Revision NotesAS

circulatory system — A system that carries fluids around an organism’s body.

In mammals, this is a closed double circulatory system involving the heart, blood vessels, and blood. Its primary role is to transport oxygen, nutrients, hormones, and waste products, much like a city's plumbing system with a central pump maintaining flow.

closed blood system — A circulatory system made up of vessels containing blood.

In a closed system, blood always remains within the blood vessels, ensuring efficient transport and maintaining high pressure. This is similar to a sealed central heating system where fluid stays within pipes, never spilling out, though exchange still occurs across capillary walls.

double circulation — A circulatory system in which the blood passes through the heart twice on one complete circuit of the body.

This system consists of two loops: the pulmonary circulation (heart to lungs and back) and the systemic circulation (heart to body and back). This allows for higher blood pressure in the systemic circuit for efficient delivery to tissues, like two separate train lines starting and ending at the same station.

systemic circulation — The part of the circulatory system that carries blood from the heart to all of the body except the gas exchange surface, and then back to the heart.

This circuit delivers oxygenated blood and nutrients to body tissues and collects deoxygenated blood and waste products. It operates at a higher pressure than the pulmonary circulation, acting like the main highway system of a country.

pulmonary circulation — The part of the circulatory system that carries blood from the heart to the gas exchange surface and then back to the heart.

This circuit transports deoxygenated blood from the right ventricle to the lungs to pick up oxygen and release carbon dioxide, then returns oxygenated blood to the left atrium. It operates at a lower pressure to protect delicate lung capillaries, much like a short shuttle bus route to an airport.

artery — Vessel with thick, strong walls that carries high-pressure blood away from the heart.

Arteries have thick, elastic, and muscular walls to withstand and smooth out the high-pressure, pulsed flow of blood from the heart. Their structure allows them to stretch and recoil, maintaining blood flow, similar to a high-pressure fire hose.

vein — Vessel with relatively thin walls that carries low-pressure blood back to the heart.

Veins have thinner walls and larger lumens compared to arteries, as they carry blood at much lower pressure. They contain semilunar valves to prevent backflow of blood, especially against gravity, functioning like a drainage pipe.

arteriole — Small artery.

Arterioles contain a lot of smooth muscle in their walls, allowing them to constrict (vasoconstriction) or dilate (vasodilation) to control blood flow to capillary beds and reduce blood pressure before it enters capillaries, acting like a tap controlling water flow.

venule — Small vein.

Venules are formed when capillaries join together, collecting blood from the capillary beds and gradually merging to form larger veins. They have thin walls, reflecting the low blood pressure, much like small streams joining to form larger rivers.

capillary — The smallest blood vessel, whose role is to deliver oxygen and nutrients to body tissues, and to remove their waste products.

Capillaries have extremely thin walls, only one cell thick (endothelium), and a very narrow lumen, just wide enough for red blood cells to pass in single file. This structure facilitates rapid exchange of substances between blood and tissue fluid, like a fine mesh net.

endothelium — A tissue that lines the inner surface of a structure such as a blood vessel.

It consists of a single layer of flat cells (squamous epithelium) that provides a very smooth surface, minimizing friction with moving blood. In capillaries, this single layer forms the entire wall, facilitating diffusion, much like a smooth, non-stick lining of a pipe.

squamous epithelium — One or more layers of thin, flat cells forming the lining of some hollow structures, e.g. blood vessels and alveoli.

In blood vessels, a single layer of squamous epithelium forms the endothelium, providing a smooth surface and a short diffusion distance for substance exchange in capillaries. Its thinness is key to its function, like a single layer of thin, flat tiles.

smooth muscle — A type of muscle that can contract steadily over long periods of time.

Found in the walls of arteries and arterioles, smooth muscle allows for sustained contraction or relaxation, altering the diameter of the vessels. This controls blood flow and pressure to different tissues, responding to nerve impulses and hormones, similar to the slow contraction of a sphincter muscle.

elastic arteries — Relatively large arteries, which have a lot of elastic tissue and little muscle tissue in their walls.

