathes organs before returning to the heart (e.g. grasshopper). In a closed system, blood remains within vessels and exchange occurs at capillaries (e.g. earthworm, human).
Q: A patient has type B- blood. Which blood types can they safely receive?
A: B- and O-. They have anti-A antibodies, so they cannot receive any A or AB blood. They are Rh-, so they should not receive Rh+ blood.
Q: Describe the difference between granulocytes and agranulocytes and give one example of each.
A: Granulocytes have visible cytoplasmic granules when stained (e.g. neutrophils, which phagocytise bacteria). Agranulocytes lack visible granules (e.g. lymphocytes, which include B and T cells of the adaptive immune system).
Q: Place these clotting events in the correct order: fibrin mesh forms, vascular spasm, platelet plug, prothrombin converts to thrombin.
A: Vascular spasm, platelet plug, prothrombin converts to thrombin, fibrin mesh forms.
This material connects to the immune system (Ch. 45 or equivalent) because white blood cells are part of both the circulatory and immune systems. Understanding how blood delivers immune cells to infection sites bridges these chapters. Blood types and Rh factor also link to genetics, since ABO alleles follow codominance (I^A and I^B) and Rh follows simple dominance.
Related Terms / Search Tags
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