Circulatory System


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In today’s Biology class, We will be learning about the Circulatory System. We hope you enjoy the class!



  • Protective Function of Blood
  • Circulatory System in Mammals
  • Blood Vessels and the Heart



The blood performs two basic functions in mammals and

These are:

  • Transportation of various substances
  • Protection of body against diseases


The blood defends the body in major ways

  1. Antibody production (Clumping): The lymphocytes produce antibodies which are any of a large variety of proteins normally present in the body or produced in response to an antigen which it neutralizes, thus producing an immune response. This can also be ensured by injecting a small dose of the weaker or dead pathogens into the body. This process is known as vaccination or immunization
  2. Neutralization: The whole blood will also produce antitoxins which neutralize the toxins produced by the pathogens.
  3. Phagocyte Action (Engulfing): Phagocytes engulf the pathogens and digest them.
  4. Clotting: The blood platelets clot the blood to prevent germs from entering the body and also prevent loss of blood.


MECHANISM OF CLOTTING                     

When a blood vessel is damaged and exposed to air, platelets in the bloodstream release an enzyme thrombokinase. The thrombokinase then converts prothrombin (inactive blood protein) to an enzyme called thrombin. The thrombin then converts the soluble fibrinogen in the plasma to insoluble fibrin in the presence of calcium ions. The threadlike fibrin then forms a network or mesh on the surface of the wound and blood cells are trapped within the network or mesh to form a clot. The clot dries to scab over the wound.

Blood clotting


  1. Describe the mechanism of blood clotting.
  2. What is vaccination?



The circulatory system in animals can be:

  • Closed or open
  • Single or double

Closed Circulatory System: This involves the blood vessel called arteries which divides capillaries which later join up with other vessels called veins. Blood is therefore limited to the vessels and the heart it does not have direct contact with the body cells this system is observed in annelids and vertebrates.


Opened Circulatory System: The blood vessels lead out of the heart but end in blood spaces called haemocoels within the body cavity. The blood has direct contact with the cells after which it is returned to the heart. Arthropods and some molluscs have an open circulatory system

Open+vs.+Closed+Circulatory+Systems biology classnotesng


Single Circulatory System: The blood passes through the heart once in a complete movement round the body e.g. fishes because of their two-chambered heart have a single circulatory system.


Double Circulatory System: The blood passes twice in the heart every time it makes one complete movement around the body. Each time it passes through a separate pathway e.g. mammals. Double circulation gives rise to pulmonary and systematic circulation.

single vs double urculation biology classnotesnng 



  • Arteries: These are vessels that carry oxygenated blood away from the heart to the body’s organs except for pulmonary artery that carries deoxygenated blood.
  • Arteriole: A branch of an artery that gives rise to capillaries.
  • Veins: These are large vessels that carry deoxygenated blood toward the heart except for pulmonary vein which carries oxygenated blood.
  • Venule: Small vessel that carries blood from the capillaries to the veins.
  • Capillaries: They link the arteries with the veins around the tissues and organs. They are tiny and thin-walled to facilitate easy exchange of gasses nutrients and waste products between the cells and the blood.



  1. Define (a) artery (b) venule (c) capillary
  2. What is a double circulatory system?



                      ARTERY VEIN
1 It has a thick (muscular walls) It has a thin wall
2 Elastic wall Non-elastic wall
3 Carries blood away from the heart Returns blood to the heart
4 Carries oxygenated blood except for pulmonary artery Carries deoxygenated blood except for the pulmonary vein
5 Blood is pink or bright in colour Blood is dark red in colour
6 Situated deep in muscles Superficially situated
7 Has small lumen Large lumen
8 Pressure is high Pressure is low
9 Pulse is readily detectable Pulse is not easily detected
10 It has no valve except semilunar valves It has valves




The heart is a muscular and powerful organ responsible for pumping blood in the system of mammals. It is located within the chest cavity and protected in the ribs and sternum. The pumping action of the heart is known as the heartbeat. Heartbeat per second varies from animal to animal and is often increased at moments of excitement e.g. Humans’ heart is about 72 beats per minutes.

The special muscles making up the heart is known as cardiac muscles and the heart is enclosed in a two-layered tough protective membrane called the pericardium. Pericardial fluid fills the space between the heart and pericardium. It reduces the friction arising from the pumping movement of the heart against its surrounding tissues.



The human heart is divided into four chambers, the right and left auricles, the right and left ventricles. The walls of the ventricles are often thicker than those of the auricles. The left ventricles especially have a thick wall because it pumps blood out to all other parts of the body and this requires more pressure.

The heart is divided into two halves by a central barrier called the septum. Bicuspid valves separate the left auricle and the left ventricle. This ensures that blood flows only in one direction i.e. from the auricles to the ventricles. Similarly, the tricuspid valve exists between the right auricle and the right ventricle. It serves the same function as the former. These bicuspid valves are held in place by special fibres, non-elastic cords known as chordae tendineae.



The heartbeat consist of alternate contractions and relaxation of the right and left auricles as well as the right and left ventricle. The human heartbeat is about 72 beat per minutes this can be divided into two phases, namely

  • Diastole: This is the first stage of the heartbeat; the two auricles contract forcing blood into the ventricles and oxygenated blood into the left ventricle. As the ventricles get filled up, the cuspid valves are pushed up and closed.
  • Systole: This is the second phase of the heartbeat. The ventricles contract sending blood out to the two tracks of the main arteries and out of the heart. Deoxygenated blood from the right ventricles passes into the pulmonary artery while oxygenated blood is sent into the aorta. The sequence repeats itself.



Blood vessel supply


Blood Vessel (Artery)                 Organ Supplied

  • Carotid artery …….               Head
  • Pulmonary artery ……..            Lungs
  • Hepatic artery ……..              Liver
  • Gastric artery ……..             Stomach
  • Mesenteric artery ……..            Intestine
  • Renal artery ………     kidney
  • Gonadal artery ………          Gonads
  • Intercostals artery   ………        Wall of thorax

Corresponding veins accompany the arteries stated above



  1. Draw a well-labelled diagram of the heart
  2. Differentiate between arteries and veins
  3. Explain systole and diastole.
  4. Draw the cross-section of the artery
  5. What is pulmonary circulation?


Reading Assignment

College Biology, Chapter 7, Page 136 – 157




  1. The following are the basic functions of blood except A.Transportation of substances B. Reproduction         C. Protection of the body D. Transportation of hormones
  1. ………. is actively involved in preventing excessive loss of blood at injury A. Red blood cell B. White blood cell C. Platelet D. Serum
  1. The following except one have closed circulatory system. A. Snail B. Earthworm C. man D. lion
  2. The semilunar valve is present in A. Arteries B. Veins C. Capillaries D. Bladder
  3. The human heartbeat is …….. per minute A. 60 B. 66 C. 72 D. 80



  1. With a well-labelled diagram only, describe the structure of the heart.
  2. State the differences between arteries and veins




We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

In our next class, we will be learning about Transport System in Higher Plants. We are very much eager to meet you there.


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