These arteries, like the aorta, are close to the heart and stretch to accommodate the high-pressure blood surge from the ventricles. Their recoil helps to 'even out' blood flow and maintain pressure during diastole, like a large, strong rubber band.

muscular arteries — Arteries that are closer to the final destination of the blood inside them than elastic arteries, with more smooth muscle in their walls which allows them to constrict and dilate.

These arteries regulate blood flow to specific regions of the body by contracting or relaxing their smooth muscle. This control is vital for diverting blood to active tissues or reducing flow to less active ones, like adjustable pipes in a plumbing system.

vasoconstriction — The narrowing of a muscular artery or arteriole, caused by the contraction of the smooth muscle in its walls.

This process reduces blood flow to a particular area and increases blood pressure upstream. It is controlled by nerve impulses and hormones, allowing the body to divert blood to other tissues, similar to squeezing a garden hose.

vasodilation — The widening of a muscular artery or arteriole, caused by the relaxation of the smooth muscle in its walls.

This process increases blood flow to a particular area and decreases blood pressure upstream. It is also controlled by nerve impulses and hormones, allowing for increased supply to active tissues, like opening a tap fully.

semilunar valve — A half-moon shaped valve, such as the ones in the veins and between the ventricles and arteries.

These valves prevent the backflow of blood. In veins, they ensure unidirectional flow towards the heart, especially against gravity. In the heart, they are found at the exits of the ventricles (aortic and pulmonary valves) to prevent blood from flowing back into the ventricles during diastole, acting like a one-way gate.

plasma — The liquid component of blood, in which the blood cells float; it carries a very large range of different substances in solution.

Plasma is mostly water (about 95%) and contains dissolved nutrients, waste products, hormones, and plasma proteins. It plays a crucial role in transport and heat distribution throughout the body, much like the water in a river carrying various dissolved substances.

plasma proteins — A range of several different proteins dissolved in the blood plasma, each with their own function; many of them are made in the liver.

These proteins, such as albumin, are too large to easily escape capillaries and are crucial for maintaining the water potential of the blood. They contribute to osmotic pressure, drawing water back into capillaries from tissue fluid, like large, non-diffusible molecules creating an osmotic pull.

tissue fluid — The almost colourless fluid that fills the spaces between body cells; it forms from the fluid that leaks from blood capillaries.

Tissue fluid is similar to blood plasma but contains far fewer protein molecules and no red blood cells. It acts as the medium for exchange of materials (oxygen, nutrients, waste) between blood and body cells, like a 'soup' surrounding individual cells.

neutrophil — One type of phagocytic white blood cell; it has a lobed nucleus and granular cytoplasm.

Neutrophils are the commonest type of phagocyte, destroying invading microorganisms by phagocytosis. They are a key component of the innate immune system, rapidly responding to infection, like 'first responder' police officers.

monocyte — The largest type of white blood cell; it has a bean-shaped nucleus; monocytes can leave the blood and develop into a type of phagocytic cell called a macrophage.

Monocytes circulate in the blood and then migrate into tissues, where they differentiate into macrophages. Macrophages are powerful phagocytes and also act as antigen-presenting cells, linking innate and adaptive immunity, like a 'reserve' police officer transforming into a detective.

macrophage — Phagocytic cell found in tissues throughout the body; they act as antigen-presenting cells (APCs).

Macrophages engulf and digest pathogens and cellular debris. They also present antigens from pathogens to lymphocytes, initiating specific immune responses. They are crucial for both innate immunity and activating adaptive immunity, like a 'clean-up crew' that also collects evidence.

lymphocyte — A white blood cell with a nucleus that almost fills the cell, which responds to antigens and helps to destroy the antigens or the structure that is carrying them.

