Topic 8: Transport In Mammals
8.1 The circulatory system
Students should be able to:
1)state that the mammalian circulatory system is a closed double circulation consisting of a heart, blood and blood vessels including arteries, arterioles, capillaries, venules and veins
2) describe the functions of the main blood vessels of the pulmonary and systemic circulations, limited to pulmonary artery, pulmonary vein, aorta and vena cava
3) recognise arteries, veins and capillaries from microscope slides, photomicrographs and electron micrographs and make plan diagrams showing the structure of arteries and veins in transverse section (TS) and longitudinal section (LS)
4) explain how the structure of muscular arteries, elastic arteries, veins and capillaries are each related to their functions
5) recognise and draw red blood cells, monocytes, neutrophils and lymphocytes from microscope slides, photomicrographs and electron micrographs
6) state that water is the main component of blood and tissue fluid and relate the properties of water to its role in transport in mammals, limited to solvent action and high specific heat capacity
7) state the functions of tissue fluid and describe the formation of tissue fluid in a capillary network
1. The Mammalian Circulatory System as a Closed Double Circulation
The mammalian circulatory system is described as a closed double circulation system, meaning that blood remains enclosed within blood vessels at all times and passes through the heart twice in each complete circuit around the body. This system consists of a heart, blood, and blood vessels (including arteries, arterioles, capillaries, venules, and veins).
In the pulmonary circulation, deoxygenated blood is pumped from the right side of the heart to the lungs via the pulmonary artery, where gas exchange occurs — carbon dioxide is exhaled, and oxygen is absorbed. The now oxygenated blood returns to the left side of the heart through the pulmonary veins. The systemic circulation then carries this oxygen-rich blood from the left ventricle through the aorta to all tissues of the body, delivering oxygen and nutrients and collecting carbon dioxide and wastes. The deoxygenated blood returns to the right atrium via the venae cavae.
This double system ensures efficient separation of oxygenated and deoxygenated blood, maintains high pressure in the systemic circuit for rapid delivery of oxygen, and allows lower pressure in the pulmonary circuit to protect delicate lung capillaries from damage.
2. Functions of the Main Blood Vessels of Pulmonary and Systemic Circulations
The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs. It is the only artery that transports deoxygenated blood in the body. Once in the lungs, carbon dioxide diffuses out, and oxygen diffuses into the blood. The pulmonary vein then returns oxygenated blood from the lungs to the left atrium — it is the only vein in the body that carries oxygenated blood.
The aorta is the body’s main systemic artery. It carries oxygenated blood from the left ventricle at high pressure and distributes it through smaller arteries and arterioles to all parts of the body. The vena cava consists of the superior vena cava, which collects blood from the upper body, and the inferior vena cava, which drains the lower body. Both carry deoxygenated blood into the right atrium of the heart. Together, these vessels maintain the unidirectional, continuous flow of blood necessary for oxygen and nutrient transport and waste removal.
3. Recognition of Arteries, Veins, and Capillaries in Microscopy
Under the microscope, these blood vessels show characteristic structural differences.
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Arteries have thick walls composed of three layers: an inner endothelium (tunica intima), a middle layer of smooth muscle and elastic fibres (tunica media), and an outer collagen layer (tunica externa). The lumen is narrow and circular.
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Veins have thinner walls, less smooth muscle and elastic tissue, and a larger lumen. Valves are often visible as crescent-shaped flaps projecting into the lumen.
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Capillaries consist of a single layer of endothelial cells, forming an extremely thin wall to facilitate diffusion. Their diameter allows the passage of red blood cells in single file.
In plan diagrams of transverse sections (TS), arteries appear circular with thick walls and a narrow lumen, while veins appear irregular and collapsed with a thin wall and large lumen. Longitudinal sections (LS) show the elongated arrangement of fibres and sometimes visible valves in veins.
4. Structure–Function Relationships in Blood Vessels
Each type of blood vessel is structurally adapted for its specific role in the circulatory system.
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Elastic arteries (such as the aorta) have thick walls rich in elastic fibres to withstand and smooth out the high pressure generated by ventricular contraction. They stretch during systole and recoil during diastole, maintaining a continuous blood flow.
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Muscular arteries have a higher proportion of smooth muscle in their walls. They can constrict or dilate (vasoconstriction and vasodilation) to regulate blood flow to specific organs.
