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Anatomy & Physiology II · Blood, Cardiovascular System, and Circulation

Composition and Functions of Blood

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Blood is a connective tissue with a liquid matrix. This section covers its composition — and (red cells, white cells, platelets) — its major functions, and key measures like .

Why this matters

Blood transports oxygen, nutrients, wastes, hormones, and immune cells, and helps regulate temperature and pH. Understanding its parts is the foundation for interpreting blood tests (the most common lab work in medicine) and for topics like anemia, clotting, and transfusion.

The college version

Blood is a liquid connective tissue. As introduced in histology, blood consists of cells suspended in a fluid matrix, meeting the definition of connective tissue. When blood is spun in a centrifuge, it separates into layers: a large bottom layer of red blood cells, a thin middle "buffy coat" of white cells and platelets, and a top layer of plasma.

Plasma (~55% of blood). Plasma is about 90% water, with dissolved proteins (like albumin, clotting factors, and antibodies), nutrients (glucose, amino acids, lipids), electrolytes (sodium, potassium, calcium), wastes (urea), gases, and hormones. Plasma is the medium that carries everything the blood transports. (Plasma without clotting factors is called serum.)

Formed elements (~45% of blood). The cellular portion has three types:

  • Erythrocytes (red blood cells): by far the most numerous; carry oxygen (and some carbon dioxide) using hemoglobin.
  • Leukocytes (white blood cells): the body's mobile defense cells, fighting infection.
  • : cell fragments essential for blood clotting.

Functions of blood. Blood serves three broad roles:

  • Transport: oxygen and carbon dioxide, nutrients, wastes, hormones, and heat.
  • Regulation: helps maintain body temperature (distributing heat), pH (via buffers like bicarbonate), and fluid balance.
  • Protection: clotting to prevent blood loss, and immune defense by white cells and antibodies.

Hematocrit. The hematocrit is the percentage of blood volume made up of red blood cells (roughly 40–50% in men and 36–46% in women, varying by source and individual). It's a common, quick indicator: a low hematocrit suggests anemia (too few red cells or too little hemoglobin), while a high value can indicate dehydration or excess red cell production.

How it works

Reading a spun blood sample:

Top layer: plasma (~55%) — water + proteins, nutrients, wastes, hormones
Thin middle: buffy coat — white cells + platelets (<1%)
Bottom layer: red blood cells (~45%) — the hematocrit

Comparisons

Component~% of bloodRole
Plasma55%Liquid transport medium
Red blood cells45%Oxygen transport (hematocrit)
White cells + platelets<1%Defense + clotting (buffy coat)
FunctionExamples
TransportO₂, CO₂, nutrients, wastes, hormones, heat
RegulationTemperature, pH, fluid balance
ProtectionClotting, immune defense

Common confusions

  • Plasma vs serum. Serum is plasma without clotting factors (what remains after clotting).
  • Formed elements aren't all cells. Platelets are cell fragments, not whole cells.
  • Hematocrit measures red cells only, not total blood cells.
  • Blood is connective tissue — cells in a liquid matrix.

Memory aids

  • "Plasma on top, red cells on the bottom, buffy coat between."
  • Functions: "Transport, Temperature/pH regulate, Protect."
  • Hematocrit = "how packed with red cells."

Quick review

  • Blood is a liquid connective tissue = plasma (~55%, water + proteins/nutrients/wastes/hormones) + formed elements (~45%: red cells, white cells, platelets).
  • Functions: transport (gases, nutrients, wastes, hormones, heat), regulation (temperature, pH, fluid), protection (clotting, immunity).
  • Hematocrit = % red blood cells; low suggests anemia.
  • The CBC blood test is built on these components.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Simple idea

Blood is a liquid tissue that carries oxygen, food, and defenders around your body — it's part liquid and part tiny cells.

Analogy

Think of blood like a busy river delivery system. The water of the river is the plasma — it carries dissolved food, messages (hormones), and trash (wastes) to and from every part of the body. Floating in the river are three kinds of "boats": lots of red boats carrying oxygen (red blood cells), fewer white boats that are the body's soldiers fighting germs (white blood cells), and tiny repair rafts that patch leaks (platelets). If you let the river settle in a tube, the heavy red boats sink to the bottom, and how much of the tube they fill is called the hematocrit.

What is actually happening

Blood does three big jobs: transport (oxygen, food, waste, heat), regulation (keeping your temperature and acidity balanced), and protection (clotting to stop bleeding and fighting infection). Doctors check the "boat counts" with a common blood test called a CBC — too few red boats means anemia (you feel tired and weak), lots of white boats can mean an infection, and too few repair rafts means you might bruise or bleed easily.

Where the analogy stops

A river just flows downhill, but your blood is pumped in a complete loop by your heart and is constantly being refreshed — old cells are removed and new ones made in your bones all the time.

Key takeaways

  • ### High-Yield Pre-Nursing Connections
  • The complete blood count (CBC) — one of the most ordered lab tests — measures red cells, white cells, platelets, hemoglobin, and hematocrit. A low hematocrit/hemoglobin signals anemia; a high white count suggests infection; a low platelet count raises bleeding risk. Plasma proteins like albumin affect fluid balance (low albumin can cause edema). Understanding blood composition underlies interpreting nearly every routine blood test.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice Anatomy & Physiology II

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe the two main components of blood: plasma and formed elements.
  • List the functions of blood (transport, regulation, protection).
  • Define hematocrit and what it measures.
  • Recall that blood is a connective tissue.

Key vocabulary

Plasma
the liquid matrix of blood (mostly water with dissolved substances).
Formed elements
the cells and cell fragments: erythrocytes, leukocytes, platelets.
Erythrocytes (RBCs)
red blood cells; carry oxygen.
Leukocytes (WBCs)
white blood cells; defense.
Platelets (thrombocytes)
cell fragments; clotting.
Hematocrit
the percentage of blood volume that is red blood cells.

Sources & references

  1. OpenStax, *Anatomy and Physiology 2e*, Chapter 18.1: An Overview of Blood. https://openstax.org/details/books/anatomy-and-physiology-2e
  2. U.S. National Library of Medicine, MedlinePlus — Blood; Complete Blood Count. https://medlineplus.gov/blood.html

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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