This blog post provides readers with the following objectives. The reader will be able to:

o     Explain the concept “Transport” and its need in mammals.
o     Describe the structure of the mammalian heart.
o     Explain the mechanism of heart excitation and contractions.
o     Describe the structure of blood vessels.
o     Describe the composition of blood.
o     State the functions of blood.
o     Describe circulation of blood of a mammal.
o     Explain the formation of lymph.
o      Outline the functions of lymph. 

Mammalian Heart


The circulatory system consists of two main divisions;

¨      Cardiovascular System: consists of the heart and the blood vessels which pump and carry blood respectively.

¨      Lymphatic System: Consists of the lymphatic vessels and lymphoid tissues within the spleen thymus, tonsils and lymph nodes.

Types of Circulatory System

Open Circulatory System

In an open circulatory system, blood is pumped from the heart through blood body cavities, where it flows over tissues. Blood flows slowly because there is little blood pressure. Arthropods and most mollusks have an open circulatory system. 

Closed Circulatory System

In a closed circulatory system, blood is contained within blood vessels, pressure is high, and blood is therefore pumped faster. Valves prevent the backflow of blood within the blood vessels. This type of circulation is found in vertebrates and some invertebrates such as annelids, squids.


Blood is a fluid connective tissue consisting of cells and plasma. It carries oxygen and nutrients to and waste materials away from all body tissues. It is slightly alkaline, with a pH between 7.35 – 7.45. Blood accounts for approximately 8% of the body weight.

Functions of the Blood  

The functions of the blood are categories into transport, protection and regulation.


o  carries oxygen from the lungs to other organs

o  carries nutrients from the digestive system to all parts of the body

o  carries carbon dioxide from cells to the lungs for removal

o  carries hormones from the endocrine glands to their target cells

o  carries other wastes to the liver and kidneys for detoxification and excretion

o  carries metabolic heat to the skin for removal; and help to stabilize body heat

o  carries platelets and blood proteins


o  Leukocytes destroy some microorganisms and cancerous cells. 

o Antibodies and complement proteins neutralize toxins or help to destroy microorganisms.

o  Clotting of blood to minimize excessive blood loss.


o  Blood transfers water to and from body tissues which help to stabilize water content.

o  Regulation of acid-base balance: Blood helps to stabilize body pH (7.35-7.45).

o  Regulate body temperature; warm blood is transported from the interior to the surface of the body, where heat is released from the blood.

Components of Blood

Blood is the only fluid tissue in the body.  It has both Cellular and liquid components. That is, the blood consists of living blood cells suspended in a nonliving fluid matrix called plasma

Types of blood cells (Cellular Components)

o   Erythrocytes (Red Blood Cells)

o  Leukocytes (White Blood Cells)

o   Thrombocytes (Platelets)


Plasma is the liquid part of blood.  It is a pale yellow fluid that consists of about 91% water and 9% other substances, such as proteins, ions, nutrients, gases and waste products.

o     Plasma proteins: constitute 7% to 9% of the plasma.  The three types of proteins are; Albumins, Globulins and Fibrinogen. Other Proteins/Regulatory Substances include metabolic enzymes, antibacterial proteins and hormones.

o  Waste Products: such as by-products of cellular metabolism such as urea, uric acid, creatinine and ammonium salts.

o    Nutrients (Organic Materials): these are materials absorbed from digestive tract and transported for use throughout the body.  They include glucose and other simple carbohydrates, amino acids, fatty acids, glycerol and vitamins.

o   Electrolytes/ ions: such as cations and anions.  The cations are sodium, potassium, calcium, magnesium.  The anions include chloride, sulphate and bicarbonate.

o   Gases: include oxygen, carbon dioxide and nitrogen. They are necessary for aerobic respiration; electron-transport chain and also help buffer blood.

Erythrocytes (Red Blood Cells)

Red blood cell is a small, flattened, biconcave disc shape.  It lacks nucleus and mitochondrion. It contains pigmented protein called hemoglobin that binds oxygen. The biconcave shape increases the surface area for more absorption of oxygen.

Absence of nucleus enables the cell to pack more haemoglobin. It also makes the cell more flexible to squeeze through blood capillaries.  Red blood cell is manufactured by stem cell in the bone marrow. It has a short life span of about 120 days..

Human red blood cells
Scanning electron micrograph of human red blood cells

Hemolysis of Red blood cell

Red blood cell ruptures (haemolysed) by Kupffer cells of the liver or macrophages in the spleen and bone marrow. Iron in the cell is released and stored in the liver or carried to the bone marrow for the production of new red blood cell.

Liver cells (hepatocytes) pick-up the remaining portion of the hemoglobin, convert into bilirubin and release it in the bile.

Leukocytes (White Blood Cells)

White blood cell is large, irregular shape cell.  It contains nuclei, mitochondria and can move in an amoeboid fashion.  Amoeboid ability enables the cell to squeeze through pores in capillary walls.  White blood cells are classified according to their staining properties.  Leukocytes that have granules in the cytoplasm are called granulocytes or granular Leukocytes without visible granules are called Agranulocytes.


