Follow the exchange between blood, tissue fluid and lymph.
A cell cannot walk to the lungs for oxygen. Materials have to reach it. Start by following the fluid around a single cell.
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Tissue fluid
The liquid around most body cells. Oxygen, nutrients and wastes pass through it between cells and the blood.
For example
Oxygen leaves a tiny blood vessel, crosses this fluid and reaches a nearby cell.
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Tissue fluid
The liquid around most body cells. Oxygen, nutrients and wastes pass through it between cells and the blood. Oxygen leaves a tiny blood vessel, crosses this fluid and reaches a nearby cell.
Capillary
A very small blood vessel with a thin wall suited to exchange. Nutrients move from capillary blood towards cells; many wastes move the other way.
Plasma / lymph
Plasma is the liquid portion of blood. Lymph is fluid collected inside lymphatic vessels. Excess tissue fluid becomes lymph when it enters a lymphatic capillary.
Homeostasis
The body’s ongoing adjustments that keep conditions such as temperature and water balance within a suitable range. Sweating helps cool a warm body. Homeostasis allows small changes; it does not freeze every value at one number.
Deliver supplies to cells
Cells need oxygen and nutrients, the useful substances obtained from food. Blood transports these around the body. Most cells exchange with the fluid around them, called tissue fluid or interstitial fluid. Tiny blood vessels called capillaries provide a thin wall for this exchange.
The four main parts of blood
Plasma is the liquid that carries blood cells and dissolved substances. Red blood cells use a protein called haemoglobin to carry most oxygen. White blood cells help defend the body. Platelets are small cell fragments that help stop bleeding by joining a plug at a damaged vessel.
A return route matters too
Arteries carry blood away from the heart; veins carry it back. These names describe direction. The pulmonary arteries, for example, carry blood low in oxygen towards the lungs. Lymphatic vessels collect excess tissue fluid and return it to the blood. Inside these vessels the fluid is called lymph.
Explore the illustration
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The components of blood
A magnified teaching view. Cells and fragments are suspended in plasma; colours are illustrative.
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01 / 4 Red blood cell
Red cells and platelets drift through plasma in this slowed teaching view. The enlarged details stay still for comparison. Select a label, then open the 3D study below to inspect the shape. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.
Red blood cell
The flattened disc has a thinner centre. Haemoglobin inside carries most blood oxygen.
Blood cells in 3D
Each model opens as a cross-sectional cutaway. Study the interior first, then switch to the whole cell to compare.
Go deeper
In-depth reading
Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.
Worked example: an oxygen delivery to your toe
In the lungs, oxygen enters the blood. Most attaches to haemoglobin inside red blood cells. Blood returns to the left side of the heart, which pumps it into arteries leading towards the body tissues.
Near a toe cell, oxygen leaves the blood through a capillary wall, crosses tissue fluid and enters the cell. The red blood cell normally stays inside the vessel: it delivers oxygen without entering the toe cell.
Blood returns through veins to the right heart and then travels to the lungs. Meanwhile, lymphatic vessels recover excess tissue fluid. Keeping the fluid around cells suitable for life is part of homeostasis: maintaining internal conditions within useful ranges.
Explain it yourself
Does an artery always carry oxygen-rich blood?
Show the explanation
No. An artery carries blood away from the heart. Pulmonary arteries go from the right heart to the lungs with blood relatively low in oxygen.
More detail — when you are ready
These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.
What blood carries
Plasma is the liquid component of blood. It is mostly water.
It carries nutrients, chemical messages called hormones, proteins and wastes. It also carries electrolytes: dissolved mineral particles, such as sodium and potassium, needed for nerve and muscle activity.
Erythrocytes is another name for red blood cells. They contain haemoglobin, the protein carrying most oxygen.
Leukocytes are white blood cells; their different types contribute to defence. Platelets are cell fragments that participate in stopping bleeding.
A platelet plug and the clotting proteins work together; platelets are not oxygen carriers. The fluid between cells is interstitial fluid.
Cells exchange materials with this local environment rather than all touching circulating blood directly.
Two circuits, one continuous pathway
The right heart sends blood through the pulmonary circulation to the lungs. After gas exchange, pulmonary veins return blood to the left heart.
The left heart supplies the systemic circulation, and systemic veins return blood to the right heart. Arterioles are small arteries leading into capillary networks.
By narrowing or widening, they help adjust how much blood enters. Capillaries have thin walls that let substances pass between blood and tissues.
An artery is defined by flow away from the heart, even in the pulmonary circuit where its blood contains less oxygen. The muscular heart generates pressure; vessels distribute the flow.
Why lymphatic return matters
Some fluid and proteins leave blood capillaries and are collected by blind-ended lymphatic capillaries. Once inside lymphatic vessels, this fluid is called lymph.
It passes through larger vessels and lymph nodes before returning to venous blood near the neck. Nodes filter lymph and provide sites for immune responses; the spleen surveys blood rather than filtering lymph flowing along these vessels.
If drainage is impaired, fluid can accumulate in tissues. Transport therefore depends on both blood circulation and recovery of fluid from the interstitial space.
Further reading — optional
Free external resources in English. Internet connection required to open the links. Choose the topic you want to explore; these readings are not required to complete the section.
Trace tissue fluid into lymphatic vessels and back to blood. Then read how lymph nodes support immune defence.
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Sources and illustration notes
Bilingual teaching notes organised from your supplied anatomy reference pages. Medical SVG artwork is credited beneath each illustration; guided sequences and process overlays support slow, step-by-step study. Anatomy and translation await expert review.