The heart and circulation

Why Animals Need a Heart and Circulation

Many animals need a heart and circulation because mass transport delivers substances at speeds that diffusion alone could never achieve over long distances.

Diffusion [the net movement of particles from a high to a low concentration] works well only across short distances. For a single-celled organism, oxygen and nutrients reach every part of the cell directly through the cell surface. As an organism grows larger, its surface area to volume ratio decreases. Cells deep inside the body sit too far from the surface for diffusion to keep up with demand.

Active animals also have high metabolic demands, so their cells need a fast and constant supply of substances. Mass transport meets these demands through the bulk flow of fluid carrying dissolved substances at speed.

The mammalian system has two main parts. The heart is a muscular pump that creates the pressure needed to drive blood. Blood vessels form a network that distributes blood to every tissue and back again. Together they deliver oxygen, glucose, hormones, and heat, while removing carbon dioxide and other waste products.

Structure and Function of Blood Vessels

Each type of blood vessel has a structure that directly matches its specific function in carrying blood under different conditions of pressure and exchange.

Blood leaves the heart through arteries, passes through capillaries where exchange happens, and returns to the heart through veins. The pressure, speed, and purpose of blood flow change along this route, and the vessels are built to suit each role.

Arteries have thick walls with lots of elastic tissue and smooth muscle. The elastic tissue stretches when the heart pumps blood and recoils between beats. This recoil smooths blood flow and maintains pressure. The smooth muscle controls vessel diameter, helping to direct blood. The narrow lumen keeps blood pressure high.

Veins carry blood at low pressure back to the heart. They have thinner walls, less muscle and elastic tissue, and a wider lumen. Valves in veins prevent blood flowing backwards, since the pressure is too low to keep blood moving in one direction on its own. Skeletal muscles around veins squeeze them, pushing blood towards the heart.

Capillaries are the smallest vessels and the only ones thin enough for exchange. Their walls are a single layer of endothelium, which is one cell thick. This short diffusion distance lets oxygen, glucose, and waste pass quickly between blood and tissue cells. The lumen is so narrow that red blood cells must squeeze through one at a time, pressing close to the wall and improving exchange.

The Heart and the Cardiac Cycle

The mammalian heart is a four-chambered muscular pump that drives double circulation, with separate flows for the lungs and the body.

The heart has the following structures:

• Right atrium receives deoxygenated blood from the body through the vena cava
• Right ventricle pumps this blood through the pulmonary artery to the lungs
• Left atrium receives oxygenated blood from the lungs through the pulmonary vein
• Left ventricle pumps oxygenated blood through the aorta to the body
• The septum separates the two sides, preventing mixing of oxygenated and deoxygenated blood
• Atrioventricular (AV) valves lie between atria and ventricles (tricuspid on the right, bicuspid on the left)
• Semilunar valves lie at the base of the pulmonary artery and aorta

The left ventricle has a much thicker muscular wall than the right ventricle. This is because it must generate enough pressure to push blood around the entire body. The right side only needs to pump to the nearby lungs.

The cardiac cycle is one heartbeat, divided into three stages:

1. Atrial systole — the atria contract together. Atrial pressure rises above ventricular pressure. The AV valves stay open, and blood is pushed from atria into ventricles.
2. Ventricular systole — the ventricles contract. Ventricular pressure rises sharply, closing the AV valves (producing the first heart sound, “lub”). When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open, and blood is forced out.
3. Cardiac diastole — all four chambers relax. Ventricular pressure falls below arterial pressure, so the semilunar valves close (producing the second heart sound, “dub”). Atria refill passively from the veins, and most ventricular filling also happens at this stage.

Examiner InsightIn questions on the cardiac cycle, marks are awarded for linking pressure changes to valve actions. Always state which chamber’s pressure rises or falls. Then explain that valves open or close because of pressure differences, not muscle contraction.

Exam TipAlways say “valve closes because pressure in [X] exceeds pressure in [Y]”.

Haemoglobin and Gas Transport

Haemoglobin is a conjugated globular protein found in red blood cells that transports oxygen and helps carry carbon dioxide around the body.

Each haemoglobin molecule consists of four polypeptide chains (two alpha and two beta). Each chain holds a haem group containing an iron(II) ion. This means one haemoglobin molecule can bind up to four oxygen molecules, forming oxyhaemoglobin.

Oxygen loading and unloading depend on the partial pressure of oxygen (pO₂) [the concentration of oxygen in a mixture of gases]:

• In the lungs, where pO₂ is high, haemoglobin has a high affinity for oxygen and binds it to form oxyhaemoglobin (loading).
• In respiring tissues, where pO₂ is low, haemoglobin has a lower affinity, and oxygen dissociates from oxyhaemoglobin (unloading).

Carbon dioxide is transported in three ways:

1. About 5% dissolves directly in the plasma.
2. About 10–20% binds to amino groups of haemoglobin, forming carbaminohaemoglobin.
3. About 75–85% is carried as hydrogencarbonate ions (HCO₃⁻) in the plasma.

The hydrogencarbonate route happens inside red blood cells. The enzyme carbonic anhydrase [enzyme catalysing the reaction between CO₂ and water] catalyses the reaction:

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

Hydrogencarbonate ions then diffuse out of the red blood cell into the plasma. To maintain electrical balance, chloride ions (Cl⁻) move into the cell. This is called the chloride shift. Hydrogen ions (H⁺) bind to haemoglobin, which buffers the blood and prevents pH from falling sharply.

The Oxygen Dissociation Curve and Bohr Effect

The oxygen dissociation curve plots the percentage saturation of haemoglobin against partial pressure of oxygen, and its sigmoid shape reveals how haemoglobin loads and unloads oxygen efficiently.

The curve is S-shaped (sigmoid) because oxygen binds cooperatively. When the first oxygen molecule binds to one haem group, the haemoglobin shape changes slightly. This makes it easier for the second and third oxygen molecules to bind. The fourth oxygen binds more slowly, which gives the curve its flattening top.

The shape gives haemoglobin two ideal properties:

• At high pO₂ (in the lungs): the curve is almost flat near the top. Haemoglobin is nearly fully saturated even if pO₂ varies slightly. This guarantees efficient loading.
• At low pO₂ (in respiring tissues): the curve is steep, so a small fall in pO₂ causes a large fall in saturation. This means lots of oxygen is unloaded where it is needed.

The Bohr Effect

The Bohr effect describes how the oxygen dissociation curve shifts to the right when carbon dioxide concentration increases, or pH decreases.

In active tissues, more carbon dioxide is produced. CO₂ reacts with water to form H⁺ ions, which lower pH. These H⁺ ions bind to haemoglobin and change its shape, lowering its affinity for oxygen. As a result, oxygen unloads more readily exactly where respiration is fastest.

MisconceptionMany students say the Bohr effect “reduces oxygen binding” or “makes haemoglobin release oxygen”. The curve does not stop oxygen binding altogether. It lowers haemoglobin’s affinity at any given pO₂, so more oxygen unloads in actively respiring tissues.

Exam TipSay “lowers oxygen affinity at the same pO₂” to gain the precision mark.

Fetal Haemoglobin

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. Its dissociation curve is shifted to the left of the adult curve. At the placenta, maternal blood has already given up much of its oxygen, so pO₂ is relatively low. At this low pO₂, fetal haemoglobin can still load oxygen while adult haemoglobin would be unloading it. This allows oxygen to transfer efficiently from mother to fetus.