Granulocytes are large, roughly spherical and have a short life span.  They have rounded nuclei masses connected by thin nuclear material. Granulocyte are phagocytes.

Types of granulocytes; neutrophils, basophils and eosinophils.

o    Neutrophils: the granules take up both basic (blue) and acidic (red) dyes color. The nuclei consist of three to six lobes. Neutrophils are involved in engulfing and destroying foreign invaders e.g. bacteria. They also secrete lysozymes, enzymes which destroy certain bacteria.

o      Eosinophils (Acidiophils): contain granules that stain bright red with eosin, an acidic stain. They produce enzymes that destroy inflammatory chemicals like histamine.  They also release toxic chemicals that attack certain worm e.g. tapeworms, flukes, hookworms.

o    Basophils: contain large cytoplasmic granules that stain blue or purple with basic dyes.  They contain histamine, which release in tissues to increase inflammation.  They also release heparin, which inhibits blood clotting. 


They include lymphocytes and monocytes.  They lack visible cytoplasmic granules.  The nuclei are spherical or kidney shaped.

o     Lymphocytes: are produced by the lymph glands or the lymph nodes. They have large rounded nucleus with small amount of non-granular cytoplasm. Lymphocytes produce antibodies which fight against viruses.

o     Monocytes: are largest leukocytes, have kidney shaped nuclei.  They are transformed into macrophages and migrate through tissues. Macrophages are phagocytic in nature and they defend the body against viruses and certain intracellular bacterial parasites.

Platelets (Thrombocytes)

Platelets are actually not true cells. They are small fragments of large cells called megakaryocytes. They consist of small amount of cytoplasm surrounded by a plasma membrane.  They are ovoid, irregular or disk-like in shape. The cytoplasm contains actin and myosin which cause contraction of platelet. Platelets are produced within the red bone marrow. Platelets play an important role in blood clotting.


When there is cut or wound, coagulation or blood clotting, results in the formation of a clot. 

Blood clot: is a network of thread like protein fibers, called fibrin that traps blood cells and fluid. Blood clotting depends on proteins, called coagulation factors, found within plasma. Coagulation factors are in an inactive state and activated by ions to produce a clot.

Mechanism of Blood Clotting

1.   At cut or wound, the damaged tissues and blood platelets release an enzyme called thromboplastin also known as tissue factor.

2.  The thromboplastin together with calcium ions and vitamin K, convert (inactive) protein prothrombin to active enzyme thrombin.

3.  The thrombin then converts soluble protein fibrinogen into insoluble threads of fibrin.

4.  The fibrin threads form a mesh or network to trap blood cells to form a clot.

5.  White blood cells within the clot fight against invaders at the cut or wound. 

mechanism of blood clotting


The heart is a pear-shaped, hollow muscular organ that pumps blood throughout the blood vessels. It is located within the middle thoracic cavity. It is enclosed by a double -walled Sac called the pericardium or pericardial SacThe pericardium contains pericardial fluid which nourishes the heart and prevents shocks. The heart consists of four chambers; two upper chambers with thin called atria or (auricles) and two lower chambers with thick walls ventricles. The right side is separated from the left side by a muscular wall called median septum.

The heart also has valves. 

o   Tricuspid valve lies between the right atrium and the right ventricle. It consists of three flaps (hence the name). The flaps are held in position by tendons.

o   Bicuspid valve (or mitral valve) with two flaps lies between the left atrium and the left ventricle. The valves prevent backflow of blood from the ventricles to the atria.

Several large veins; superior vena cava and the inferior vena cava carry blood from the body to the right atrium and pulmonary veins carry blood from the lungs to the left atrium. Two arteries, the aorta and the pulmonary artery, exit the heart. Pulmonary artery carries blood from the right ventricle to the lungs, whilst the aorta carries blood from the left ventricle to body tissues. 

Structure of Mammalian heart

Mode of Action of the Heart / Cardiac cycle

The cardiac cycle refers to repeated pattern of contraction and relaxation of the heart.  The contraction phase is called systole and relaxation phase is called diastole. When the two atria contract the blood is forced into the relaxed ventricles. After a slight pause, the two ventricles contract, forcing the blood into the arteries. The backflow of blood into the atria is prevented by the sudden closing of the tricuspid and the bicuspid valves. The closing of these valves produces a loud “lub” sound which we can hear in a heartbeat.

After the ventricles have fully contracted, they start to relax. As they relax, the blood in the arteries tends to flow back into the ventricles. This is prevented by the sudden closing of the semi-lunar valves which produces a soft “dub” sound. Ventricular contraction (systole) makes a “lub” sound. Ventricular relaxation (diastole) makes a “dub” sound. A systole and a diastole make up one heartbeat. There is a short pause between two heartbeats. The average normal heart beat of an adult is about 72 times per minute.



