Transport in Animals¶

Part of Module 3: Exchange and transport.

This topic extends the logic of exchange surfaces to internal transport. Once an animal becomes multicellular and metabolically active, diffusion alone cannot move substances quickly enough between the exchange surface and every cell in the body. The solution is a circulatory system — a network of vessels, a pump, and a transport fluid — that delivers oxygen and nutrients while removing waste products.

Learning Objectives¶

Why a Transport System is Needed¶

Multicellular organisms cannot rely on diffusion for internal transport for two reasons. First, diffusion distances from the outer surface to interior cells are too large; exchange rates fall sharply as distance increases. Second, larger, more active organisms have higher metabolic rates, demanding rapid delivery of oxygen and glucose and rapid removal of carbon dioxide and other wastes. Simple diffusion across the body surface is simultaneously too slow and too limited in total capacity. A dedicated circulatory system solves both problems.

Types of Circulatory System¶

Open and Closed Systems¶

In an open circulatory system (found in insects), blood — more accurately called haemolymph — is not fully enclosed in vessels. It flows freely through the body cavity (haemocoel), bathing organs directly. Blood returns to a simple tubular heart through valves. This low-pressure system is adequate for small animals but cannot sustain high oxygen delivery rates. In a closed circulatory system (vertebrates), blood remains within vessels at all times. Arteries distribute oxygenated blood; veins return deoxygenated blood to the heart. Closed systems can sustain much higher pressures and therefore faster delivery.

Single and Double Circulations¶

Fish have a single closed circulation: blood passes through the heart once per circuit. The heart pumps deoxygenated blood to the gills, where it picks up oxygen, and it then flows directly to the body before returning to the heart. Blood pressure is significantly reduced after passing through the gill capillaries, limiting the speed of delivery to body tissues.

Mammals have a double closed circulation: blood passes through the heart twice per complete circuit. The right side of the heart pumps deoxygenated blood to the lungs (pulmonary circuit), and the oxygenated blood returns to the left side. The left side then re-pressurises the blood before pumping it through the aorta to the body tissues (systemic circuit). This separation allows the systemic circuit to operate at a much higher pressure than the pulmonary circuit, enabling faster delivery to distant or metabolically demanding tissues such as the kidneys and muscles.

The two sides of the mammalian heart are separated by the septum, which prevents mixing of oxygenated and deoxygenated blood.

Blood Vessels¶

There are five main types of blood vessel, each adapted to its position and role in the circuit.

Arteries¶

Arteries carry blood away from the heart at high pressure. Their walls contain three layers: an outer layer of collagen (providing structural strength to prevent bursting), a thick middle layer of smooth muscle and elastic fibres containing elastin, and an inner endothelial lining. The elastic fibres are critical: during systole, the vessel wall stretches to accommodate the surge of blood, and during diastole it recoils, smoothing the pressure wave into a more continuous flow. Smooth muscle allows arteries to constrict (vasoconstriction) or dilate (vasodilation), redistributing blood between organs according to demand.

Arterioles¶

Arterioles are smaller than arteries and have a proportionally larger lumen. They have more smooth muscle and less elastic tissue than arteries, because they do not need to withstand the same peak pressures. As the final gatekeepers before capillary beds, arterioles control local blood flow through vasoconstriction and vasodilation.

Capillaries¶

Capillaries form extensive networks within tissues. Their walls are just a single endothelial cell thick, giving a very short diffusion distance. Their lumen is so narrow that red blood cells must pass through in single file, pressed close to the capillary wall — reducing diffusion distance still further. The highly branched architecture of capillary networks creates an enormous total surface area for exchange. Oxygen, glucose, amino acids, hormones, and waste products all diffuse between capillary blood and tissue fluid across this thin wall.

Venules and Veins¶

Venules collect blood from capillaries and drain into veins. Both have thin walls with little smooth muscle or elastic tissue — blood pressure is low at this stage of the circuit and thick walls are unnecessary. The thinner walls also mean veins are easily compressed by surrounding skeletal muscle during movement, which helps push blood toward the heart. Pocket valves in venules and veins prevent backflow when the vessel is compressed. The wide lumen of veins offers low resistance to flow, aiding venous return to the heart.

