The circulatory system - The Heart.

Circulatory system

[heart]


# Circulatory system was discovered by Aristotle.

# Closed type of circulatory syatem [blood-vascular system] was discovered by William Harvey.


Protists, porifers, coelenterates etc have a large surface area to volume ratio, so, diffusion or cytoplasmic streaming(cyclosis) is sufficient for exchange of materials between the organisms and the surrounding.
In the multicellular organisms, a specific circulatory system is needed to transport nutrients, gases, wastes, hormones etc. A typical circulatory system has 3 distinct features –
i) a circulating fluid [blood, lymph]
ii) a pumping organ [heart- human heart acts as a compression pump ]
iii)asystem op tubes through which the circulating fluid move.


TYPES OF CIRCULATORY SYSTEM

There are two types – open circulatory system and closed circulatory system.

1.Open type of circulatory system:
In open circulatory system the blood not only flows through the blood vessels but also through open spaces called lacunae and sinuses.The flow is slow and under low pressure and it is less efficient then closed type of circulatory system.this type of circulation is present in arthropods, most molluscs.

2.Closed type of circulatory system:
Blood flows in vessles only and it doesn’t come in direct contact with the tissue.The exchange of materials is mediated by INTERSTITIAL FLUID.The flow is fast and under high pressure.This type of circulation is more efficient to exchange materials as the flow is fast.Present in most annelids, cephalopods, all vertebrates, echinoderms.



Mammalian heart

The heart is a conical, muscular, hollow, mesodermal organ lying in the thorax above the diaphragm between the lungs in the middle mediastinum towards the left of the body with broad base upwards and pointed apex facing downwards.
Size of human heart: 12 cm by 9 cm by 6cm
Weight:300 gm in male, 250 gram in female.
Capacity : about 120 ml.
COVERINGS OF THE HEART:Heart is covered by a fibroserous covering known as pericardium.PERICARDIUM is composed of two layers –

1.outerfibrous layer , the fibrous pericardium, which is continuous with the tunica externa/adventitia of the blood vessles entering and leaving the heart.It prevents excessive distention of the heart.

2.inner serous layer, the serous pericardium.It is composed of two layers –the outer PARIETAL LAYER and the inner VISCERAL LAYER. The parietal pericardium is continuous with the parietal covering of the thorax .the visceral paericardium is continuous with the visceral layer of the thorax.Visceral pericardium is also known as EPICARDIUM. In between these two layers of serous pericardium is present PERICARDIAL FLUID, about 60 ml. It protects the heart from friction, mechanical shock, and external injury.

THE WALL OF THE HEART

The heart wall is composed of three layers
i) PERICARDIUM
ii) MYOCARDIUM: It is made of cardiac muscle.Cardiac muscle is involuntary musle.It has faint but regular cross-striations.It occurs as a network pattern called the syncytium.It is usually UNINUCLEATE [but sometimes binucleate].There is presence of INTERCALATED DISCS, large number of mitochondria, abundant blood supply. Contraction of these muscles help heart pump the blood..The heart starts to beat in 4th week of intra-uterine life[28 days of featal life], and continuously beat in rhythmic pattern throughout the life.THUS HEART IS SAID TO BE THE BUSIEST ORGAN OF THE BODY.The cardiac myocytes contract rhythmically and continuously.The ions required for contraction of cardiac myocytes are Na, K, and Ca.The Energy required is derived from carbohydrates (35%), aminoacids(5%), and fatty acids(60%).Automaticity/autorhythmicity, conductivity, frank-starling law[heart pumps all the blood it receives], resting membrane potential [ responsible for automaticity of cardiac muscles] of the cardiac myocytes is about -85 to -95 mV.
iii) ENDOCARDIUM: It consists of the flat epithelial cells.this layer is continuous with the endothelium of the blood vessels entering and leaving the heart.



THE HEART CONSISTS OF 4 CHAMBERS : 2 ATRIA AND 2 VENTRICLES.
THE ATRIA ARE SEPARATED FROM THE VENTRICLES BY ANNULAR PAD INTERNALLY AND EXTERNALLY THERE IS PRESENCE OF A CORANARY SULCUS.
$ THE RIGHT ATRIUM AND VENTRICLE IS ALSO KNOWN AS THE PULMONARY/VENOUS HEART.
$THE LEFT ATRIUM AND VENTRICLE IS ALSO KNOWN AS THE SYSTEMIC/ARTERIAL HEART.

THUS MAMMALIAN CIRCULATION IS CALLED AS DOUBLE CIRCULATION. DOUBLE CIRCULATION IS MORE EFFICIENT THAN TH E SINGLE CIRCULATION FOUND IN THE FISHES[ HEART OF FISHES ONLY GETS VENOUS BLOOD, HEART OF PRAWN ONLY GETS ARTERIAL BLOOD]


1.ATRIA: ATRIA gets blood from different parts of the body.The right atrium gets deoxygenated blood from all over the body and the left atrium gets oxygenated blood from the lungs.The right and left atria are separated by INTER-ATRIAL SEPTUM.There is presence of FOSSA OVALIS, which is the remnant of the FORAMEN OVALE present in the featal heart.