Blood vessels are closed tubes through which blood circulates around the body. There are three main types of blood vessels: arteries, veins and capillaries.

artery and vein


Arteries are thick, strong, elastic vessels that carry blood away from the heart. Arteries give rise to thinner tubes or branches called arteriolesThey have narrow lumen with no valves. Thick wall enables the artery to withstand high pressure of blood from the heart.

The wall of an artery consists of three distinct layers. 

o   The innermost layer (tunica interna) made of simple epithelium called endothelium. Endothelium prevent blood clotting by provide a smooth surface that allows blood cells and platelets to flow through without being damaged. 

o   The middle layer (tunica media) has thick layer of smooth muscles and elastic fibers. The fibers enable the arteries to dilate or stretch when the heart pumps blood into them at high pressure. 

o   The outer layer (tunica externa) is thin and consists of connective tissue with elastic and collagen fibers.  This layer attaches the artery to the surrounding tissues. 


Veins are vessels that carry blood from capillaries back to the heart. Veins have thin wall, less muscular and less elastic. They have wide lumen with valves to prevent backflow of blood. The blood flows with less resistance. Smallest veins are called venules.


Capillary is small, narrow tube with one-cell-thick walls and narrow lumen.  It composed only of endothelium with a basement membrane. Capillaries supply cells with materials and oxygen and take away waste products. Capillaries link the arterioles to the venules within the tissues.

Structure of blood vessels artery, vein and capillary

Differences between Arteries and Veins 



Carry blood away from the heart

Carry blood towards the heart

Transports blood under high pressure

Transports blood under lower pressure

Blood flows fast

Blood flows more slowly and smoothly

Have relatively narrow lumens

Have relatively wide lumens

Have more muscles/ elastic tissue

Have relatively less muscles/elastic tissue

Have no semi lunar valves

Have semi lunar valves to prevent back flow of blood

Carry red oxygenated blood (except: pulmonary arteries)

Carry bluish-red deoxygenated blood (except: pulmonary veins)

Pathways of Blood Circulation  

Illustration of double circulation of blood in human

In mammals, there is a double circulation, i.e. blood passes through the heart twice in one complete circuit. The double circulation consists of two parts:

Pulmonary circulation

From the heart, the pulmonary arteries carry the deoxygenated blood to the lungs. In the lungs the blood gains oxygen and at the same time releases carbon dioxide. The oxygenated blood is then returned to the heart by the pulmonary veins.

Systemic circulation

From the left side of the heart, the oxygenated blood is then distributed by arteries to all parts of the body (except the lungs).  The blood releases the oxygen to be used for tissue respiration and at the same time gains carbon dioxide. Deoxygenated blood from body parts is then carried back to the right side of the heart by veins.


Lymphatic system is part of circulatory system which consists of the lymphatic vessels, lymph nodes and lymphoid organ (the spleen, thymus, and tonsils).

o      Lymphatic Vessels: are similar to veins, including the presence of valves. They depend on the movement of skeletal muscles to move the fluid inside. They contain fluid called lymph. They empty into the circulatory system via the thoracic duct and the right lymphatic duct. The thoracic duct is much larger than the right lymphatic duct.

o      Lymph Nodes: are small, spherical or ovoid structures that are connected to lymphatic vessels. They contain open spaces (sinuses), each with many leukocytes which function to remove infectious pathogens and foreign particles.

o      Spleen: stores blood. It helps purify blood that passes through it by removing bacteria and worn-out or damaged red blood cells.

Functions of lymphatic system

o      It takes up excess tissue fluid and return it to the circulatory system

o      It absorbs and transports fats from the intestinal villi to the circulatory system

o      It transports white blood cells to and from the lymph nodes into the bones

o      It defends against disease

Formation Lymph and Tissue Fluid

The cells in the walls of the blood capillaries do not fit together exactly. So there are small gaps between them. Plasma and white blood cells leak out of the blood capillaries. Red blood cells however too large to pass through these gaps. The fluid formed in this way is called tissue fluid. It surrounds all the cells in the body.

In the tissues, together with the blood capillaries are the lymphatic capillaries. The tissue fluid slowly drains into the lymphatic capillaries. The fluid is now called lymph. The lymphatic capillaries join up to form larger lymphatic vessels. The lymphatic vessels carry the lymph to the sub-clavian veins where it enters the venous blood. The lymph vessels have valves to ensure that movement is only in one direction. Lymph flows much more slowly than blood.

As the lymph flows from the tissues to the sub-clavian vein, it passes through several lymph nodes and lymph tissues (e.g. spleen). Lymph nodes contain large numbers of white cells. Most bacteria or toxins in the lymph are destroyed by these cells.

formation o tissue fluid and lymph from blood

Functions of Lymph

 It supplies cells with oxygen and food nutrients diffuse from the blood

It also removes waste products from the body cells back into the blood 

Carries pathogens to the lymph nodes to be phagocytized 

Returns tissues fluids to the blood stream

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