CIRCULATORY SYSTEM

(A) Open Circulatory System in Insect, (B) Closed Circulatory System in Earth Worm

The circulatory system serves to transport blood to where it can receive oxygen and remove waste products. Circulation then carries newly oxygenated blood to the tissues of the body as oxygen and other chemicals diffuse out of the blood cells into surrounding body fluid, waste product diffuse into the blood cells to be carried away to where they can be eliminated. The system serves to maintain body temperature, PH and homeostasis.

There are two types of circulatory system

• Open circulatory system: In this type of circulatory system, there is no heart like in human, rather, blood vessels acts as pump to force the blood along. These vessels join with open sinuses to force a combination of blood and interstitial fluid known as “hemolymph”, into the body cavity called  hemocoel. Open circulation occur in invertebrates such as crustaceans, insect and mollusk.

• Closed circulatory system: Here blood is pumped through closed system of arteries, vein and capillaries, this ensures that all cell receives equal amount of oxygen and eliminates waste products

Functions of blood

Blood performs many important functions; it is regarded as the major transport medium of the body and helps to transport

1. Soluble organic compounds (digested foods) are transported from the small intestine to various parts of the body where they are stored or assimilated, for example glucose s carried in the blood from the liver to the muscle.

2. Maintains constant body temperature (Homeostasis) by distributing excess heat from deeply seated tissue.

3. End product of metabolism to organs of excretion for example, urea made in the liver is transported to the kidney for excretion.

4. Hormone from glands to target organs, for example insulin is transported from the liver to the pancreases. The above mention functions of blood are performed by the plasma.

 Other important roles of blood includes the followings

5.Oxygen to tissues and carbon dioxide to the lung for excretion in the reverse reaction

6. Promotes clothing by transporting platelets and fibrinogen to wound thereby preventing excessive blood losses.

7. White blood cells is defend the body against diseases by engulfing and digesting bacteria by the process of phagocytosis.

Components of Blood

The components of blood are

• Plasma: This is a straw colored fluid containing substances of higher and lower molecular weight that constitute 10% of its volume, these substances includes, Water, ions such as Ca²⁺, Na⁺, CL^-, HCO₃^­-, etc and proteins.

The concentration of plasma protein in the blood is about 6 – 8gm/100ml and account for about 7% of the volume.

Albumen – 4.5gm/100ml

Globulin – 2.5gm/100ml

Prothrombin - 0.4gm/100ml

Fibrinogen – 0.25gm/100ml

• Red blood cells: This is also known as erythrocytes, they are circulating anucleated biconcave disk shape cells parked with oxygen (O²) carrying protein haemoglobin.  Production in children is by red marrow of long and flat bone and in adults, by flat bones of the rib, sternum, pelvis and end of long bone. They survive for 120days and are destroyed in the spleen of the liver. Their diameter is 2.5µ with a width of 2.0µ, their Packed Cell Volume (PVC) is 45, and their surface contains agglutigens which give rise to blood group.

 Erythropoietin stimulates red cell production in bone marrow. Iron, vitamin B₁₂ and folic acid are also required for red blood cell production and maturation.

Erythrocyte population is about 3.9.5million per micro liter in women and 4.1-6 per micro liter in men.

A reduction in the number of red blood cell is usually caused by anemia, some of the factors that leads to decrease in the number of this component of blood includes iron deficiency (hypochromic anemia), B₁₂ and foliate deficiency (megaloblastic anemia),  other causes are poor nutrition, abnormal destruction of red blood cell (haemolylic anemia) caused by malaria infection or abnormal haeme synthesis  such as sickle cell disease  and thalassemia from inherited blood cell disorder in which abnormal hemoglobin pigments are produced, therr is more red blood cell with insufficient amount heomoglobin,  (Aplastic anemia) also known as bone marrow failure,  is either inherited or acquired from  toxins such as those used in the production of pesticide and arsenic, radiation cancer treatments, some  antibiotics, steroids and from certain infections. 

On the other hand, increased number of erythrocytes or polycythemia occurs in people who live at high attitude when oxygen tension is low. 

• White Blood Cells: White blood cell or Leukocyte are larger than red blood cells and fewer in number  about 7000 per micro liter of blood, they are nucleated and plays an important role in body defence against disease.

They have a short life span of 13-20days and are destroyed in the lymphatics. White blood cells exhibits amoeboid movement which allows them to alter their shape and squeeze through capillary pores to get to the tissue and sight of infection………. See more.

•  Platelets: Are irregular shaped membrane bound cell fragments that has no nuclei. Platelets are formed from megakaryotes in the bone marrow. They have a diameter of 2-4µm and there are about 260,000 per mm^3,, this blood component have a short life span of 5-10days.

Platelets appears as clump in stained blood smear, it plays a key role in blood clothing and help repair gaps in the wall of blood vessels.