Blood Composition¶

Blood is a connective tissue consisting of plasma and formed elements suspended within it.

• Plasma: the liquid component; mostly water. Transports dissolved gases, nutrients (glucose, amino acids, fatty acids), hormones, metabolic waste (urea, CO₂), heat, and clotting factors.
• Red blood cells (erythrocytes): biconcave, anucleate cells packed with haemoglobin. Carry oxygen from the lungs to respiring tissues.
• White blood cells (leucocytes): immune cells involved in defence; includes phagocytes and lymphocytes.
• Platelets (thrombocytes): cell fragments involved in blood clotting.

Tissue Fluid and Lymph¶

Formation of Tissue Fluid¶

Tissue fluid (interstitial fluid) is the medium through which cells actually exchange substances with the blood. It fills the spaces between cells and has a composition similar to plasma, but without most large plasma proteins, which are too large to pass through the capillary wall, and with fewer red and white blood cells.

Tissue fluid forms at the arterial end of capillary beds. Here, hydrostatic pressure — generated by the pumping action of the heart — is high enough to force water, dissolved gases, and small solutes out through the capillary wall into the surrounding tissue space. Proteins and cells are retained in the blood.

At the venous end of capillary beds, hydrostatic pressure has fallen. The large plasma proteins that were retained in the blood have been concentrated by the loss of fluid, so oncotic pressure (the osmotic pull exerted by dissolved proteins) is relatively high. Water potential in the capillary blood is therefore lower than in the tissue fluid, and water moves back into the capillary by osmosis. However, not all tissue fluid is reabsorbed by this route.

Lymph¶

Excess tissue fluid — the fraction not reabsorbed by capillaries — drains into blind-ended lymph capillaries (lymphatic vessels). Once inside the lymphatic system, this fluid is called lymph. Lymph is transported through lymph vessels by the compression of surrounding muscles, with valves preventing backflow. It passes through lymph nodes, where pathogens are filtered out and lymphocytes can act on foreign material. Lymph is ultimately returned to the blood at the subclavian veins near the base of the neck.

Lymph has a composition similar to tissue fluid, but with more white blood cells (lymphocytes), more fatty acids (absorbed from the small intestine via lacteals), and less oxygen and nutrients than fresh tissue fluid.

Haemoglobin and Oxygen Transport¶

Structure and Loading¶

Haemoglobin is a globular protein with a quaternary structure: four polypeptide subunits, each associated with a haem prosthetic group. Each haem group contains an iron ion (Fe²⁺) that can bind one molecule of oxygen reversibly. A single haemoglobin molecule can therefore carry up to four oxygen molecules, forming oxyhaemoglobin.

Oxygen loading in the lungs and unloading in the tissues depends on the partial pressure of oxygen (pO₂, measured in kPa). At the high pO₂ in the pulmonary capillaries, haemoglobin has high affinity for oxygen and loads it readily. At the low pO₂ in actively respiring tissues, affinity decreases and oxygen dissociates.

The Oxygen Dissociation Curve¶

The relationship between pO₂ and haemoglobin saturation is shown by the oxygen dissociation curve. This curve is S-shaped (sigmoidal), a shape explained by cooperative binding: when the first oxygen molecule binds to one haem group, it causes a conformational change in the whole molecule that makes it easier for the next three to bind. Conversely, once three oxygens are bound, it becomes harder to load the fourth, and saturation approaches 100% asymptotically. In the steep portion of the curve, small changes in pO₂ produce large changes in saturation — this region corresponds to the pO₂ found in respiring tissues, maximising oxygen release exactly where it is needed.

