· Mammals have a closed type of circulatory system.
· Circulatory system is the transport system of the body.
· Circulatory system of mammals consists of
    1. Blood vascular system
    2. Lymphatic system
· Blood circulates through Blood vascular system and lymph circulates in the lymphatic system.

Blood Vascular System

· This system consists of the heart, blood vessels (arteries, veins & capillaries) and blood.

· Heart is muscular, pear-shaped reddish central and sucking and pumping organ.
· It lies in mediastinal space between the lungs slightly inclined to the left.
· It is Myogenic in nature i.e. wave of contraction originate from SA node.
· Pericardium: Outer coverings of the heart
· It is the outermost covering of the heart made up of connective tissue and a layer of Mesothelium.
· Pericardium has two layers – outer Parietal Pericardium and the inner Visceral Pericardium the inner layer.
· Pericardial fluid is present in between these two thin membranes which ensure the free movement of the heart, keep the heart moist, protect from external injuries and absorb shock.

· Heart has 3 layers of tissues: Epicardium, Myocardium & Endocardium.
a. Epicardium

b. Myocardium
· Myocardium or cardiac muscle forms the bulk of the heart.
· Cardiac muscles are present only in the heart.
· Presence of intercalated disc and gap junction between cardiac myofibrils are the unique features of cardiac muscle.
c. Endocardium
· It is the innermost layer of the heart which is continuous with the tunica interna of blood vessels.

· Heart is divided into the upper articular part and large lower ventricular part by coronary sulcus or auriculo-ventricular groove
· In Rabbit, the Left ventricle is larger than the right ventricle.
1. Auricular appendix is a swollen muscular flap found behind each auricle which slightly carries the ventricle of the left side
2. Pectinate muscle is found only in auricles.
3. A shallow longitudinal inter-ventricular groove divides two ventricles externally.
· Right ventricle is smaller than the left ventricle.

Internal Structure of Heart

· Heart is four-chambered i.e. 2 Auricles and 2 Ventricles.

A. Auricles or Atria:
· Inter-auricular septum separates right and left auricle internally.
· Inter-auricular septum has a small and slightly depressed oval area called the fossa ovalis which during the embryonic period was represented by an opening called the foramen ovale.
· Inner lining of auricles form a network of low muscular ridges called musculi pectinati.
· Left auricle has a thicker wall than the right auricle.
· The right auricle receives deoxygenated blood only by two superior vena cava or left and right precaval and one inferior vena cava or post-caval vena cava.
· A small and spongy mass of specialized cardiac muscle form Sino-auricular node which is present near the opening of the superior vena cava.
· Pulmonary vein which carries oxygenated blood from the lungs opens into the left atrium.
· In Human heart has only one superior and one inferior vena cava.

B. Ventricles:
· Ventricles are also divided by a thick oblique interventricular septum into right and left ventricles.
· Walls of ventricles are thicker than walls of atria and the wall of the left ventricle is much thicker than the wall of the right ventricle because the left ventricle has to pump the blood into the aorta.
· Bundles of muscles project into the lumen of both ventricles from the wall of the ventricles which are called Columnae cornea or Papillary muscle.

Valves of the Heart

· Right auriculo-ventricular aperture is guarded by a tricuspid valve which consists of three membranous flaps or pockets whereas the Left auricle opens into the left ventricle through the left auriculo-ventricular aperture guarded by Bicuspid or Mitral valve.
· It consists of two membranous flaps or pockets.
· Mitral valve allows the flow of oxygenated blood from the left auricle to the left ventricle but not in reversed direction.
· Atrioventricular valve is held in position by fibrous tissues called Chordae tendineae.
· Eustachian valves guard the opening of the inferior vena cava into the right and left auricle which on one hand are attached to the lower side of the valve and on the other hand to the Columnae cornea.
· Tricuspid valve allows the flow of blood from the right auricle to the right ventricle, not the reverse or backflow.
· The opening of the right ventricle into the pulmonary aorta is guarded by three pockets like a semilunar valve which allows blood flow only from the right ventricle to the pulmonary trunk, not in the reverse direction.
· The Carotico systemic-aorta or Systemic arch or aorta arises from the upper end of the left ventricle and the opening of the left ventricle into the aorta is guarded by three semi-lunar valves (Aortic Valve) which allow the blood to flow from the left ventricle to aorta and not in reverse order.
· Carotico-systemic aorta crosses the pulmonary aorta. At this point of crossing, these two aortae are connected together by a muscular band called ligament arteriosum in adult mammals. But in the embryonic stage, they represent a single artery called ductus arteriosum or ductus Botalli.