Lymphocytes are central to adaptive immunity, recognising specific antigens. They include B cells (producing antibodies) and T cells (cell-mediated immunity). They are smaller than most phagocytes and have a large, round nucleus, acting like 'special forces' soldiers.

haemoglobin — A protein with a quaternary structure, found in red blood cells, that transports oxygen.

Haemoglobin is a respiratory pigment containing iron, which reversibly binds to oxygen. Its structure allows for cooperative binding, meaning the binding of one oxygen molecule increases the affinity for subsequent oxygen molecules, making it highly efficient for oxygen transport.

partial pressure — A measure of the concentration of a gas.

In a mixture of gases, the partial pressure of a specific gas is the pressure it would exert if it alone occupied the volume. It is used to describe gas concentrations in blood and alveoli, influencing gas diffusion, like how much of the total noise in a room is made by one group.

percentage saturation — The degree to which the haemoglobin in the blood is combined with oxygen, calculated as a percentage of the maximum amount with which it can combine.

This value indicates how much oxygen haemoglobin is carrying relative to its full capacity. It varies with the partial pressure of oxygen, as shown by the dissociation curve, and is crucial for efficient oxygen transport, like a battery charge indicator.

dissociation curve — A graph showing the percentage saturation of a pigment (such as haemoglobin) with oxygen, plotted against the partial pressure of oxygen.

The S-shaped curve illustrates haemoglobin's affinity for oxygen: high affinity at high partial pressures (lungs) for loading, and lower affinity at low partial pressures (tissues) for unloading. The shape reflects cooperative binding, like a graph showing how many seats on a bus are filled at different stops.

Bohr shift — The decrease in affinity of haemoglobin for oxygen that occurs when carbon dioxide is present.

High partial pressures of carbon dioxide (and thus lower pH) cause haemoglobin to release oxygen more readily. This is advantageous in actively respiring tissues where CO2 is high and oxygen is needed, shifting the dissociation curve to the right, like a taxi driver eager to drop off passengers in a busy area.

carbonic anhydrase — An enzyme found in the cytoplasm of red blood cells that catalyses the reaction between carbon dioxide and water to form carbonic acid.

This enzyme rapidly converts CO2 into carbonic acid, which then dissociates into hydrogen ions and hydrogencarbonate ions. This reaction is crucial for efficient carbon dioxide transport and buffering blood pH, acting like a super-fast chemical 'mixer'.

chloride shift — The movement of chloride ions into red blood cells from blood plasma, to balance the movement of hydrogencarbonate ions into the plasma from the red blood cells.

As hydrogencarbonate ions move out of red blood cells into the plasma for transport, chloride ions move into the red blood cells to maintain electrical neutrality across the cell membrane. This is essential for continuous CO2 transport.

carbaminohaemoglobin — A compound formed when carbon dioxide binds with haemoglobin.

A small proportion of carbon dioxide is transported in the blood by binding directly to the amine groups of haemoglobin, forming carbaminohaemoglobin. This binding is reversible and contributes to the overall transport of CO2.

cardiac muscle — The type of muscle that makes up the walls of the heart.

Cardiac muscle is a specialised type of muscle found only in the heart. It is myogenic, meaning it can contract and relax rhythmically without nervous stimulation, and its continuous, involuntary contractions are essential for pumping blood throughout the body.

coronary arteries — Arteries that branch from the aorta and spread over the walls of the heart, supplying the cardiac muscle with nutrients and oxygen.

The heart muscle itself requires a constant supply of oxygen and nutrients to sustain its continuous pumping action. The coronary arteries ensure this vital supply, branching directly from the aorta to deliver oxygenated blood to the cardiac muscle tissue.

septum — The layer of tissue that separates the left and right sides of the heart.

The septum is a muscular wall that divides the heart into two distinct halves, preventing the mixing of oxygenated and deoxygenated blood. This separation is crucial for maintaining the efficiency of the double circulatory system in mammals.

atrium — One of the chambers of the heart that receives low-pressure blood from the veins.

