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Circulation notes

MDCAT Biology

Circulation is the transport system that moves oxygen, carbon dioxide, nutrients, hormones and wastes through the body. This chapter covers the human heart, cardiac cycle, blood vessels, blood composition, lymphatic system and selected links with transport in plants and other organisms.

Need for Circulation and Composition of Blood

Large multicellular organisms need a transport system because most cells are far from the external environment. Diffusion alone is too slow to supply all cells with oxygen and nutrients and to remove their wastes.

Blood is a connective tissue consisting of plasma and formed elements. Plasma carries dissolved substances, while red blood cells, white blood cells and platelets perform specialised functions.

  • Plasma is the liquid part of blood and contains water, salts, nutrients, hormones, gases and plasma proteins.
  • The main plasma proteins are albumin, globulins and fibrinogen.
  • Albumin helps maintain osmotic pressure. Globulins include antibodies and transport proteins. Fibrinogen helps in blood clotting.
  • Red blood cells contain haemoglobin and transport oxygen. They also carry part of the carbon dioxide.
  • White blood cells protect the body against pathogens and foreign substances.
  • Platelets are cell fragments that help form blood clots.
  • Haemoglobin is a respiratory pigment. Oxygen binds with the iron present in the haem group of haemoglobin.
  • In earthworms, haemoglobin is dissolved in the plasma instead of being enclosed in red blood cells.

Structure of the Human Heart

The human heart is a muscular, hollow organ in the thoracic cavity between the lungs. It is slightly tilted, so the heartbeat is heard more clearly on the left side. The ventricles, especially the left ventricle, extend towards the left side.

A septum separates the right and left sides of the heart. The right side receives and pumps deoxygenated blood, while the left side receives and pumps oxygenated blood.

  • The four chambers are the right atrium, right ventricle, left atrium and left ventricle.
  • The right atrium receives deoxygenated blood from the superior and inferior venae cavae.
  • The left atrium receives oxygenated blood from the four pulmonary veins.
  • The right ventricle pumps blood into the pulmonary trunk.
  • The pulmonary trunk divides into the left and right pulmonary arteries.
  • The left ventricle pumps oxygenated blood into the aorta.
  • The tricuspid valve lies between the right atrium and right ventricle. The bicuspid or mitral valve lies between the left atrium and left ventricle.
  • Semilunar valves are present at the bases of the pulmonary trunk and aorta. Valves prevent backflow of blood.

Route of Blood Through the Heart and Body

Deoxygenated blood from body tissues returns through veins to the right atrium. It passes into the right ventricle and is then sent to the lungs for gas exchange. This part of the circulation is called pulmonary circulation.

Oxygenated blood returns from the lungs to the left atrium. It passes into the left ventricle and is pumped through the aorta to all body tissues. This is systemic circulation.

  • Body tissues to heart: body veins, venae cavae, right atrium, right ventricle.
  • Heart to lungs: pulmonary trunk, pulmonary arteries, lung capillaries.
  • Lungs to heart: pulmonary veins, left atrium, left ventricle.
  • Heart to body: aorta, arteries, arterioles, tissue capillaries, venules and veins.
  • Pulmonary arteries carry deoxygenated blood, while pulmonary veins carry oxygenated blood.
  • The femoral veins carry deoxygenated blood from the lower limbs into the iliac veins.
  • Blood from the gut passes through the hepatic portal vein to the liver before returning to the heart.
  • The hepatic portal system allows absorbed nutrients to reach the liver for storage, processing or detoxification.

Cardiac Cycle and Heartbeat

The cardiac cycle is the sequence of events during one complete heartbeat. It includes atrial systole, ventricular systole and a period of general diastole. Systole means contraction and diastole means relaxation.

During atrial systole, the atria contract and push blood into the ventricles. The ventricles remain relaxed. During ventricular systole, the ventricles contract and force blood into the pulmonary trunk and aorta.

  • Atrial systole occurs while the ventricles are in diastole.
  • During ventricular systole, the atrioventricular valves close to prevent backflow into the atria.
  • During ventricular systole, the semilunar valves open and blood leaves the ventricles.
  • During general diastole, both atria and ventricles are relaxed and the chambers fill with blood.
  • The first heart sound, often described as lub, is produced mainly by closure of the atrioventricular valves.
  • The second heart sound, often described as dub, is produced mainly by closure of the semilunar valves.
  • The sinoatrial node acts as the natural pacemaker and initiates the heartbeat.
  • The atrioventricular node receives the impulse and helps conduct it towards the ventricles through conducting fibres.

ECG and Control of Heart Activity

An electrocardiogram, or ECG, records the electrical changes produced by the heart during a cardiac cycle. It helps relate electrical activity to contraction and relaxation of the chambers.

The autonomic nervous system can modify the rate and force of the heartbeat. Sympathetic stimulation generally increases heart rate, while parasympathetic stimulation generally decreases it. Hormones such as adrenaline can also increase cardiac activity.