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Veins operate under low pressure, so their thin walls and wide lumens minimise resistance to blood flow. They contain valves to prevent backflow, and surrounding skeletal muscles aid in moving blood toward the heart.
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Capillaries are extremely thin (one cell thick) with a narrow lumen to bring blood cells close to tissues, reducing diffusion distances for efficient exchange of gases, nutrients, and wastes.
5. Recognition and Function of Blood Cells
In blood smears or micrographs, four main types of blood cells are recognised:
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Red blood cells (erythrocytes) are small, biconcave discs without nuclei. Their shape provides a large surface area to volume ratio for oxygen diffusion and flexibility to pass through narrow capillaries.
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Monocytes are large, with a kidney-shaped nucleus. They develop into macrophages that engulf and digest pathogens.
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Neutrophils have a lobed nucleus (often three to five lobes) and granular cytoplasm. They are short-lived phagocytic cells that form part of the body’s first line of defence.
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Lymphocytes have a large, round nucleus occupying most of the cell. They are involved in the specific immune response, producing antibodies and coordinating other immune cells.
6. Water as the Main Component of Blood and Tissue Fluid
Water is the principal component of blood plasma and tissue fluid, acting as a solvent and a medium of transport. Its polar nature allows it to dissolve a wide range of ionic and polar substances such as glucose, amino acids, and salts, enabling them to be transported efficiently through the bloodstream.
Water also has a high specific heat capacity, meaning it can absorb or release large amounts of heat with minimal temperature change. This property stabilises the internal temperature of the body, protecting enzymes and cells from harmful fluctuations during metabolic activity.
7. Formation and Function of Tissue Fluid
Tissue fluid is formed when plasma is forced out of capillaries due to hydrostatic pressure at the arterial end of a capillary bed. Small molecules such as oxygen, glucose, and amino acids pass through the capillary wall to bathe body cells, allowing diffusion of nutrients into and waste products out of the cells.
At the venous end of the capillary bed, hydrostatic pressure falls while oncotic pressure (due to plasma proteins) draws much of the fluid back into the capillaries by osmosis. The remaining excess fluid enters the lymphatic system, becoming lymph, which eventually returns to the bloodstream. Tissue fluid thus ensures constant exchange between blood and cells and helps maintain the internal environment.
8.2 Transport of oxygen and carbon dioxide
Students should be able to:
1) describe the role of red blood cells in transporting oxygen and carbon dioxide with reference to the roles of:
• haemoglobin
• carbonic anhydrase
• the formation of haemoglobinic acid
• the formation of carbaminohaemoglobin
2) describe the chloride shift and explain the importance of the chloride shift
3) describe the role of plasma in the transport of carbon dioxide
4) describe and explain the oxygen dissociation curve of adult haemoglobin
5) explain the importance of the oxygen dissociation curve at partial pressures of oxygen in the lungs and in respiring tissues
6) describe the Bohr shift and explain the importance of the Bohr shift
8. The Role of Red Blood Cells and Haemoglobin in Gas Transport
Red blood cells contain the pigment haemoglobin, a globular protein with four polypeptide chains, each containing a haem group with an iron ion capable of binding one molecule of oxygen. In the lungs, oxygen binds to haemoglobin to form oxyhaemoglobin; this reaction is reversible.
The enzyme carbonic anhydrase, present in red blood cells, catalyses the reversible reaction of carbon dioxide with water to form carbonic acid, which dissociates into hydrogencarbonate ions (HCO₃⁻) and hydrogen ions (H⁺). Some CO₂ also binds directly to haemoglobin to form carbaminohaemoglobin, while some hydrogen ions combine with haemoglobin to form haemoglobinic acid (HHb). This buffering action prevents harmful pH changes in the blood.
9. The Chloride Shift and Its Importance
As hydrogencarbonate ions (HCO₃⁻) diffuse out of red blood cells into the plasma, chloride ions (Cl⁻) move into the cells to maintain electrical neutrality — a process known as the chloride shift. Without this exchange, the electrochemical balance across the red blood cell membrane would be disrupted, impeding the continued transport of carbon dioxide. This mechanism is vital for efficient CO₂ transport and acid–base balance.