The atria are the upper chambers of the heart. The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs. They act as collecting chambers before pumping blood into the ventricles.

ventricle — One of the chambers of the heart that receives blood from the atria and then pushes it into the arteries.

The ventricles are the lower, more muscular chambers of the heart. The right ventricle pumps deoxygenated blood to the lungs, while the left ventricle pumps oxygenated blood to the rest of the body. The left ventricle has a thicker muscular wall to generate higher pressure for systemic circulation.

atrioventricular valve — A valve between the atria and ventricles that closes when the ventricles contract and stops backflow of blood into the atria.

These valves, the bicuspid (mitral) on the left and tricuspid on the right, ensure unidirectional blood flow from the atria to the ventricles. They prevent blood from being forced back into the atria during ventricular contraction.

bicuspid valve — The atrioventricular valve on the left side of the heart.

Also known as the mitral valve, the bicuspid valve is located between the left atrium and the left ventricle. It has two cusps and prevents the backflow of oxygenated blood into the left atrium when the left ventricle contracts.

tricuspid valve — The atrioventricular valve on the right side of the heart.

The tricuspid valve is located between the right atrium and the right ventricle. It has three cusps and prevents the backflow of deoxygenated blood into the right atrium when the right ventricle contracts.

cardiac cycle — The sequence of events that takes place during one heartbeat.

The cardiac cycle involves a coordinated series of contractions (systole) and relaxations (diastole) of the atria and ventricles. This cycle ensures efficient filling and emptying of the heart chambers, maintaining continuous blood flow throughout the body.

atrial systole — The stage of the cardiac cycle when the muscle in the walls of the atria contracts.

During atrial systole, the atria contract, pushing the remaining blood into the ventricles. This ensures that the ventricles are fully filled before they begin to contract, maximising the efficiency of blood ejection.

ventricular systole — The stage of the cardiac cycle when the muscle in the walls of the ventricles contracts.

Ventricular systole involves the powerful contraction of the ventricular walls, forcing blood out into the pulmonary artery (from the right ventricle) and the aorta (from the left ventricle). During this phase, the atrioventricular valves close to prevent backflow into the atria, and the semilunar valves open.

diastole — The stage of the cardiac cycle when the muscle in the walls of the heart relaxes.

During diastole, all chambers of the heart relax, allowing blood to flow from the veins into the atria and passively into the ventricles. This relaxation phase is crucial for the heart to refill with blood before the next contraction cycle.

myogenic — A word used to describe muscle tissue that contracts and relaxes even when there is no stimulation from a nerve.

The heart's cardiac muscle is myogenic, meaning it generates its own electrical impulses that initiate contraction. This intrinsic ability allows the heart to beat rhythmically and continuously without direct nervous input, though nerve signals can modify the rate.

sinoatrial node (SAN) — A patch of cardiac muscle in the right atrium of the heart which contracts and relaxes in a rhythm that sets the pattern for the rest of the heart muscle.

The SAN is the natural pacemaker of the heart. It generates electrical impulses that spread across the atria, causing them to contract. This rhythmic impulse generation is fundamental to the heart's myogenic activity.

atrioventricular node (AVN) — A patch of tissue in the septum of the heart which transmits the wave of excitation from the walls of the atria and transmits it to the Purkyne tissue.

The AVN receives the electrical impulse from the SAN and introduces a crucial delay before transmitting it to the ventricles. This delay ensures that the atria complete their contraction and empty blood into the ventricles before ventricular contraction begins.

Purkyne tissue — A bundle of fibres that conduct the wave of excitation down through the septum of the heart to the base (apex) of the ventricles.

The Purkyne tissue (also known as the Bundle of His and Purkyne fibres) rapidly conducts the electrical impulse from the AVN down the septum and then upwards through the ventricular walls. This ensures that the ventricles contract from the apex upwards, efficiently pushing blood into the arteries.