NEET Biology Notes Body Fluids and Circulation Blood
Blood
Blood is the most common body fluid and a connective tissue, which transports nutrients, oxygen, essential substances and collects harmful substances for excretion from tissues. The later are continuously being removed for healthy functioning of fluids.
Composition of Blood
It is a special connective tissue containing a fluid matrix, plasma and formed elements.
Plasma is a coloured, viscous fluid constituting about 55% of blood. It contains fibrinogen, globulins, albumins, small amount of minerals like Na + , , Ca2+, Mg2+ ,
glucose, amino acids, lipids, etc., and factors for coagulation.
Formed elements in blood are erythrocytes, leucocytes and platelets, which are about 45% of blood.
Erythrocytes or Red Blood Cells (RBCs) are most abundant. RBCs are devoid of nucleus (nucleated in animals like frog) and biconcave in shape. RBCs are formed in red bone marrow. A healthy person has 12-16 g of haemoglobin in every 100 mL of blood. RBCs have an average life span of 120 days, after which they are destroyed in the spleen.
Leucocytes or White Blood Cells (WBCs) are nucleated and less in number of (average 6000-8000/mm3 of blood).
Platelets or thrombocytes are cell fragments produced by megakaryocytes (special cells in bone marrow). Blood contains about 1,500,00-3,500,00 platelets mm-3. Platelets are involved in clotting of blood.
Blood Groups
In humans, two types of blood grouping, i.e. ABO and Rh are widely used all over the world. ABO blood group is based on the presence or absence of two surface antigens on the RBCs, i.e. A and B. The plasma also contains two natural antibodies.
Rh Group
The Rh-antigen similar to that present in rhesus monkey is also observed on the surface of RBCs of about 80% of humans. Such persons are called Rh positive (Rh+) and in those, antigen is lacking are called Rh negative (Rh-). Rh group should be matched before transfusion. An Rh- person, if exposed to Rh+ blood, will form specific antibodies against the Rh antigens.
The Rh- blood of a pregnant mother do not mix with Rh+ antigens of the foetus in the first pregnancy as two bloods will be separated by the placenta. However, during delivery of the first child maternal blood may be exposed with the Rh+ blood of the foetus. In such cases, the mother starts preparing antibodies against Rh antigen in her blood. In case of further pregnancies, the Rh antibodies of mother (Rh-) can leak into the blood of the foetus (Rh+) and destroy the foetal RBCs.
This could be fatal to the foetus and the condition is called erythroblastosis foetalis. This can be avoided by administering anti-Rh antibodies – to the mother immediately after the delivery of first child.
Coagulation of Blood
‘ In an injured or cut site, a clot or coagulation is formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.
Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin. Thrombins are formed from prothrombin. An enzyme, thrombokinase is required for this reaction.
Coagulation of blood occurs by a series of linked enzyme reactions (cascade process) involving a number of factors present in plasma. Calcium ions also play an important role in clotting.
Mechanism of Clotting
The pathway of mechanism of blood clotting are of two types, i.e. extrinsic pathway and intrinsic pathway. It can be summarised as follows :
Composition of Lymph
The fluid collected in the spaces between the cells of tissues is called lymph or interstitial fluid or tissue fluid.
Lymph contains water and same mineral contents as in plasma. The network of vessels called lymphatic system collects this fluid and drains it back to the major veins. Lymph is a colourless fluid containing special lymphocytes responsible for the immune responses of the body.
Functions
- Exchange of nutrients, gases, hormones, etc., occur through it.
- It is responsible for the immune responses of the body.
- Fats are absorbed through lymph in the lacteals present in the intestinal villi.
Circulatory System
In higher and multicellular organisms, there is no direct supply of useful materials and removal of wastes from the body cells so, they need a transport system called circulatory system. The circulatory system are generally of two type :
1. Open Circulatory System
- In open circulatory system, blood pumped by the heart passes through the large vessels into open spaces or body cavities called sinuses. This system is found in leeches among the annelids, cockroach, prawns, insects, spiders, starfish, etc. Blood flows with very slow velocity and at low pressure. In cockroach, blood circulation is completed in 5-6 min.