The Bohr Effect¶

At higher partial pressures of carbon dioxide (pCO₂), haemoglobin has lower affinity for oxygen. This is the Bohr effect, and it arises through a precise biochemical mechanism. In respiring tissues, CO₂ enters red blood cells and is converted to carbonic acid by carbonic anhydrase:

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

The hydrogen ions produced bind to haemoglobin (forming haemoglobinic acid), causing a conformational change that lowers oxygen affinity and promotes oxygen release. On the oxygen dissociation curve, the Bohr effect shifts the curve to the right: for any given pO₂, saturation is lower. This means that in tissues producing large quantities of CO₂ — precisely the tissues with the highest oxygen demand — more oxygen is unloaded. The system is therefore self-regulating: greater metabolic activity produces more CO₂, which drives greater oxygen unloading.

Fetal Haemoglobin¶

Fetal haemoglobin (HbF) differs from adult haemoglobin (HbA) in its protein subunit composition. HbF has a higher affinity for oxygen than HbA at any given pO₂. This means the fetal oxygen dissociation curve lies to the left of the adult curve. At the placenta, both fetal and maternal blood are present. As the mother's haemoglobin releases oxygen in response to the relatively low pO₂ and the Bohr effect, the fetal haemoglobin — with its higher affinity — picks it up. This ensures efficient transfer of oxygen from maternal to fetal blood across the placenta.

Carbon Dioxide Transport¶

Carbon dioxide is transported from respiring tissues to the lungs in three ways:

1. Dissolved in plasma (~7%): a small proportion diffuses directly into the plasma and is carried in solution.
2. As carbaminohaemoglobin (~23%): CO₂ binds directly to the globin chains of haemoglobin (not to the haem groups), forming carbaminohaemoglobin.
3. As hydrogencarbonate ions (HCO₃⁻) in plasma (~70%): the major route. CO₂ enters red blood cells, and carbonic anhydrase catalyses its conversion to carbonic acid, which dissociates to H⁺ and HCO₃⁻. The HCO₃⁻ exits the red blood cell into plasma. To maintain electrical balance, chloride ions move into the red blood cell — the chloride shift. The H⁺ ions are buffered by binding to haemoglobin, which also explains the Bohr effect.

At the lungs, where pCO₂ is low, these reactions run in reverse: HCO₃⁻ re-enters red blood cells, H⁺ is released from haemoglobin, and they recombine to form CO₂, which diffuses into the alveoli and is exhaled.

Heart Structure¶

The mammalian heart is a four-chambered muscular pump. The two atria (left and right) are upper, thin-walled chambers that receive blood from veins. The two ventricles (left and right) are lower chambers with much thicker muscular walls that pump blood into arteries.

The left ventricle wall is thicker than the right ventricle wall. Both ventricles contract simultaneously and generate the same stroke volume, but the left ventricle must pump blood through the long, high-resistance systemic circuit to all organs in the body, requiring substantially higher pressure than the right ventricle, which only pumps blood the short distance to the nearby lungs.

The septum divides the two sides of the heart, ensuring no mixing of oxygenated and deoxygenated blood.

Heart Valves¶

Four valves prevent backflow and ensure unidirectional blood flow:

• Tricuspid valve: between right atrium and right ventricle. Prevents backflow into the right atrium during ventricular contraction.
• Bicuspid (mitral) valve: between left atrium and left ventricle. Prevents backflow into the left atrium during ventricular contraction.
• Both atrioventricular valves are held by tendinous cords (chordae tendineae) attached to papillary muscles, preventing them from inverting.
• Semilunar valves: one in the aorta (aortic valve) and one in the pulmonary artery (pulmonary valve). Both prevent backflow of blood from the arteries into the ventricles when the ventricles relax.

Blood Vessels of the Heart¶

• Pulmonary vein: carries oxygenated blood from the lungs to the left atrium.
• Aorta: carries oxygenated blood from the left ventricle to the body.
• Vena cava: carries deoxygenated blood from the body to the right atrium.
• Pulmonary artery: carries deoxygenated blood from the right ventricle to the lungs.

Note that the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — the naming convention follows direction relative to the heart, not oxygen content.

The Cardiac Cycle¶

The cardiac cycle is the repeating sequence of contraction (systole) and relaxation (diastole) that drives blood around the body. It has three main stages:

Stage 1 — Atrial systole: The atria contract, raising atrial pressure. The atrioventricular valves open (atrial pressure exceeds ventricular pressure) and blood is pushed into the ventricles. The ventricles are relaxed (diastole).

