Transport

Transport is one of the fundamental functions of blood. Continuous circulation through the cardiovascular system allows substances to move rapidly between the lungs, gastrointestinal tract, liver, kidneys, endocrine organs, and peripheral tissues.

Blood transports oxygen, carbon dioxide, nutrients, hormones, electrolytes, metabolic waste products, plasma proteins, immune components, and heat. Some substances are dissolved directly in plasma, while others require specialized carrier proteins or are transported within blood cells.

This transport function allows individual organs to operate as parts of an integrated physiological system. Cells can receive oxygen and nutrients, endocrine signals can reach distant targets, and metabolic products can be delivered to organs responsible for processing or excretion.

Major Substances Transported by Blood

Substance	Major Source	Major Destination or Role
Oxygen	Lungs	Peripheral tissues
Carbon dioxide	Metabolically active tissues	Lungs
Nutrients	Gastrointestinal tract and storage organs	Cells and metabolic organs
Hormones	Endocrine cells and glands	Target tissues
Metabolic wastes	Peripheral tissues	Kidneys, liver and lungs
Electrolytes	Diet, tissues and body stores	Distributed among body compartments
Heat	Metabolically active tissues	Redistributed throughout body and toward skin

Transport Mediums Within Blood

Blood uses several mechanisms to carry substances. Water-soluble molecules may circulate dissolved in plasma, hydrophobic molecules frequently bind carrier proteins or lipoproteins, and respiratory gases are transported using specialized chemical mechanisms.

Plasma

Plasma is the liquid component of blood and provides the principal medium for transporting dissolved substances.

Its high water content allows ions, nutrients, metabolic products, hormones, proteins, and many other molecules to circulate between organs.

Red Blood Cells

Red blood cells provide specialized transport of respiratory gases, particularly oxygen.

Their high concentration of hemoglobin allows blood to carry far more oxygen than could be transported dissolved in plasma alone.

Plasma Proteins as Carriers

Several plasma proteins bind and transport substances that are poorly soluble, biologically active, or require controlled distribution.

Albumin and specialized binding proteins are particularly important carrier molecules.

Oxygen Transport

Oxygen enters the blood across the pulmonary respiratory membrane and is transported from the lungs to tissues throughout the systemic circulation.

Most oxygen is carried reversibly bound to hemoglobin inside erythrocytes.

Hemoglobin

Hemoglobin is an iron-containing protein specialized for respiratory gas transport.

Each hemoglobin molecule contains four heme groups, allowing reversible binding of up to four oxygen molecules.

Oxyhemoglobin

When oxygen binds hemoglobin, the resulting form is called oxyhemoglobin.

Oxygen binding increases in pulmonary capillaries where oxygen partial pressure is relatively high and decreases in systemic tissues where conditions favor oxygen unloading.

Dissolved Oxygen

A small proportion of blood oxygen is transported physically dissolved in plasma.

Although quantitatively much smaller than hemoglobin-bound oxygen, dissolved oxygen determines the oxygen partial pressure measured in plasma.

Oxygen Delivery to Tissues

Oxygen delivery depends on arterial oxygen content and blood flow.

Hemoglobin concentration, hemoglobin saturation, pulmonary gas exchange, cardiac output, and regional perfusion therefore all influence tissue oxygen delivery.

Carbon Dioxide Transport

Carbon dioxide produced by cellular metabolism diffuses into blood and is transported toward the lungs for elimination.

It travels in three principal forms: bicarbonate, carbamino compounds, and physically dissolved carbon dioxide.

Bicarbonate Transport

Most carbon dioxide is ultimately transported in blood in the form of bicarbonate ions.

Within erythrocytes, carbonic anhydrase rapidly catalyzes reactions that facilitate conversion between carbon dioxide and bicarbonate-related products.

Carbonic Anhydrase

Carbonic anhydrase is abundant in erythrocytes and accelerates the reversible hydration of carbon dioxide.

This enzyme is essential for rapid carbon dioxide transport and contributes to acid-base regulation.