- Closed Circulatory System
In closed circulatory, system the blood pumped by the heart is always circulated through a closed network of blood vessels. It is found in earthworm, Neries, molluscs and all vertebrates. Blood flows with high speed and at high pressure. Exchange of materials occurs through the tissue fluids.
Human Circulatory System
The blood vascular system transports nutrients like glucose, fatty acids, vitamins, etc., from the site of absorption to different parts of the body. It transports nitrogenous wastes like ammonia, urea, uric acid, etc., from different parts of the body to the organ of excretion.
It transports hormone from the endocrine glands to target organs. It transports water and other chemical
substances all over the body. It transports oxygen to different cells and tissues of the body from the lungs. Blood vascular system consists of three components,
i. e. heart, blood vessels and blood.
Blood Vessels
Blood vessels are generally of three types :
- Arteries
Thick-walled blood vessels, carry the blood away from the heart to various body parts.
Deep seated in body and have no valves in them. Carry oxygenated blood in them, except the pulmonary artery, which carries deoxygenated blood to the lungs. Blood flows at high pressure with high speed.
- Veins
Thin-walled blood vessels. Carry blood away from various body parts to the heart. Superficial in position and have valves in, them to prevent back flow of blood in them, blood flows at low pressure with slow speed.
Carry deoxygenated blood in them except the pulmonary vein, which carries oxygenated blood to the heart.
- Capillaries
Thinnest blood vessels. Capillaries connect arteries to the veins. Each capillary is lined by a single layer of flat cells. These help in exchange of materials like nutrients, gases, waste products, etc., between blood and cells.
Structure of Human Heart
Thick, muscular, automatic pulsating and contractile organ. A fish has only two-chambered heart (one auricle and one ventricle). In amphibians, heart is three-chambered.
Reptilian heart is structurally three-chambered but functionally four-chambered (i.e. incomplete four-chambered) except in crocodile. In crocodiles, birds and mammals, the heart is divided into four chambers (two auricles and two ventricles).
In cockroaches the heart is present between 7, 9, 12 and 13 segments.
The human heart is four-chambered, i.e. two auricles and two ventricles. Heart is made up of cardiac muscles. A nodal tissue (specialised cardiac musculature) is also distributed in heart. A patch of nodal tissue is present in the right upper comer of the right atrium called the Sino-Atrial Node (SAN). Another mass of this tissue is present in lower left comer of the right atrium close to the atrio-ventricular septum called Atrio-Ventricular Node (AVN).
A bundle of nodal fibres, atrio-ventricular bundle (AV node) continues from the AVN and divides into a right and left bundle. These branches give rise to minute fibres called Purkinje fibres. These fibres along with right and left bundles are known as bundle of His. The nodal musculature is autoexcitable. The SAN can generate the maximum number of action potentials, i.e. 70-75 min-1 and is responsible for rhythmic contractile activity of the heart. Therefore, it is called pacemaker. The human heart beats about 70-75 times per minute.
Right Auricle
It receives deoxygenated blood from superior vena cava and inferior vena cava. Right auricle opens in right ventricle through a wide circular right auriculoventricular aperture guarded by tricuspid valve. Tricuspid valve regulates unidirectional flow of blood from right auricle to right ventricle.
Right Ventricle
It gives off a large blood vessel called pulmonary artery leading to the lungs, which carries deoxygenated blood to the lungs.
Left Auricle
It receives oxygenated blood from lungs through pulmonary vein. They have no valve. Left auricle opens in left ventricle by an aperture called left auriculoventricular aperture. This aperture is guarded by bicuspid valve (mitral valve). Bicuspid valve prevents back flow of blood from left auricle to left ventricle.
Left Ventricle
It gives off a large blood vessels called the aorta. It carries oxygenated blood to various body parts. The thickest part of the human heart is the wall of the left ventricle.