Difficulty: Introductory | Prerequisites: Part 1 of these notes (Circulatory Systems and Blood Flow Dynamics).
Once you understand how circulatory systems generate flow (Part 1), the next question is: what is actually flowing, and through what? This section covers the composition of blood (the transport medium), the different types of blood cells and their jobs, the structure of blood vessels, and the lymphatic system that recovers fluid lost from capillaries. You should already be comfortable with open vs. closed circulatory systems and the concept of bulk flow before tackling this.
Blood is a complex tissue that carries gases, nutrients, waste, hormones, immune cells, ions and clotting factors. About 55% is plasma and 45% is cells (mostly red blood cells). Blood vessels are named by their direction of flow relative to the heart, and the lymphatic system collects the interstitial fluid that leaks out of capillaries and returns it to the blood.
Blood
A complex connective tissue that serves as the circulating transport medium in a closed circulatory system. Carries respiratory gases (O₂, CO₂), nutrients, waste products, hormones, antibodies, salts and ions, red and white blood cells, and platelets.
Plasma
The liquid component of blood, making up roughly 55% of total blood volume. Contains water, dissolved proteins, electrolytes, nutrients, hormones and waste. Think of it as the "solvent" that carries everything else.
Erythrocytes (red blood cells, RBCs)
Make up roughly 45% of blood volume. Contain haemoglobin, the protein that binds and transports oxygen. In simple terms, these are the oxygen delivery trucks.
Haemoglobin
The iron-containing protein inside red blood cells that binds oxygen in the lungs and releases it in the tissues. Each molecule can carry four O₂ molecules.
Leukocytes (white blood cells, WBCs)
Make up less than 1% of blood volume. Responsible for immune defence. Several subtypes, including lymphocytes and granulocytes.
Lymphocytes
A type of white blood cell responsible for adaptive immunity: the targeted, specific immune response (e.g. producing antibodies, killing virus-infected cells).
Granulocytes
White blood cells that engulf invading particles or produce enzymes to destroy them. Part of the innate (non-specific) immune response.
Thrombocytes (platelets)
Cell fragments involved in blood clotting. They aggregate at wound sites and help form a plug to stop bleeding.
Hemocytes
A general term for blood cells, particularly used in invertebrate biology. Made in bone marrow in vertebrates.
Blood vessels
The tubes through which blood flows. Named by the direction of flow relative to the heart. Have complex, layered walls surrounding a central lumen. They are not simple continuous tubes.
Lumen
The central, hollow space inside a blood vessel through which blood flows.
Arteries
Vessels that carry blood away from the heart. Thick-walled, elastic, and carry blood under high pressure.
Arterioles
Small arteries that regulate blood flow into capillary beds. Primary site of vascular resistance.
Capillaries
The smallest vessels, with walls just one cell thick. Where gas, nutrient and waste exchange occurs between blood and tissues. Have the greatest total cross-sectional area, so blood velocity is lowest here.
Venules
Small veins that collect blood from capillary beds and feed into larger veins.
Veins
Vessels that carry blood back toward the heart. Thinner-walled than arteries, often contain valves to prevent backflow.
Interstitial fluid
Extracellular fluid that directly bathes the tissues. Formed when plasma is filtered across capillary walls. Not the same as blood: blood is fluid that circulates inside vessels in a closed system.
Lymphatic system
A network of vessels and nodes that collects interstitial fluid (which, once inside the lymphatic vessels, is called lymph) and returns it to the blood. Also plays a role in immune surveillance.
Blood is a complex tissue, not just a simple fluid
It carries: respiratory gases (O₂ and CO₂), nutrients, waste products, hormones, antibodies, salts and ions, red blood cells, white blood cells, and platelets
Blood circulates inside vessels in a closed circulatory system; the fluid that leaks out across capillary walls and bathes tissues directly is called interstitial fluid, not blood
Plasma: roughly 55% of total blood volume. The liquid portion, carrying dissolved substances.
Red blood cells (erythrocytes): roughly 45% of blood volume. Contain haemoglobin for oxygen transport.
White blood cells (leukocytes): less than 1% of blood volume. Immune defence.