Chloride Shift

As bicarbonate leaves erythrocytes in systemic capillaries, chloride can enter through an anion exchanger to maintain electrical balance.

This process is known as the chloride shift.

Carbaminohemoglobin

Some carbon dioxide binds to amino groups on hemoglobin and other proteins, forming carbamino compounds.

Carbon dioxide bound to hemoglobin is commonly referred to as carbaminohemoglobin.

Forms of Carbon Dioxide Transport

Form	Transport Mechanism
Bicarbonate	Carbon dioxide converted through reactions involving carbonic anhydrase
Carbamino compounds	Carbon dioxide binds to proteins, including hemoglobin
Dissolved CO2	Physically dissolved in plasma and intracellular fluid

Nutrient Transport

Blood distributes nutrients absorbed from the gastrointestinal tract and mobilized from storage tissues.

Important transported nutrients include glucose, amino acids, fatty acids, lipids, vitamins, minerals, and other metabolic substrates.

Glucose Transport

Glucose is water soluble and circulates primarily dissolved in plasma.

Blood delivers glucose to tissues for energy production, biosynthesis, or storage.

Amino Acid Transport

Amino acids absorbed from the gastrointestinal tract circulate in plasma and are delivered to tissues.

They are used for protein synthesis, production of biologically active molecules, and energy metabolism.

Lipid Transport

Many lipids are poorly soluble in water and cannot circulate freely in substantial quantities.

Triglycerides and cholesterol are transported primarily within lipoprotein particles, while nonesterified fatty acids circulate largely bound to albumin.

Lipoproteins

Lipoproteins package hydrophobic lipids with proteins and other lipid components, allowing them to travel through aqueous plasma.

Major classes include chylomicrons, very-low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins.

Vitamin Transport

Water-soluble vitamins can circulate dissolved in plasma or associated with specific proteins.

Fat-soluble vitamins generally require association with lipoproteins or specialized carrier proteins for effective transport.

Mineral Transport

Blood transports minerals and trace elements between the gastrointestinal tract, storage sites, kidneys, bones, and tissues.

Some circulate freely as ions, while others bind specific carrier proteins.

Iron Transport

Plasma iron is transported primarily bound to transferrin.

This carrier delivers iron to tissues, particularly developing erythroid cells in bone marrow, while limiting the amount of unbound circulating iron.

Calcium Transport

Plasma calcium circulates in several forms.

A portion is ionized and biologically active, while another fraction is bound mainly to albumin or complexed with small anions.

Hormone Transport

Blood provides the principal route by which endocrine hormones travel from sites of secretion to distant target tissues.

The mode of transport varies according to the chemical properties of the hormone.

Water-Soluble Hormones

Peptide hormones and catecholamines are generally sufficiently water soluble to circulate primarily in plasma without extensive carrier binding.

Some nevertheless interact with plasma proteins to varying degrees.

Lipid-Soluble Hormones

Steroid hormones and thyroid hormones circulate extensively bound to plasma proteins.

Protein binding increases their solubility in plasma and influences their distribution, half-life, and availability.

Hormone Carrier Proteins

Examples of hormone-binding proteins include thyroxine-binding globulin, corticosteroid-binding globulin, and sex hormone-binding globulin.

Albumin also binds many hormones with lower specificity.

Transport of Metabolic Waste

Cellular metabolism continuously produces substances that must be processed, recycled, or eliminated.

Blood carries these products from tissues toward the kidneys, liver, lungs, and other organs.

Urea

Urea is produced primarily by the liver during nitrogen metabolism and circulates dissolved in plasma.

It is transported to the kidneys, where much of it is filtered from the blood and ultimately excreted in urine.

Creatinine

Creatinine is generated from creatine-related metabolism, particularly in skeletal muscle.

It circulates in plasma and is transported to the kidneys for excretion.

Uric Acid

Uric acid is produced during purine metabolism.

Blood transports it to the kidneys and other sites involved in its handling and elimination.

Bilirubin

Bilirubin is produced from heme degradation, particularly following breakdown of aged erythrocytes.

