What Is Blood?

Blood is a specialized connective tissue that circulates through the heart and blood vessels.

Blood performs several major functions:

• transport
• defense
• regulation
• clotting
• maintenance of internal conditions

Blood connects different organs and tissues by transporting substances between them.

For example:

lungs → oxygen → blood → body cells

small intestine → nutrients → blood → body cells

body cells → carbon dioxide → blood → lungs

tissues → wastes → blood → kidneys

Blood is therefore much more than a red liquid. It is a complex living tissue containing cells, cell fragments, water, proteins, ions, nutrients, wastes, and many other substances.

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The Major Components of Blood

Blood has four major components:

• plasma
• red blood cells
• white blood cells
• platelets

Each component has a different role.

Plasma → transports dissolved substances and distributes heat

Red blood cells → transport oxygen

White blood cells → help defend the body against pathogens and abnormal cells

Platelets → help blood clot

Together, these components allow blood to perform its many functions.

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Plasma

Plasma is the liquid component of blood.

It makes up a little more than half of blood volume in a typical healthy adult, although the exact proportion varies.

Plasma is mostly water.

Dissolved or suspended within plasma are substances including:

• plasma proteins
• glucose
• amino acids
• mineral ions
• hormones
• carbon dioxide and bicarbonate
• urea
• antibodies
• nutrients
• other metabolic products

Plasma acts as the body's main liquid transport medium.

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Why Is Water Important in Plasma?

Water is an excellent transport medium because many substances dissolve in it.

This allows blood to carry dissolved substances throughout the body.

For example:

glucose absorbed from the digestive system can be transported to cells.

Urea produced from amino acid metabolism can be transported toward the kidneys.

Hormones released by endocrine glands can travel to target tissues.

The high water content of plasma also allows blood to distribute thermal energy around the body.

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Plasma Proteins

Plasma contains several important groups of proteins.

These include:

• albumin
• globulins
• fibrinogen and other clotting proteins

Albumin helps maintain the osmotic conditions of blood.

Some globulins include antibodies involved in immune defense.

Fibrinogen participates in blood clotting.

This means plasma contributes not only to transport but also to:

fluid balance + defense + clotting

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Red Blood Cells

Red blood cells, also called erythrocytes, are specialized primarily for oxygen transport.

They are the most numerous formed elements in human blood.

Their structure is highly specialized for their function.

Mature human red blood cells:

• have a biconcave disc shape
• contain large amounts of haemoglobin
• lack a nucleus
• lack most organelles
• are flexible
• have a relatively large surface-area-to-volume ratio

These adaptations allow efficient oxygen transport.

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The Biconcave Shape

A red blood cell is thinner in the centre than around its edge.

This creates its characteristic biconcave shape.

This shape:

• increases surface area relative to volume
• creates short diffusion distances
• helps oxygen move into and out of the cell efficiently
• contributes to flexibility

Red blood cells can deform as they pass through narrow capillaries.

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Haemoglobin

Red blood cells contain haemoglobin, an iron-containing protein that can bind oxygen.

In the lungs:

oxygen enters the blood

↓

oxygen enters red blood cells

↓

oxygen binds to haemoglobin

The oxygenated form is called oxyhaemoglobin.

A simplified representation is:

haemoglobin + oxygen ⇌ oxyhaemoglobin

The reversible arrow is important.

Haemoglobin must be able to bind oxygen and later release it.

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Oxygen Loading in the Lungs

Air entering the lungs reaches structures called alveoli.

Oxygen diffuses across the thin alveolar-capillary barrier and enters the blood.

Much of the oxygen then binds to haemoglobin inside red blood cells.

The blood transports this oxygen through the heart and into systemic circulation.

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Oxygen Unloading in Tissues

When oxygenated blood reaches active tissues, oxygen is released from haemoglobin under appropriate local conditions.

Oxygen then diffuses toward cells.

Cells use oxygen during aerobic respiration.

A simplified equation is:

glucose + oxygen → carbon dioxide + water + energy released

The circulatory and respiratory systems therefore work together to support cellular respiration.

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Why Mature Red Blood Cells Lack a Nucleus

Mature human red blood cells do not contain a nucleus.