All blood cells (hemocytes) are made in the bone marrow
Erythrocytes: contain haemoglobin; primary role is oxygen transport
Lymphocytes: a type of WBC; responsible for adaptive immunity (targeted, specific immune responses)
Granulocytes: a type of WBC; engulf invading particles or produce enzymes to destroy them (innate immunity)
Thrombocytes (platelets): cell fragments involved in blood clotting
Named based on the direction of flow relative to the heart (away = arteries; toward = veins)
Have complex, layered walls surrounding a central lumen
They are not simple, continuous, uniform tubes
The sequence from heart and back: arteries, arterioles, capillaries, venules, veins
Arterioles are the primary site of vascular resistance regulation
Capillaries are the exchange site: thin walls, greatest total cross-sectional area, lowest velocity
Collects interstitial fluid that has been filtered across capillary walls
Important distinction: interstitial fluid is the extracellular fluid that directly bathes the tissues; blood is the fluid that circulates inside vessels
Once collected by lymphatic vessels, the fluid is called lymph
Lymph nodes filter the fluid and house immune cells
The lymphatic system returns this recovered fluid to the blood, maintaining fluid balance
Also plays a role in immune surveillance (lymph nodes screen for pathogens)
The distinction between blood cell types is the basis of a complete blood count (CBC), one of the most common clinical lab tests. An elevated white blood cell count suggests infection or inflammation; a low platelet count raises the risk of uncontrolled bleeding. The lymphatic system is also why cancer staging uses "lymph node involvement" as a key indicator: tumour cells that reach the lymph nodes can spread throughout the body.
Students sometimes think arteries always carry oxygenated blood and veins always carry deoxygenated blood. The naming is about direction relative to the heart, not oxygen content. Pulmonary arteries carry deoxygenated blood to the lungs; pulmonary veins carry oxygenated blood back.
Students often confuse interstitial fluid with blood. Blood is inside vessels; interstitial fluid is outside them, bathing the tissues. They have different compositions.
Students sometimes assume that because WBCs are less than 1% of blood volume, they are unimportant. Their low volume is misleading: they are critical for immune defence and their absence (as in certain immunodeficiency conditions) is life-threatening.
Students sometimes forget that the lymphatic system has a circulatory role, not just an immune one. Without lymphatic return, fluid would accumulate in the tissues (oedema).
⚠️ Know the percentage breakdown of blood: 55% plasma, 45% RBCs, less than 1% WBCs.
⚠️ Be able to name the blood cell types and match each to its function (erythrocytes = O₂ transport, lymphocytes = adaptive immunity, granulocytes = innate immunity, thrombocytes = clotting).
⚠️ Know the vessel sequence: arteries, arterioles, capillaries, venules, veins, and what distinguishes each.
⚠️ Understand the distinction between blood, interstitial fluid, and lymph.
⚠️ The lymphatic system's dual role: fluid recovery and immune surveillance.
Fill in the blank: Plasma makes up roughly ___% of blood volume.
True or false: Arteries are defined as vessels that carry oxygenated blood.
Fill in the blank: The fluid that bathes tissues directly outside of blood vessels is called ______ fluid.
True or false: Granulocytes are responsible for adaptive immunity.
Fill in the blank: Once interstitial fluid enters lymphatic vessels, it is called ______.
Answers: 1. 55%. 2. False (arteries are defined by carrying blood away from the heart, regardless of oxygenation). 3. Interstitial. 4. False (granulocytes handle innate immunity; lymphocytes handle adaptive immunity). 5. Lymph.
Q: A patient's lab results show a very low platelet count. What clinical risk does this create, and why?
A: Increased risk of bleeding, because platelets (thrombocytes) are essential for clot formation. Without enough platelets, the body cannot effectively plug wounds or stop internal bleeding.
Q: Explain why blood is described as a "complex tissue" rather than simply a fluid.
A: Blood contains multiple cell types (erythrocytes, leukocytes, thrombocytes) suspended in plasma. These cells have distinct structures and perform specialised functions (oxygen transport, immune defence, clotting), which makes blood a tissue with diverse components, not a simple solution.
Q: A capillary bed is leaking more fluid into the surrounding tissue than the lymphatic system can recover. What happens?
A: Fluid accumulates in the interstitial space, causing oedema (swelling). This is precisely what the lymphatic system normally prevents by collecting excess interstitial fluid and returning it to the blood.
Q: Why are blood vessels described as "not simple continuous tubes"?
A: Because they have complex, multi-layered walls (endothelium, smooth muscle, connective tissue in various proportions), and different vessel types have different structures adapted to their function. Capillaries have walls just one cell thick for exchange; arteries have thick elastic walls for high-pressure flow.
Blood composition links directly to gas exchange (how O₂ binds haemoglobin in erythrocytes) and to immunology (the roles of lymphocytes and granulocytes). The vessel sequence connects to Part 1's discussion of cross-sectional area and velocity. The lymphatic system will come up again in immune defence topics and in discussions of tissue fluid balance and oedema.
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