Because unconjugated bilirubin is poorly water soluble, it is transported in plasma primarily bound to albumin on its way to the liver.

Transport to the Kidneys

The kidneys receive a large blood flow that allows continuous filtration and regulation of plasma composition.

Blood delivers metabolic wastes, excess electrolytes, water, and numerous other substances to the kidneys for selective processing.

Transport to the Liver

The liver receives substances from both the systemic arterial circulation and the hepatic portal circulation.

Blood delivers nutrients, drugs, toxins, hormones, metabolic products, and other compounds for processing, storage, synthesis, or elimination.

Hepatic Portal Circulation

The hepatic portal system carries venous blood from much of the gastrointestinal tract and associated abdominal organs to the liver before that blood returns to the systemic circulation.

This arrangement allows absorbed nutrients and other intestinal substances to undergo hepatic processing.

Transport from the Gastrointestinal Tract

Many absorbed nutrients enter blood capillaries within the intestinal mucosa and travel through portal veins toward the liver.

Most long-chain dietary lipids follow a different initial route through intestinal lymphatic vessels before entering the bloodstream.

Electrolyte Transport

Blood distributes sodium, potassium, chloride, bicarbonate, calcium, magnesium, phosphate, and other ions throughout the body.

Their plasma concentrations are regulated through coordinated interactions among tissues, kidneys, gastrointestinal tract, bones, and endocrine systems.

Plasma Protein Transport

Plasma itself carries a wide range of proteins produced primarily by the liver, immune cells, and other tissues.

These include albumin, immunoglobulins, complement proteins, coagulation factors, enzymes, carrier proteins, and signaling molecules.

Immune Component Transport

Blood transports leukocytes, antibodies, complement proteins, cytokines, and other immune components.

This allows immune defenses to be distributed rapidly and enables leukocytes to reach sites of infection or tissue injury.

Leukocyte Transport

For many leukocytes, circulation is a temporary transport phase rather than their final functional location.

Cells can adhere to vascular endothelium and migrate into tissues through transmigration when appropriate signals are present.

Antibody Transport

Immunoglobulins circulate within plasma and extracellular fluids.

This distribution allows antibodies to recognize antigens at sites distant from the plasma cells that produced them.

Drug Transport

After absorption or administration, many drugs enter the bloodstream and are distributed to tissues.

Some circulate freely, while others bind extensively to albumin or other plasma proteins.

Protein Binding and Distribution

Binding to plasma proteins can influence the distribution, clearance, and effective free concentration of transported molecules.

Only the unbound fraction of many substances can readily cross certain biological membranes or interact with receptors.

Heat Transport

Blood carries heat generated by metabolically active tissues and redistributes it throughout the body.

This function is especially important during exercise, fever, and exposure to changing environmental temperatures.

Transport of Heat to the Skin

Increasing blood flow through cutaneous vessels transfers more internal heat toward the body surface.

Heat can then be lost to the environment through radiation, convection, conduction, and evaporation.

Blood Flow and Transport Rate

The amount of a substance delivered to a tissue depends partly on its concentration in blood and the rate of blood flow through that tissue.

Changes in regional perfusion therefore provide an important mechanism for adjusting transport according to tissue demand.

Cardiac Output

Cardiac output determines the total volume of blood pumped into the systemic circulation per unit time.

Changes in cardiac output alter the overall rate at which oxygen, nutrients, hormones, heat, and other substances can be distributed.

Regional Blood Flow

Blood flow is not distributed equally among all tissues.

Local vascular resistance and systemic regulatory mechanisms continuously adjust perfusion according to metabolic activity and physiological priorities.

Capillary Exchange

Capillaries are the principal sites where many transported substances move between blood and tissues.

Exchange occurs through diffusion, filtration, reabsorption, transcytosis, and other mechanisms depending on the substance and capillary type.

Diffusion

Small molecules such as oxygen and carbon dioxide move largely by diffusion down concentration or partial-pressure gradients.

The thin capillary wall minimizes the distance between blood and surrounding tissues.