This leaves more internal space available for haemoglobin.

However, losing the nucleus and most organelles also means mature red blood cells have limited ability to repair themselves or divide.

They therefore have a limited lifespan and must continually be replaced.

New blood cells are produced primarily in red bone marrow.

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White Blood Cells

White blood cells, or leukocytes, are involved in immune defense.

Unlike mature human red blood cells, white blood cells contain nuclei.

There are several types of white blood cell, and they perform different functions.

These include:

• neutrophils
• lymphocytes
• monocytes
• eosinophils
• basophils

At an introductory level, their major shared role is:

protecting the body against disease and harmful biological agents.

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Phagocytosis

Some white blood cells can engulf microorganisms and other particles.

This process is called phagocytosis.

A simplified sequence is:

1. White blood cell detects signals associated with infection.

2. It moves toward the affected area.

3. It surrounds a microorganism.

4. The microorganism is engulfed.

5. Enzymes help break down the engulfed material.

Phagocytosis is an important part of innate immune defense.

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Lymphocytes and Antibodies

Some lymphocytes participate in highly specific immune responses.

B lymphocytes can develop into plasma cells that produce antibodies.

Antibodies bind specifically to particular molecular targets called antigens.

Other lymphocytes, including T cells, have different roles such as coordinating immune responses or destroying infected cells.

Blood allows many immune cells and molecules to move rapidly around the body.

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Blood and Immune Surveillance

Blood constantly moves through tissues.

White blood cells and immune molecules can therefore:

• detect signs of infection
• travel toward damaged tissues
• attack pathogens
• transport antibodies
• participate in inflammatory responses

The circulatory system effectively provides a transport network for immune defense.

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Platelets

Platelets are small cell fragments involved in blood clotting.

They are produced from large cells called megakaryocytes in the bone marrow.

When a blood vessel is damaged, platelets become activated and contribute to formation of a temporary platelet plug.

They also participate in reactions that lead to formation of a stronger blood clot.

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Blood Clotting

Blood clotting is a complex sequence of reactions.

A simplified process is:

blood vessel damaged

↓

platelets become activated

↓

platelets accumulate at the damaged area

↓

clotting reactions are activated

↓

fibrin forms

↓

a fibrin network strengthens the clot

Fibrin forms a network that traps blood cells and helps stabilize the clot.

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Why Is Clotting Important?

Clotting helps:

• reduce blood loss
• maintain blood pressure and circulating volume
• seal damaged blood vessels
• reduce entry of microorganisms
• create conditions that support tissue repair

Without effective clotting, even relatively small injuries could result in dangerous blood loss.

However, inappropriate clotting inside undamaged vessels can also be dangerous.

Blood clotting must therefore be carefully regulated.

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Transporting Oxygen

Most oxygen in the blood is transported by haemoglobin inside red blood cells.

A much smaller amount is dissolved directly in plasma.

The overall pathway is:

lungs → blood → heart → arteries → capillaries → tissues

At tissues, oxygen leaves the blood and becomes available to cells.

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Transporting Carbon Dioxide

Carbon dioxide is produced during cellular respiration.

It must be transported from tissues to the lungs.

Carbon dioxide is transported in several forms.

Most is converted into bicarbonate ions (HCO₃⁻).

Smaller amounts:

• dissolve directly in plasma
• bind to haemoglobin and other proteins

The general pathway is:

body cells → blood → heart → lungs → exhaled air

Carbon dioxide transport is also closely connected to regulation of blood pH.

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Transporting Glucose

Glucose is absorbed from the digestive system and transported dissolved in plasma.

The pathway can be simplified as:

small intestine → blood → liver and other tissues → cells

Cells can use glucose for cellular respiration.

The liver can also store glucose indirectly as glycogen when appropriate.

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Transporting Amino Acids

Proteins are digested into amino acids.

Amino acids are absorbed through the small intestine and enter the bloodstream.

Plasma carries them to tissues.

Cells can use amino acids to build proteins needed for:

• growth
• repair
• enzymes
• receptors
• transport proteins
• many other cellular structures

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Transporting Lipids

The transport of lipids is more complicated because many lipids do not dissolve readily in water.