Stage 2 — Ventricular systole: The ventricles contract, raising ventricular pressure above atrial pressure. The atrioventricular valves close, producing the first heart sound ("lub"). As ventricular pressure rises further to exceed pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected into the arteries. The atria are relaxed and begin to fill with returning venous blood.

Stage 3 — Diastole: Both atria and ventricles relax. Ventricular pressure falls below arterial pressure, causing the semilunar valves to close (second heart sound, "dub"). Blood flows passively from the veins into the atria and gradually into the relaxed ventricles. The cycle then repeats.

Cardiac Output¶

Cardiac output (CO) is the total volume of blood ejected by the left ventricle per minute:

Cardiac output (cm³ min⁻¹) = heart rate (min⁻¹) × stroke volume (cm³)

A typical resting cardiac output is about 5000 cm³ min⁻¹ (70 beats min⁻¹ × ~70 cm³ stroke volume). Both heart rate and stroke volume can increase during exercise to meet higher metabolic demands.

Coordination of the Heartbeat¶

Cardiac muscle is myogenic: it can initiate its own contractions without external nervous stimulation. The co-ordination of the heartbeat depends on a sequence of electrical excitation through specialised conducting tissues:

1. Sino-atrial node (SAN): located in the wall of the right atrium; acts as the pacemaker. Spontaneously generates electrical impulses that spread across both atria, causing atrial systole.
2. Layer of non-conducting collagen fibres: prevents direct electrical spread from the atria to the ventricles, ensuring the atria finish contracting before the ventricles begin.
3. Atrio-ventricular node (AVN): at the junction of atria and ventricles. Picks up the electrical signal from the atria and imposes a short delay (approximately 0.1 s) before passing it on. This delay allows ventricular filling to complete.
4. Bundle of His (atrioventricular bundle): a band of specialised conducting tissue that runs down through the interventricular septum to the apex of the heart.
5. Purkyne fibres (Purkinje fibres): branch from the bundle of His and spread throughout the ventricular walls. They conduct excitation rapidly, causing ventricular contraction to begin at the apex and spread upward, efficiently ejecting blood toward the aorta and pulmonary artery.

The ECG¶

An electrocardiogram (ECG) records the electrical events of the cardiac cycle using surface electrodes. The trace shows three main features:

• P wave: corresponds to atrial systole (depolarisation of the atria).
• QRS complex: corresponds to ventricular systole (depolarisation of the ventricles). The Q and S deflections are small; the R wave is the large upward spike.
• T wave: corresponds to diastole (repolarisation of the ventricles).

The interval between successive P waves gives the heart rate. ECGs can reveal abnormalities:

• Tachycardia: abnormally rapid heart rate; peaks closer together.
• Bradycardia: abnormally slow heart rate; peaks further apart.
• Ectopic heartbeats: extra beats arising from a focus outside the SAN, creating irregular spikes.
• Atrial fibrillation: rapid, disorganised atrial activity replacing the regular P wave with irregular baseline activity; QRS complexes are irregularly spaced.

Common Confusions¶

Blood flow, blood pressure, and electrical coordination of the heart are related but distinct concepts. Pressure drives the opening and closing of valves; the electrical signal initiates contraction; contraction generates the pressure. Understanding each of these separately and then connecting them is more reliable than conflating them.

The Bohr effect is specifically about haemoglobin's response to CO₂ concentration. It should not be described simply as "CO₂ pushes oxygen off haemoglobin" — the mechanism runs through carbonic acid, H⁺ ions, and conformational change.

Tissue fluid differs from plasma because proteins cannot cross the capillary wall; it differs from lymph because lymph is enriched in lymphocytes and fatty acids. None of these three fluids is the same as the others.

