Chapter 15: Body Fluids and Circulation

The components of the formed elements in blood are as follows:

• Erythrocytes or red blood cells – They carry oxygen and contain the pigment called haemoglobin. Haemoglobin reacts with oxygen to form Oxyhaemoglobin that carries oxygen to areas that are deprived of oxygen in the body.
• Leucocytes or white blood cells – Lymphocytes are known to synthesise antibodies that neutralise or kill germs. Neutrophils act as a defence mechanism against bacteria, known as phagocytosis.
• Thrombocytes or blood platelets – They aid in the coagulation of blood.

Some of the plasma proteins and their significance are given below.

• Globulins – They are involved in the defence mechanism of the body and are also referred to as immunoglobulins.
• Albumins –They aid in maintaining the osmotic balance of the body.
• Fibrinogens – It plays a significant role in blood coagulation.

Blood is a connective tissue as it is mesodermally derived and contains an extra-cellular matrix known as plasma. It is an abundant and widely distributed tissue in the body. Connective tissues link and bind, providing support to other organs of the body, thereby transporting oxygen and other nutrients within the body, eliminating waste products from the body and flowing throughout the body. Hence, it is considered a connective tissue.

The difference between lymph and blood is as follows:

Double circulation, as the name suggests, is where the blood circulates twice in the heart. Double circulation is possible, as the heart is divided into four chambers – the right and the left halves – by the atrioventricular septum.

The two circulations are

1. Pulmonary circulation
   • Blood in the right ventricle is pumped into the pulmonary arteries.
   • For oxygenation, these pulmonary arteries transport deoxygenated blood to the lungs.
   • The oxygenated blood is then sent to the left atrium from the lungs through the pulmonary veins.
   • This type of circulation of blood is referred to as pulmonary circulation, where blood is pumped via pulmonary blood vessels.

1. Systemic circulation
   • It is a term used to refer to the major circulation of the body.
   • Oxygenated blood is pumped from the left ventricle into the aorta.
   • Furthermore, it is carried by the arteries, arterioles and the linkage of blood capillaries.
   • Simultaneously, deoxygenated blood is accumulated in the right atrium through the inferior and superior vena cava.
   • This circulation supplies nutrients and oxygen and carries away carbon dioxide and other toxic substances for elimination.

Importance of double circulation

• This type of circulation checks and prevents the mixing of oxygenated and deoxygenated blood.
• In double circulation, oxygen is utilised efficiently.

The differences are as follows:

(a) Blood and Lymph

(b) Open and Closed system of circulation

(c) Systole and Diastole

(d) P-wave and T-wave

An evolutionary change in the pattern of heart among the vertebrates has been observed through careful analysis. Vertebrates possess a muscular heart, it is chambered. They have evolved from having a two-chambered heart (fish) to possessing a four-chambered heart (mammals).

A fish has a two-chambered heart. It pumps deoxygenated blood to the gills, where it is oxygenated and sent to the body. The blood that is oxygenated is then carried to the heart.

Three-chambered hearts are found in amphibians – a ventricle and 2 atria (left atrium and right atrium). The left atrium receives oxygenated blood from the respiratory organs, while deoxygenated blood is received by the right atrium from the organs of the body. But, both types of blood are eventually mixed in the ventricle; hence, the body receives mixed blood.

Half septum in reptiles divides the ventricle partially. But in birds, crocodiles and mammals, the heart is completely segregated into halves hence deoxygenated, and oxygenated blood is separated.

A structural modification in the hearts of fish up to mammals is observed, checking that oxygen-rich blood is supplied to the body while the four-chambered heart ensures that the blood flow is synchronised. As the structure of the heart has evolved, the type of circulation also depends on it, if it is single or double circulation.

The term ‘myo’ refers to muscle, and ‘genic’ refers to originating from. The altered cardiac muscles or the nodal tissues of the heart – the sino-atrial or the sinus node (SA node) – is capable of generating an impulse that extends over the heart wall, which results in the heartbeat. As the cardiac impulse initiates in the cardiac muscles, it is referred to as myogenic.

The sino-atrial, or the sinus node (SAN), is a specialised bundle of neurons generating action potential which produces a cardiac impulse without any exterior stimuli, i.e., it is auto-excitable. SAN can generate a maximum action potential of approximately 70 to 75 in a minute. It is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Due to these capabilities, SAN is referred to as the pacemaker.