Products of fat digestion are absorbed by intestinal cells and are commonly packaged into particles called chylomicrons.

These initially enter lymphatic vessels called lacteals.

They eventually enter the bloodstream.

Other lipid transport particles, called lipoproteins, also transport lipids through blood.

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Transporting Hormones

Hormones are chemical messengers produced by endocrine glands.

They are released into the blood and transported to target tissues.

Examples include:

• insulin
• adrenaline
• thyroid hormones
• reproductive hormones

Blood may carry hormones throughout the body, but only cells with appropriate receptors respond strongly to them.

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Transporting Urea

Urea is produced mainly in the liver from nitrogen-containing compounds generated during amino acid metabolism.

It is transported dissolved in plasma.

The simplified pathway is:

liver → blood → kidneys

The kidneys remove urea from the blood and excrete it in urine.

This demonstrates how circulation connects metabolism with excretion.

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Blood and Heat Transport

Blood also transports thermal energy.

Active tissues such as muscles can produce considerable heat during respiration.

Blood distributes this heat throughout the body.

Changes in blood flow near the skin help regulate heat loss.

This is an important contribution to thermoregulation.

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Blood and Body Temperature

When body temperature rises, blood vessels near the skin can widen.

This is called vasodilation.

More warm blood moves near the surface, increasing heat transfer to the environment.

When body temperature falls, these vessels can narrow.

This is called vasoconstriction.

Blood therefore acts as part of the body's heat-distribution system.

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Blood and Homeostasis

Homeostasis is the regulation of internal conditions within ranges that support normal cell function.

Blood plays a central role because it continuously connects different tissues and organs.

It helps regulate:

• temperature
• pH
• glucose concentration
• water balance
• ion concentrations
• oxygen availability
• carbon dioxide levels

Blood both transports substances and provides a medium whose composition can be monitored and adjusted.

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Blood Glucose Regulation

After eating a carbohydrate-containing meal:

digestion releases glucose

↓

glucose is absorbed into blood

↓

blood glucose rises

↓

pancreas detects the change

↓

insulin is released

↓

blood transports insulin to target cells

↓

cells alter glucose uptake and storage

The circulatory system is therefore essential for both transporting glucose and transporting the hormones that regulate it.

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Blood and pH Regulation

Human blood normally operates within a narrow pH range.

Changes outside this range can interfere with proteins, enzymes, membrane transport, and cellular function.

Blood contains buffer systems that resist rapid changes in pH.

One particularly important system involves:

• carbon dioxide
• carbonic acid
• bicarbonate ions

The lungs regulate carbon dioxide removal, while the kidneys contribute to longer-term regulation of acids and bicarbonate.

Blood connects these organs into one regulatory system.

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Blood and Water Balance

Plasma contains a large amount of water.

Water moves between:

• blood
• tissue fluid
• cells
• digestive system
• kidneys

The kidneys regulate the amount of water removed in urine.

Hormones help coordinate this process.

Blood therefore contributes to maintaining appropriate water concentrations and blood volume.

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Blood and Ion Balance

Blood carries ions including:

• sodium
• potassium
• calcium
• chloride
• bicarbonate

These ions are important for:

• nerve impulses
• muscle contraction
• fluid balance
• pH regulation
• cellular processes

The kidneys and other organs regulate their concentrations.

Circulation transports these ions between the organs and tissues that use or regulate them.

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Tissue Fluid

Blood does not normally directly contact most body cells.

Instead, substances move between blood and cells through tissue fluid.

At capillary beds, some plasma components move across capillary walls and contribute to tissue fluid.

Tissue fluid surrounds cells and allows exchange.

For example:

blood → oxygen/glucose → tissue fluid → cells

and:

cells → carbon dioxide/wastes → tissue fluid → blood

Much of the fluid returns to the circulation, while some enters the lymphatic system before eventually returning to the bloodstream.

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Capillary Exchange

Capillaries are highly suited to exchange because they have:

• very thin walls
• a large total surface area
• extensive networks close to cells
• blood flow conditions that permit exchange

Substances can move between blood and tissues through processes including diffusion and bulk fluid movement.

This is where the transport function of blood connects directly with individual cells.