Bulk Fluid Movement

Water and dissolved substances can move between plasma and interstitial fluid according to hydrostatic and osmotic forces and the permeability characteristics of the capillary wall.

Lymphatic vessels return excess interstitial fluid and proteins to the circulation.

Continuous Transport Between Organs

Organ or Tissue	Major Blood Transport Relationship
Lungs	Add oxygen and remove carbon dioxide
Gastrointestinal tract	Adds absorbed nutrients and water
Liver	Processes nutrients, wastes, drugs and plasma proteins
Kidneys	Regulate water, electrolytes and metabolic wastes
Endocrine glands	Release hormones into circulation
Peripheral tissues	Receive oxygen and nutrients and release metabolic products
Skin	Participates in heat exchange

Clinical Significance

Anemia

Reduced hemoglobin concentration or red cell mass can decrease the oxygen-carrying capacity of blood.

Tissue oxygen delivery then depends increasingly on compensatory changes such as altered cardiac output and regional blood flow.

Carbon Monoxide Exposure

Carbon monoxide binds hemoglobin with high affinity and interferes with normal oxygen transport.

This can impair tissue oxygen delivery even when dissolved oxygen measurements do not fully reflect the reduction in functional hemoglobin availability.

Hypoxemia

Hypoxemia refers to abnormally low oxygen levels in arterial blood.

Depending on severity and cause, it can reduce oxygen loading onto hemoglobin and compromise tissue oxygen delivery.

Dehydration

Loss of body water can reduce plasma volume and alter concentrations of transported substances.

Significant volume depletion can also impair tissue perfusion and therefore reduce transport efficiency.

Circulatory Shock

Shock involves inadequate effective tissue perfusion and can severely compromise delivery of oxygen and nutrients and removal of metabolic products.

The underlying mechanisms vary according to the type of shock.

Renal Dysfunction

When renal function is impaired, substances normally removed or regulated by the kidneys can accumulate or become imbalanced in blood.

This demonstrates the dependence of blood transport on functioning destination organs.

Liver Dysfunction

Liver disease can alter processing of nutrients, bilirubin, drugs, toxins, hormones, and plasma proteins.

Changes in hepatic function can therefore affect both the composition of blood and the transport of numerous substances.

Transport Forms of Major Substances

Substance	Principal Transport Form
Oxygen	Bound mainly to hemoglobin
Carbon dioxide	Mainly bicarbonate, plus carbamino and dissolved forms
Glucose	Dissolved in plasma
Free fatty acids	Largely bound to albumin
Triglycerides	Within lipoproteins
Iron	Bound to transferrin
Unconjugated bilirubin	Bound to albumin
Steroid hormones	Largely protein bound
Urea	Dissolved in plasma

Key Features of Blood Transport

Feature	Key Point
Primary liquid transport medium	Plasma
Primary oxygen carrier	Hemoglobin in erythrocytes
Major carbon dioxide transport form	Bicarbonate
Major nonspecific plasma carrier	Albumin
Iron carrier	Transferrin
Lipid transport particles	Lipoproteins
Major exchange vessels	Capillaries
Endocrine transport	Hormones carried from secretory cells to targets
Waste transport	Delivery toward kidneys, liver and lungs
Thermal transport	Redistribution of heat by circulating blood

Anatomical and Physiological Importance

The transport function of blood allows anatomically separated organs to operate as an integrated system. The lungs supply oxygen and remove carbon dioxide, the gastrointestinal tract contributes absorbed nutrients, endocrine organs release hormones, the liver processes circulating substances, and the kidneys regulate and eliminate numerous plasma constituents.

Different components of blood are specialized for different transport tasks. Erythrocytes and hemoglobin provide highly efficient oxygen transport, plasma carries water-soluble molecules, plasma proteins transport many poorly soluble substances, and lipoproteins allow hydrophobic lipids to circulate through the aqueous bloodstream.

Continuous circulation and capillary exchange ultimately connect these transport pathways with individual cells. Blood therefore provides the essential internal distribution network through which tissues receive required materials, communicate through chemical signals, release metabolic products, and exchange heat with the rest of the body.