  • The P wave represents electrical excitation of the atria and is followed by atrial systole.
  • The QRS complex represents electrical excitation of the ventricles and is associated with ventricular systole.
  • The T wave represents recovery or repolarisation of the ventricles.
  • The QRS complex is larger than the P wave because the ventricles have more muscle mass.
  • The pacemaker is located in the wall of the right atrium near the opening of the superior vena cava.
  • The left ventricle has a thicker muscular wall than the right ventricle because it pumps blood through the whole body.
  • The right ventricle pumps blood only to the lungs, so its wall is comparatively thinner.

Blood Vessels and Blood Pressure

Arteries carry blood away from the heart. Their walls are thick, elastic and muscular because blood leaves the heart under high pressure. Arterioles regulate the amount of blood entering capillary beds by constricting or dilating.

Veins carry blood towards the heart. Their walls are thinner and their internal diameter is generally larger than that of arteries. Valves in many veins prevent the backflow of blood, especially in the limbs.

  • Arteries usually carry oxygenated blood, except the pulmonary arteries, which carry deoxygenated blood.
  • Veins usually carry deoxygenated blood, except the pulmonary veins, which carry oxygenated blood.
  • Capillaries have walls only one cell thick, allowing exchange by diffusion and filtration.
  • Capillaries form networks between arterioles and venules.
  • The greatest blood pressure is observed in arteries, especially close to the heart.
  • Blood pressure decreases as blood moves from arteries to arterioles, capillaries and veins.
  • Atherosclerosis is the thickening and blockage of an artery due to the deposition of fatty material, including cholesterol.
  • The pulse is the rhythmic expansion of arteries caused by ventricular contraction.

Lymphatic System and Tissue Fluid

Some fluid filters out of blood capillaries into the spaces between cells. This fluid is called tissue fluid or interstitial fluid. It bathes cells and allows exchange of materials between cells and blood.

Lymph is formed when tissue fluid enters lymph capillaries. Lymph capillaries end blindly in body tissues. Accumulation of interstitial or extracellular fluid produces pressure that forces fluid into these capillaries.

  • Lymph capillaries have thin, permeable walls and collect excess tissue fluid.
  • Lymph vessels carry lymph towards larger ducts and finally return it to the blood circulation.
  • Lymph nodes filter lymph and contain lymphocytes that help defend the body.
  • The lymphatic system returns excess fluid and plasma proteins to the blood.
  • Lymph absorbs and transports fats from intestinal villi through specialised lymph capillaries called lacteals.
  • Lymph flow is helped by skeletal muscle contraction, breathing movements and valves in lymph vessels.
  • Unlike blood circulation, the lymphatic system has no central pump equivalent to the heart.

Related Transport Facts and Comparisons

Some transport principles are also seen in plants and in other organisms. Hydrogen bonding between water molecules produces cohesion, which helps water molecules remain together during transport in xylem. Water movement and cell turgidity depend on water potential.

Guard cells control stomatal opening by changes in their solute concentration and turgor. These plant facts are distinct from animal circulation but use related ideas of transport and pressure.

  • Hydrogen bonding between water molecules produces cohesion.
  • When a cell is fully turgid, its water potential is zero relative to the usual reference convention.
  • A plant cell reaches maximum turgidity when placed in distilled water, provided the cell membrane is intact.
  • Conversion of sugar into starch in guard cells lowers their solute concentration and causes the stomatal pore to close, often completely.
  • Conversion of starch into organic acids contributes to stomatal closing by changing guard-cell solute conditions.
  • The average velocity of sugar movement in phloem is about 1 metre per hour.
  • In Plasmodium, gametozoites are the stage associated with the mosquito rather than the human body.
  • O negative blood is commonly called the universal donor for red blood cell transfusion, but compatibility testing is still required.
  • Blood containing more carbon dioxide is darker, reddish-purple or deoxygenated in appearance than oxygen-rich blood.

Key terms

Circulation
The movement of blood or other transport fluid through a body.
Plasma
The liquid part of blood in which cells and dissolved substances are suspended.
Haemoglobin
An iron-containing respiratory pigment in red blood cells that binds oxygen.
Atrium
An upper chamber of the heart that receives blood.
Ventricle
A lower chamber of the heart that pumps blood out of the heart.
Pulmonary circulation
The circulation of blood between the heart and lungs.
Systemic circulation
The circulation of blood between the heart and the body tissues.
Cardiac cycle
The complete sequence of contraction and relaxation during one heartbeat.
Systole
The phase in which a chamber of the heart contracts.
Diastole
The phase in which a chamber of the heart relaxes.
Pacemaker
The sinoatrial node that initiates the normal heartbeat.
Artery
A blood vessel that carries blood away from the heart.
Vein
A blood vessel that carries blood towards the heart.
Capillary
A microscopic blood vessel with a wall one cell thick, specialised for exchange.
Atherosclerosis
Thickening and narrowing of an artery due to fatty deposits such as cholesterol.
Lymph
Fluid formed from tissue fluid that enters lymphatic vessels.
Interstitial fluid
Fluid present in the spaces between body cells.
Cohesion
The attraction between molecules of the same substance, such as water molecules.

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Recognising acellular organisms

If an entity has no cellular organisation and depends on a host for replication, identify it as a virus. Viruses are non-cellular and are not placed among cellular organisms.

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Use ribosomes as the common feature. Both cell types have ribosomes, but only eukaryotes have a membrane-bound nucleus and membrane-bound organelles.

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