10. Role of Plasma in Carbon Dioxide Transport
Plasma carries about 5–10% of carbon dioxide in dissolved form and acts as a transport medium for hydrogencarbonate ions, which account for the majority (around 85%) of carbon dioxide transport in the blood. Plasma also buffers pH changes and distributes dissolved gases and solutes to maintain equilibrium between tissues and lungs.
11. The Oxygen Dissociation Curve of Haemoglobin
The oxygen dissociation curve shows the relationship between the partial pressure of oxygen (pO₂) and haemoglobin saturation. It has a sigmoid (S-shaped) form because the binding of one oxygen molecule increases haemoglobin’s affinity for additional oxygen molecules — a property known as cooperative binding.
At high pO₂ (in the lungs), haemoglobin becomes almost fully saturated, whereas at low pO₂ (in respiring tissues), haemoglobin releases oxygen readily. This ensures efficient oxygen uptake and delivery according to tissue demand.
12. The Importance of the Oxygen Dissociation Curve
In the lungs, where the pO₂ is high and CO₂ is low, haemoglobin binds oxygen readily, maximising oxygen loading. In respiring tissues, where pO₂ is low and CO₂ concentration is high, haemoglobin releases oxygen efficiently. This difference is crucial for maintaining aerobic respiration and energy production in all tissues.
13. The Bohr Shift and Its Significance
The Bohr shift describes the rightward shift of the oxygen dissociation curve under conditions of increased carbon dioxide concentration, lower pH, or higher temperature. These factors reduce haemoglobin’s affinity for oxygen, promoting oxygen release where it is most needed — in actively respiring tissues producing CO₂ and heat. In the lungs, where CO₂ levels are low and pH is higher, haemoglobin’s affinity increases again, enhancing oxygen uptake. The Bohr effect thus fine-tunes oxygen delivery to match metabolic activity.
8.3 The heart
Students should be able to:
1) describe the external and internal structure of the mammalian heart
2) explain the differences in the thickness of the walls of the:
• atria and ventricles
• left ventricle and right ventricle
3) describe the cardiac cycle, with reference to the relationship between blood pressure changes during systole and diastole and the opening and closing of valves
4) explain the roles of the sinoatrial node, the atrioventricular node and the Purkyne tissue in the cardiac cycle
14. External and Internal Structure of the Mammalian Heart
Externally, the heart is a muscular organ enclosed by a protective pericardium. Four major blood vessels connect to it: the aorta, pulmonary artery, pulmonary veins, and vena cavae.
Internally, the heart is divided into four chambers: right and left atria (thin-walled) and right and left ventricles (thick-walled). Atrioventricular valves (tricuspid and bicuspid) separate atria and ventricles, while semilunar valves (pulmonary and aortic) prevent backflow from arteries into ventricles. The septum separates the two sides, preventing mixing of oxygenated and deoxygenated blood.
15. Differences in Wall Thickness of Atria and Ventricles
The atria have thin walls since they only need to push blood a short distance into the ventricles. The ventricles, however, must pump blood out of the heart — the right ventricle to the lungs and the left ventricle to the rest of the body. Therefore, the left ventricular wall is much thicker, producing the higher pressure needed to overcome greater resistance in the systemic circulation, while the right ventricle operates under lower pressure to protect the lungs.
16. The Cardiac Cycle and Valve Function
The cardiac cycle comprises:
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Atrial systole – atria contract, forcing blood into ventricles.
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Ventricular systole – ventricles contract, raising pressure; atrioventricular valves close (“lub” sound), and semilunar valves open, ejecting blood into arteries.
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Diastole – all chambers relax; semilunar valves close (“dub” sound), and blood flows into atria from veins, ready for the next cycle.
These pressure and volume changes coordinate to maintain continuous, unidirectional flow.
17. The Roles of the SAN, AVN, and Purkyne Tissue
The heartbeat is myogenic, initiated within the heart muscle itself. The sinoatrial node (SAN) in the right atrium acts as the pacemaker, generating electrical impulses that spread across atrial walls, causing them to contract. The impulse then reaches the atrioventricular node (AVN), which introduces a brief delay to ensure complete ventricular filling before contraction. The signal is then transmitted via the bundle of His and Purkyne tissue (Purkinje fibres) down the interventricular septum and up through the ventricular walls, causing coordinated contraction from the apex upward. This sequence ensures efficient ejection of blood from the ventricles.