Course of Circulation

· Mammals have a double course of circulation because pure and impure blood is circulated from different ways.
· It has pulmonary and systemic blood circulation.
· In pulmonary circulation, impure blood is carried by the pulmonary artery from the right ventricle to the lungs where blood is purified then pure blood is carried to the left auricle by pulmonary vein.
· In the systemic circulation, pure or oxygenated blood is pumped into the systemic aorta from the left ventricle then larger arteries conduct it into the arterioles and capillaries.
· Capillaries supply the oxygen into the tissues for oxidation of foodstuffs. Then deoxygenated blood is carried by venules and veins.
· Finally this impure blood pours into the right auricle by the precaval and post caval veins.
· Impure blood from tissues → into caval veins → to right auricle → to right ventricle → pulmonary aorta → lungs → pulmonary vein → left atrium → left ventricle → systemic aorta → arteries and arterioles
  capillaries for oxidation → venules and veins → caval veins to right auricle again.


· Heart performs continuous rhythmic beating without being fatigued.
· Heartbeat denotes contraction or systole and relaxation or dilation or diastole.
· The volume of blood pumped by the blood in 1 minute is called cardiac output which is about 5 litre in humans.
· The impulse of contraction originates within the heart in the SA node, a small spongy mass of specialized cardiac tissue, situated near the opening of superior vena cava (in man), or right precaval (in rabbit).
· Impulse of contraction originates from SA node spread all over auricle and auricle contracts.
· Radiating impulse on the atrium is picked up by AV node located at the upper end of the inter-ventricular septum.
· AV node → Bundle of his → Purkinje fibres → Wall of the ventricle (ventricle contract)
· A cardiac cycle is completed in 0.8 sec.
· The events during the cardiac cycle are atrial systole, atrial diastole, ventricular systole, ventricular diastole.
· Pulmonary circulation is related to the flow of blood from the heart to the lungs & again to the heart.
· Systemic circulation refers to the flow of oxygenated blood from the heart to body tissues and organs and the flow of deoxygenated blood to the right atrium.
· The cortico-systemic aorta crosses over the pulmonary aorta and the cortico-systemic aorta is connected by the muscular band known as the Ligamentum arteriosum, which during the embryonic stage was represented by a single artery known as Ductus arteriosum or Ductus Botalli.
· Heart receives blood during diastole through the coronary artery.
· Sinu-atrial node (S.A node) or natural pacemaker (lies in the right auricle) --- auricular contraction — atrioventricular node or pacesetter ( lies near the opening of tricuspid valves) --- bundle of His (lies in the interventricular septum) ----Purkinje fibres (ramified fine fibres distributed throughout ventricular wall ---- ventricular contraction.

Working of Heart

· Cardiac cycle is completed in 0.8 sec.
· It includes three phases:
A) Auricular systole

· Contract both atria but ventricle is in diastole.
· Lasts for 0.1 sec
· Closure of Eustachian valves and aperture of superior vena cava and pulmonary vein.
· Opeining of bicuspid and tricuspid valves.
· Pure and impure blood are pushed into respective ventricles from the left and right atria.

B) Ventricular systole
· Contraction of both ventricles.
· Duration – 0.3 sec
· Closing of bicuspid and tricuspid valves and hearing of first heart sound, lub.
· Closing of semilunar valve produces the second heart sound called dub.

C) Joint diastole
· Relaxation of all chambers of the heart.
· Duration – 0.4 sec

Venous System

 Deoxygenated blood from various parts of the body is collected to the heart by the venous system.
· All veins carry deoxygenated blood except the pulmonary vein.
· A right and a left precavals (also called anterior vena cava) and a single postcaval (also called posterior vena cava) drain deoxygenated blood into the right auricle.

· The right superior vena cava is formed by the union of these veins:
(i) External jugular vein – it brings impure blood from the mandible, facial region etc,
(ii) Right internal jugular vein
(iii) Right subclavian vein and
(iv) Right mammary and intercostal vein (bring blood from the thorax and the azygous cardinal vein).

· Left superior venacava: It resembles the right anterior venacava except it doesn't receive an azygous vein.

