TSPSC JL & DL Special | Pumping System of Blood with Gases

June 26, 2023 / 11:15 AM IST

HUMAN CIRCULATORY SYSTEM

LYMPH (TISSUE FLUID)

As the blood passes through the capillaries in tissues, some water along with many small water-soluble substances move out into the spaces between the cells of tissues leaving the larger proteins and most of the formed elements in the blood vessels.
This fluid released out is called the interstitial fluid or tissue fluid.
It has the same mineral distribution as that in plasma.
Exchange of nutrients, gases, etc., between the blood and the cells always occur through this fluid.
An elaborate network of vessels called the Lymphatic system collects this fluid and drains it back to the major veins.
The fluid present in the lymphatic system is called the Lymph.
Lymph is a colourless fluid con- taining specialised lymphocytes which are responsible for the immune responses of the body.
Lymph is also an important carrier for nutrients, hormones, etc.
Fats are absorbed through lymph in the lacteals present in the intestinal villi.

CIRCULATORY PATHWAYS

The circulatory patterns are of two types – open or closed.

Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.
Closed circulatory system is present in Annelids and chordates in which the blood pumped by the heart is always circulated through a closed network of blood vessels.This pattern is considered to be more advantageous as the flow of fluid can be more precisely regulated.
All vertebrates possess a muscular chambered heart.
Fishes have a 2-chambered heart with an atrium and a ventricle.
Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle,
Whereas crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles.
In fishes the heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart (single circulation).
In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood (incomplete double circulation).
In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms, hence, these animals have double circulation.

CIRCULATORY SYSTEM

Human circulatory system also called the blood vascular system consists of a muscular cham bered heart, a network of closed branching blood vessels and blood, the fluid which is circulated.
The Heart
The Heart, the mesodermally derived organ, is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left.
It has the size of a clenched fist.
It is protected by a double walled membranous bag, pericardium, enclosing the pericardial fluid.
Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles.
A thin, muscular wall called the inter-atrial septum separates the right and the left atria, whereas a thick-walled, the inter-ventricular septum, separates the left and the right ventricles.
The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum. However, each of these septa are provided with an opening through which the two chambers of the same side are connected.
The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid or mitral valve guards the opening between the left atrium and the left ventricle.
The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves.
The valves in the heart allows the flow of blood only in one direction, i.e., from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta.
These valves prevent any backward flow.
The entire heart is made of cardiac muscles.
The walls of ventricles are much thicker than that of the atria.
Nodal Tissue
A specialised cardiac musculature called the Nodal tissue is also distributed in the heart.
A patch of this tissue is present in the right upper corner of the right atrium called the sino-atrial node (SAN).
Another mass of this tissue is seen in the lower left corner of the right atrium close to the atrio-ventricular septum called the atrio-ventricular node (AVN).
A bundle of nodal fibres, atrio-ventricular bundle (AV bundle) continues from the AVN which passes through the atrio-ventricular septa to emerge on the top of the inter-ventricular septum and immediately divides into a right and left bundle.
These branches give rise to minute fibres throughout the ventricular musculature of the respective sides and are called purkinje fibres.
The Nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is auto excitable.
However, the number of action potentials that could be generated in a minute vary at different parts of the Nodal system.
The SAN can generate the maximum number of action potentials, i.e., 70-75 min–1, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker.
Our heart normally beats 70-75 times in a minute (average 72 beats min–1).

Cardiac Cycle

How does the heart function? Let us take a look. To begin with, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole.
As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and right atria. The semilunar valves are closed at this stage.
The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction – the atrial systole.
This increases the flow of blood into the ventricles by about 30 per cent.
The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of His transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract, (ventricular systole), the atria undergo relaxation (diastole), coinciding with the ventricular systole.
Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.
The Ventricles now relax (ventricular diastole) and the ventricular pressure falls causing the closure of Semilunar valves which prevents the backflow of blood into the ventricles.

Dr.Modala Mallesh
subject expert
palem, Nakrekal
Ph.9989535675