Key Terms¶

• Open circulatory system: a system in which blood is not fully enclosed in vessels and flows freely through the body cavity.
• Closed circulatory system: a system in which blood is enclosed within vessels at all times.
• Double circulation: a system in which blood passes through the heart twice per complete circuit, allowing separate pulmonary and systemic pressures.
• Vasoconstriction: narrowing of a blood vessel by smooth muscle contraction, reducing blood flow.
• Vasodilation: widening of a blood vessel by smooth muscle relaxation, increasing blood flow.
• Plasma: the liquid component of blood; mostly water; transports dissolved substances.
• Tissue fluid: fluid forced out of capillaries by hydrostatic pressure; surrounds cells and is the medium for local exchange.
• Hydrostatic pressure: the outward pressure exerted by blood against capillary walls, driving fluid into tissue spaces.
• Oncotic pressure: the osmotic pull exerted by plasma proteins that draws water back into capillaries at the venous end.
• Lymph: fluid derived from excess tissue fluid; transported through lymphatic vessels; richer in lymphocytes and fatty acids than tissue fluid.
• Haemoglobin: the iron-containing protein in red blood cells that reversibly binds oxygen.
• Oxyhaemoglobin: haemoglobin with oxygen bound to its haem groups.
• Oxygen dissociation curve: a sigmoidal graph showing the relationship between pO₂ and haemoglobin saturation; reflects cooperative binding.
• Bohr effect: the reduction in haemoglobin's affinity for oxygen at higher pCO₂, caused by H⁺ ions from carbonic acid binding to haemoglobin.
• Cooperative binding: the property of haemoglobin whereby binding one oxygen molecule increases the ease of binding subsequent oxygen molecules.
• Fetal haemoglobin (HbF): haemoglobin with higher oxygen affinity than adult haemoglobin; allows oxygen uptake from the mother's blood across the placenta.
• Carbonic anhydrase: enzyme in red blood cells that catalyses the interconversion of CO₂ and water with carbonic acid.
• Chloride shift: movement of Cl⁻ ions into red blood cells as HCO₃⁻ exits, maintaining electrical neutrality.
• Hydrogencarbonate ion (HCO₃⁻): the form in which ~70% of CO₂ is transported in blood plasma.
• Carbaminohaemoglobin: haemoglobin with CO₂ bound directly to globin chains (~23% of CO₂ transport).
• Aorta: the main artery leaving the left ventricle, carrying oxygenated blood to the systemic circuit.
• Vena cava: the major vein returning deoxygenated blood from the body to the right atrium.
• Atrium: an upper heart chamber that receives blood from veins.
• Ventricle: a lower, thicker-walled heart chamber that pumps blood into arteries.
• Septum: the muscular wall separating the left and right sides of the heart.
• Atrioventricular valve: a valve (tricuspid or bicuspid/mitral) between an atrium and a ventricle; prevents backflow during ventricular systole.
• Semilunar valve: a valve at the base of the aorta or pulmonary artery; prevents backflow into ventricles during diastole.
• Cardiac output: heart rate × stroke volume; total volume of blood pumped per minute.
• Myogenic: able to initiate contraction independently of nervous stimulation (property of cardiac muscle).
• Sino-atrial node (SAN): the pacemaker of the heart; generates the initiating electrical impulse in the right atrium.
• Atrio-ventricular node (AVN): conducts and delays the electrical impulse between atria and ventricles.
• Bundle of His: conducting tissue conveying impulses from the AVN down the interventricular septum to the apex.
• Purkyne fibres: branches from the bundle of His that rapidly conduct excitation through the ventricular walls from apex to base.
• Diastole: the relaxation phase of the cardiac cycle when chambers fill with blood.
• Systole: the contraction phase of the cardiac cycle when chambers pump blood.
• ECG (electrocardiogram): a trace of the heart's electrical activity; P wave = atrial systole; QRS = ventricular systole; T wave = diastole.
• Tachycardia: abnormally fast heart rate.
• Bradycardia: abnormally slow heart rate.
• Atrial fibrillation: irregular, disorganised atrial activity; QRS complexes irregularly spaced on ECG.

Connected Pages¶

• 3.1.1 Exchange surfaces
• 3.1.3 Transport in plants
• 2.1.2 Biological molecules