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Blood During Exercise

During exercise, muscle cells increase their metabolic activity.

They require increased delivery of:

• oxygen
• glucose and other fuels

They also produce increased amounts of:

• carbon dioxide
• heat
• other metabolic products

Cardiac output increases and blood flow is redistributed toward active muscles.

Blood therefore helps the body respond dynamically to changing demands.

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Blood and Defense

Blood contributes to defense in several ways.

White blood cells

identify and respond to pathogens and abnormal cells.

Antibodies

circulate through blood and other body fluids.

Platelets and clotting proteins

help close wounds.

Plasma

transports many immune proteins and chemical signals.

Circulation

allows immune components to travel rapidly to affected tissues.

The transport and defense functions of blood are therefore closely connected.

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Blood and Inflammation

When tissue is damaged or infected, chemical signals can cause local changes in blood vessels.

These can include:

• increased blood flow
• increased vessel permeability
• movement of immune cells into tissues

This contributes to inflammation.

Common signs include:

• redness
• warmth
• swelling
• pain

Inflammation is part of the body's coordinated response to injury and infection.

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Blood Production

Blood cells do not last forever.

New blood cells must therefore be produced continuously.

This process is called haematopoiesis.

In adults, much blood cell production occurs in red bone marrow.

Stem cells can develop along pathways that eventually produce:

• red blood cells
• different white blood cells
• platelet-producing cells

This maintains the cellular components of blood.

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Comparing the Four Major Components

Plasma

Main role: transport medium

Carries:

• nutrients
• hormones
• wastes
• ions
• proteins
• heat

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Red blood cells

Main role: oxygen transport

Important feature:

• haemoglobin

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White blood cells

Main role: defense

Functions include:

• phagocytosis
• antibody-related responses
• immune coordination
• destruction of infected cells

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Platelets

Main role:

• blood clotting

Help prevent excessive blood loss following vessel damage.

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Example 1: Oxygen Transport

A runner's muscle cells require more oxygen during exercise.

Which blood component is most directly responsible for transporting most of this oxygen?

Red blood cells

Why?

Because they contain haemoglobin, which reversibly binds oxygen.

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Example 2: Nutrient Transport

Glucose is absorbed from the small intestine.

Which component transports it through the bloodstream?

Plasma

Glucose dissolves in the watery plasma and can be carried to tissues.

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Example 3: Infection

Bacteria enter through a wound.

Which blood component has the most direct cellular role in defending against them?

White blood cells

Some can engulf microorganisms, while others participate in specific immune responses.

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Example 4: A Cut

A person cuts a finger.

Which blood component is particularly important in the initial clotting response?

Platelets

They become activated at the damaged blood vessel and contribute to formation of a platelet plug and clot.

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Example 5: Carbon Dioxide

A muscle cell produces carbon dioxide.

The carbon dioxide enters the blood.

Most of it is eventually transported as:

bicarbonate ions

The blood carries it toward the lungs, where carbon dioxide is eventually released into exhaled air.

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Example 6: Homeostasis

During exercise, body temperature rises.

Blood redistributes thermal energy and changes in skin blood flow help control heat transfer.

This shows that blood contributes to:

homeostasis

rather than simply transporting oxygen.

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Structure and Function

The components of blood provide excellent examples of the relationship between biological structure and function.

Red blood cell

biconcave + haemoglobin-rich + flexible

→ efficient oxygen transport

White blood cell

nucleus + specialized receptors and cellular machinery

→ defense and immune responses

Platelet

small reactive cell fragment

→ rapid response to damaged vessels

Plasma

water-based liquid containing dissolved substances

→ transport and regulation

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Common Mistakes

Mistake 1: Saying plasma is simply water

Plasma is mostly water but also contains proteins, ions, nutrients, hormones, wastes, and many other substances.

Mistake 2: Saying red blood cells carry all oxygen

Most oxygen is transported by haemoglobin in red blood cells, but a small amount is dissolved in plasma.

Mistake 3: Saying red blood cells contain a nucleus

Mature human red blood cells lack a nucleus.

Mistake 4: Saying white blood cells only produce antibodies

Some lymphocytes are involved in antibody production, but white blood cells have many different immune functions.