· Inferior venacava or post caval vein: takes the deoxygenated blood from hind limbs, abdominal cavity, reproductive organs, kidney, liver and diaphragm.
· The branches of inferior venacava are
(a) Internal iliac,
(b) External iliac or femoral,
(c) Ilio-lumbar,
(d) Ovarian (spermatic) veins,
(e) Renal veins,
(f) Hepatic veins.
· External jugular vein is a paired vein that collects blood from the head, tongue and muscle of the jaw.
· Internal jugular vein is a paired vein that collects blood from brain and neck muscles.
· Subclavian vein is a paired vein that collects blood from the shoulder, & forelimbs.
· Azygous vein is a single vein on the right side that collects blood from the ventral side of the thorax walls, ribs and muscle.
· Hemiazygous vein is a single vein on the left side that collects from the ventral wall of the thorax & intercostal muscles and opens into the azygous vein.
· Two external jugular veins of two precavals are connected in rabbits by transverse jugular anastomosis.
· On the left side, a few small veins from the hemizygous vein meets the azygous vein on the right side through the transverse anastomosis.
· A pair of coronary veins drain into the left precaval vein.
· Internal iliac are a pair of veins that bring blood from the back of the thigh and pelvis.
· External iliac or femoral – A pair of large vessels that collects blood from the pelvis, urinary bladder, ureter (in females).
· Iliolumbar vein – A pair of veins that drains blood from the hinder region of the abdominal wall.
· Ovarian (or spermatic) vein (Gonadial vein) collects the blood from gonads.
· Hepatic veins form the hepatic portal system.
· Hepatic portal system collects the blood from the GI tract, processes in the liver and drain into the posterior venacava. Thus the hepatic portal vein provides an opportunity to the liver for the processing of the substances.
· Pulmonary veins from each lung bring oxygenated blood and meet to open by a common aperture on the left auricle.

Arterial System

· It consists of two distinct systems:
a) Aortic System – Conveys arterial blood from the left ventricle to all parts of the body.
b) Pulmonary system carries venous blood from the right ventricle to the lungs.

· Branches from the left aortic arch are common carotid and subclavian.

1. Common carotid:

· Right and left common carotid arteries arise from systemic arch.
· Each artery is divided into
a) External carotid artery:
Supplies side of the head and face.
b) Internal carotid artery: Supplies blood to the brain.
· Usually right and left carotids arise from the innominate artery but it is not invariable and the left may arise directly from the systemic arch.

2. Subclavian arteries:
· Right subclavian artery arises either from the innominate or from the base of the right carotid artery but the left subclavian arises directly from the aorta.
a) Vertebral artery: Enters the vertebrarterial canal of the cervical vertebra. It supplies the brain and spinal cord.

b) Intermammary artery: Supplies the ventral thoracic wall and the mammary gland.

c) Brachial artery: It is the direct continuation of the subclavian artery and supplies blood to the forelimb.

d) Dorsal aorta: After turning left and backwards it continuous behind the heart as the dorsal aorta.
Branches of Dorsal Aorta:
1) Intercostal arteries: Supply blood to ribs and intercostal muscle.
2) Coeliac artery: Divides into
a) Hepatic artery: Supply liver
b) Lino-gastric artery: Supply stomach and spleen.
3) Renal arteries: Supply blood to the kidney.
4) Anterior mesenteric arteries: Supply blood to the duodenum, pancreas, small intestine & caecum.
5) Phrenic arteries (1 pair): Supply to the diaphragm.
6) Genital arteries (1 pair): Supply the gonads.
7) Posterior mesenteric (single): Supply blood to the terminal part of the rectum.
8) Lumbar arteries (1 pair): Supply the abdominal wall.
9) Iliac arteries (1 pair): In the pelvic region, the dorsal aorta divides into two iliac arteries (i.e. right and left common iliac artery) which pass into the leg to become femoral arteries.
· Caudal artery: It is a small vessel that continues in a line with the dorsal aorta into the tail.
· Common iliac divides into the external and internal iliac arteries.
· External iliac artery continues as femoral artery and supply to the hind limb.

· Internal iliac or Hypogastric artery divides into
a) Vesicular branch: Supplies blood to pelvic regions, urinary bladder, ureter, seminal vesicle in male, uterus in the female.

b) Epigastric artery: Supplies blood to the pubic region, anus, the external genitalia (penis & vagina).

Also, Read our Other Notes Related to RABBIT:

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