Circulation and Blood
The Circulatory System
Blood consists of:
- Red cells
- White cells
- Platelets
- Plasma which contains tissue fluid and lymph
Blood Vessels
1. Arteries
2. Capilliaries
3. Veins
Heart
1. Double circulation
2. Cardiac cycle
3. Cardiac muscle
The purpose of a circulatory system
Very small organisms can obtain their supplies of oxygen and nutrients by diffusion from the outside.
Large organisms like humans have a large volume in relation to their body surface. Simple diffusion
cannot cope and a transport system is needed. The circulatory system transports substances from one
part of the body to another and is necessary to supply and remove all the substances needed by the
body cells. Every cell in the body requires a supply of oxygen and food and the removal of waste
products.
Blood
A fluid containing dissolved substances and cells. The colour is given by red blood cells.
Red Blood Cells
Red Blood Cells also called erythrocytes are the most numerous cells in the blood. About four to five
million are found in 1mm cube of blood (about the size of the head of a pin).
Red blood cells have no nuclei and has bi-concave shape. They are made in the bone marrow. Vitamin
b12 is needed to make red blood cells. After about 120 days the cells are destroyed and broken down in
the liver and spleen.
The function of red blood cells is to transport oxygen around the body. They contain haemoglobin.
Oxygen diffuses into the red cells in the capillaries of the alveoli of the lungs. Here it combines loosely
with haemoglobin to make oxyhaemoglobin. Oxygen can easily split off from oxthaemoglobin again and
diffuse into body cells where it is used for respiration.
White Blood Cells
White cells or leucocytes have a nucleus and are made in the bone marrow, spleen and lymph nodes.
There are about 10000 in 1 mm cube of blood. One type of white cell is a phagocyte meaning it can
absorb particles by engulfing them. Phagocytes can pass through capillary walls into tissues and they
can engulf microorganisms in a process called phagocytosis. Other types of white blood cells are
lymphocytes which can make substances which destroy disease organisms. These substances are called
antibodies. Hence white blood cells help defend the body against infection in 2 main ways.
Plasma
Plasma is the liquid part of the blood. It is 90%water and 10% solutes. This liquid part of the blood
carries out the majority of transport functions. Nutrients from digested food diffuse from the small
intestine into capillaries in the villi. From these capillaries blood passes into small veins that join
together to form the hepatic portal vein. This vein carries the digested foods to the liver and then to the
body cells. Nitrogenous waste from proteins is transported from cells to the kidneys via blood plasma.
Hormones are also carried in blood plasma from the endocrine glands to the organs where the
hormones function. Plasma also supplies water to the tissues and helps blood carry heat around the
body.
Platelets
Platelets have no nuclei and are small fragments about ¼ diameter of erythrocytes. There are about
250000 in 1mm cube of blood. They are colourless and help in blood clotting at wounds.
Blood clotting
Skin cannot prevent the entry of bacteria if it becomes cut. A blood clot is very important to prevent the
entry of bacteria as well as prevent the loss of blood. A clot consists of a mesh of fibres which trap the
blood cells and platelets at the surface of the wound. The platelets and the damaged vessels release a
protein called thromboplastin. This acts on a blood protein called prothrombin and turns it into
thrombin with the use of calcium salts. Thrombin acts on fibrinogen and turns it into insoluble fibrin.
This fibrin forms the fibres in the clot. Red blood cells and platelets become trapped in these fibres.
Prothrombin is made in the liver. Vitamin K must be present to make prothrombin and hence its
importance in the diet.
The Functions of Blood
Red Blood Cells – Transport oxygen from lungs to tissues and some carbon dioxide from tissues to lungs
White Blood Cells – Produce Antibodies from lymphocytes in response to disease organisms and render
them harmless. Harmful substances in the blood are destroyed by phagocytosis
Platelets – helps clotting: release thromboplastin to start it, becomes trapped in fibrin threads.
Plasma – transports carbon dioxide, mainly as bicarbonate from tissues to lungs. Transports waste eg
urea from tissues to excretory organs eg kidneys. Transports hormones from ductless glands to effector
organs. Transports nutrients from intestines to liver and then to tissues. Transports heat to all parts of
the body. Supplies tissue fluid including water to tissues.
Blood Vessels
Arteries
- Thick muscular elastic walls
- Valves are absent
- Blood under high pressure and flows in spurts
- Oxygenated blood except pulmonary artery
- Carry blood away from the heart
Veins
- Thinner walls
- Valves are present
- Blood under low pressure and flows smoothly
- Deoxygenated blood except pulmonary vein
- Carry blood towards the heart
Capillaries
- Very thin walls only 1 cell thick
- Valves are absent
- Low pressure and slow flow
- Oxygen and carbon dioxide exchanged
- Substances pass through walls
- Carry blood from arterioles to venules
The structure and function of the heart and associated vessels
Heart – pumps blood to lungs and all body parts
Vena Cava – returns deoxygenated blood from body to right atrium
Right atrium – receives deoxygenated blood and pumps it to the right ventricle
Right ventricle- pumps blood to lungs
Pulmonary artery – carries deoxygenated blood to lungs
Pulmonary vein – returns oxygenated blood from lungs
Left atrium- receives oxygenated blood and pumps it to left ventricle
Left ventricle – pumps blood around the body
Aorta – carries oxygenated blood from the heart
Cardiac muscle – contracts without fatigue
Tricuspid valve/ bicuspid valve – prevent backflow of blood into atria
Valve tendons – support valves
Semilunar valves – prevent backflow of blood into ventricles
The functions of the lymphatic system
- Transport tissue fluid back into the blood
- Transport fats particularly from villi to blood
- Remove excess fluid, protein and foreign material from tissue spaces
- Body defence- microorganisms are destroyed in lymph glands by white blood cells, phagocytes
and lymphocytes
Summary
Organisms that have a large volume to surface area require a transport system
Simple diffusion could not supply and remove nutrients and wastes from large bodies so the blood
circulation system does this
The haemoglobin in red blood cells transports oxygen
Plasma transports carbon dioxide, waste, hormones, nutrients, water and heat around the body
The white blood cells have phagocytes that engulf, while lymphocytes make antibodies to destroy
bacteria.
Platelets start the clotting process. They become trapped in fibres with red cells to form a protective
clot over wounds. This is important to prevent infection and blood loss.
Arteries with thick walls carry blood in pulses under pressure from the heart.
Veins with thinner walls and valves carry blood more sluggishly to the heart
Capillaries have walls only 1 cell thick to allow diffusion of substances to and from cells
The heart made of muscle pumps blood around the body
The thick muscular wall of the left ventricle forces blood in arteries around the body
Atria have thinner walls, with valves between atria and ventricles to prevent backflow
The pulmonary circulation passes blood from heart to lungs and back, then the systemic circulation
pumps it around the rest of the body.
Hypertension is high blood pressure. This is a symptom of arteriosclerosis (artery walls harden) and
atherosclerosis ( artery walls thicken with fats) and may cause coronary heart attacks.
Tissue fluid bathes the cells transporting substances between them and the blood
Lymph drains fluids away from cells in lymph vessels and transports fats.
Lymph glands act as filters for harmful microorganisms and contain white cells to destroy them.
Carotic
art
Jugular vein – neck
Portal-
Iliac artery- supplying trunk and legs