A) THE RIGHT ATRIUM:It gets deoxygenated blood from all over the body. The blood is carried by superior vancava and inferior vanacava. Venacava is the largest vein in the body. There is presence of EUSTACHIAN VALVE at th eopening of the inferior venacava. Superior venacava is devoid of valve[ other vein without valves are internal jugular vein, pulmonary vein and brain sinuses]. Each atrium is produced behind into a swollen flap called auricular appendage.Low muscular ridges present in the right atrium are kniwn as the MUSCULI PECTINATI/PECTINI..In the mammalian heart sinus venosus is absent.The remnant of sinus venosus is represented by SA NODE(THE PACEMAKER OF HEART). There is also an opening of CORONARY SINUS in the right atrium.(Coronary sinus drains blood from the wall of the heart), the opening of coronary sinus is guarded by,THEBESISAN VALVE.In case of rabbit there are two precaval veins(superior vanacava) and one postcaval vein(inferior venacava)
B) THE LEFT ATRIUM: It gets oxygenated blood from the lungs through 4 pulmonary veins[ the other vein carrying oxygenated blood is UMBLICAL VEIN].In casse of rabbit there are only two pulmonary veins


The right atrium communicates with the left ventricle through the tricuspid valve(as it has three cusps) ant the left atrium communicates with the left ventricle by bicuspid valve/ mitral valve(as it has 2 cucsps)These valves prevent the regurgitation of the blood back to the aria.


2.VENTRICLES:Ventricle get blood from the atrium and pumps it to other parts of the body.The right ventricle pumps to the lungs through th pulmonary trunk and the left ventricle pumps to different parts of the body.The left ventricle is at least 3 times thicker than the right ventricle.The valves between the atria and ventricles are jopined to the muscles of ventricles by chordae tendinae[cords like structures] to special muscles known as papillary muscles(conical projections).TRUNCUS ARTERIOSUS AND CONUS ARTERIOSUS ARE ABSENT IN MAMMALIAN HEART.BUT PRESENT IN HEART OF THE FROG.[FROG HAS 3 CHAMBERS- 2 ATRIA AND 1 VENTRI CLE, AND IT HAS 2 ACSSECORY CHAMBERS, CONUS ARTERIOSUS AND SINUS VENISUS]

The right ventricle pumps blood to the pulmonary artey(artey carrying impure blood), the opening of the pulmonary artey is guarded by semilunar valves(3 in number).The left ventricle pumps blood to the aorta, the opening of which is guarded by semilunar valves(3 in number)



BLOOD SUPPLY OF THE HEART: CORONARY ARTERY [first branch of aorta] supplies the oxygenated blood to the heart.CORONARY SINUS and THEBESISAN VEINS drain deoxygenated blood from the heart to the right atrium.
NERVE SUPPLY OF THE HEART: PARASYMPATHETIC SUPPLY by VAGUS NERVE.
SYMPATHETIC SUPPLY by CARDIAC NERVE PLEXUS.

CONDUCTION SYSTEM OF THE HEART:The cardiac myocytes are said to be myogenic and the mammalian heart is a myogenic heart i.e. the heart beating originates in the specializedcardiacmyocytes:the SA Node, interatrial fibres, AV Node, Bundle of his/AV Bundle, Left and Right Bundle branch and Purkinje fibres are the components of conductionsystem of heart.
SA Node is said to be the pacemaker of the heart as the heart beat originates from it. It has got highest autorhythmicity [72-80/min] but lowest conductivity [0.05m/sec]. It is situated near the opening of the superior venacava, and it is remnant of sinus venosus.
Inter-atrial fibres:These fibres carry impulses to the left atrium, so that both the atria contract together.
AV Node:It is situated at the interatrial septum near the tricuspid valve.It is said to be the PACESETTER, as it conducts impulse from atria to the ventricles.
AV Bundle/Bundle of His:It lies below the AV node and is it’s continuation.It divides to give Right and Left bundle branche.The Right bundle branch supplies the right ventricle, and the left bundle branch supplies the left ventricle.The fine branches of these which supply the ventricular wall are called the PURKINJE FIBRES, they have lowest automaticity[15-45/min] but highest conduction velocity[4m/s].BOTH THE VENTRICLES CONTRACT TOGETHER.
CARDIAC OUTPUT: It is the blood pumped by each ventricle in one minute.[The blood pumped by each ventricle in one beat is called stroke volume.it is about 70 ml]. Cardiac output is about 5L.
Cardiac Output = Stroke Volume * heart rate
=70 ml * 72 /minute
=5400 ml/min.

Cardiac output is measured by using Fick’s Principle.

Heartrate in an adult is 72 bpm(beats per minute)..The normal range is 60 – 100 bpm. Below 60 bpm is called as bradycardia and above 100 bpm is called as tachycardia.
Heartrate in fetus:140-160 bpm
Newborns:120 -140 bpm
Children:80 – 120 bpm
Heart rate is controlled by the cardiac centre in the Medulla.
HeartRate is measured clinically as pulse in the Radial artery at the wrist joint.

HeartRate Is Increased in Inspiration, Excitement, fever [ but bradycardia in Typhoid fever], exercise etc and it is decreased during expiration, fear, grief etc.


The Cardiac output Received by different organs:
Liver :25%(maximum)
Kidney:maximum CO per gram tissue weight per minute.(25%)
Brain: 25% heart:5%
20%:remaining organs


CARDIAC CYCLE:The series of electrical and mechanical events that occur in the heart in one beat. The total duration of one cardiac cycle is 0.8s.
Heartbeat has two phases : contraction phase(systole) and relaxation phase(diastole)
Atrial systole:0.1 s
Atrial diastole:0.7 s
Ventricular systole:0.3s
Ventricular diastole:0.5s