Mistake 5: Saying platelets are complete red blood cells

Platelets are small cell fragments derived from megakaryocytes.

Mistake 6: Saying most carbon dioxide is carried by haemoglobin

Most carbon dioxide is transported after conversion to bicarbonate.

Mistake 7: Saying blood directly touches every cell

Most cells exchange substances with blood through tissue fluid.

Mistake 8: Saying clotting is only about stopping blood loss

Clotting also helps seal damaged vessels and contributes to protection and repair.

Mistake 9: Saying blood only transports substances

Blood also contributes to defense, clotting, temperature regulation, pH regulation, and homeostasis.

Mistake 10: Saying homeostasis means conditions remain perfectly constant

Homeostasis keeps internal conditions within controlled ranges.

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Did You Know?

A single drop of blood contains enormous numbers of red blood cells, along with smaller numbers of white blood cells and platelets suspended in plasma.

The different components work together as one integrated tissue.

Blood can simultaneously:

deliver oxygen

transport nutrients

remove wastes

carry hormones

fight infection

form clots

distribute heat

help regulate pH and fluid balance

This combination of functions makes blood essential to the survival of almost every tissue in the human body.

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Key Terms

• Blood: Specialized connective tissue responsible for transport, defense, regulation, and clotting.
• Plasma: Liquid component of blood.
• Red blood cell: Cell specialized primarily for oxygen transport.
• Erythrocyte: Scientific name for a red blood cell.
• Haemoglobin: Iron-containing protein that reversibly binds oxygen.
• Oxyhaemoglobin: Haemoglobin with oxygen bound to it.
• White blood cell: Cell involved in immune defense.
• Leukocyte: Scientific name for a white blood cell.
• Phagocytosis: Process in which a cell engulfs particles or microorganisms.
• Antibody: Protein that binds specifically to an antigen.
• Platelet: Cell fragment involved in blood clotting.
• Fibrin: Protein fibres that form a network during blood clotting.
• Clotting: Process that helps seal damaged blood vessels.
• Bicarbonate: Ion involved in carbon dioxide transport and pH regulation.
• Tissue fluid: Fluid surrounding body cells through which many substances move between blood and cells.
• Homeostasis: Regulation of internal conditions within ranges suitable for normal function.
• Haematopoiesis: Production of blood cells.

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Transport Summary

Oxygen

lungs → red blood cells/haemoglobin → tissues

Carbon dioxide

tissues → mainly bicarbonate in blood → lungs

Glucose

small intestine → plasma → cells

Amino acids

small intestine → plasma → tissues

Lipids

small intestine → lymph → blood in transport particles → tissues

Urea

liver → plasma → kidneys

Hormones

endocrine glands → plasma → target tissues

Heat

active tissues → blood → distributed throughout body and toward skin

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Key Takeaways

• Blood is a specialized transport tissue.
• The four major components are plasma, red blood cells, white blood cells, and platelets.
• Plasma is the liquid transport medium of blood.
• Red blood cells contain haemoglobin and transport most of the body's oxygen.
• Their biconcave shape, lack of nucleus, haemoglobin content, and flexibility support oxygen transport.
• White blood cells protect the body through several different immune mechanisms.
• Some white blood cells perform phagocytosis.
• Some lymphocytes participate in antibody-mediated immune responses.
• Platelets help initiate blood clotting.
• Fibrin helps strengthen a developing blood clot.
• Nutrients such as glucose and amino acids are transported mainly in plasma.
• Most carbon dioxide is transported after conversion to bicarbonate.
• Urea is carried through plasma toward the kidneys.
• Blood transports hormones between endocrine glands and target tissues.
• Blood contributes to temperature regulation by distributing heat.
• Blood supports pH, water, ion, gas, and glucose regulation.
• Blood transports immune cells and molecules around the body.
• Exchange between blood and most body cells occurs through tissue fluid.
• The composition of blood is continuously regulated by organs including the lungs, kidneys, liver, digestive system, and endocrine glands.
• Blood therefore performs three interconnected roles:

transport + protection + regulation

Together, these functions help maintain the stable internal conditions required for cells to survive and function.