The circulatory system transfers absorbed nutrients, oxygen, waste products, hormones, and more throughout the body. It has two main parts - the blood vascular system including the heart, blood vessels and blood, and the lymphatic system including lymph vessels and lymph fluid. The evolution of the vertebrate heart is traced from single-chambered hearts in primitive chordates to modern four-chambered hearts with separate pulmonary and systemic circulation in birds and mammals.

1. Circulatory System in Vertebrates
Lecture 1
Nowreen Yasmin
Assistant Professor
Department of Education
Noakhali Science and Technology University

2. Why circulatory system?
• Absorbed food in digestive track + Oxygen collected
in respiratory organs -> Transfer to allover the body
for Metabolism
• Waste product of metabolism (CO2, Nitrogenous
waste … ) -> Transfer to Excretory Organs to
eliminate from body
• Hormones from endocrine tissues, substances for
maintaining homeostasis and for circulating
nutrients all over the body

3. Parts of Circulatory System
• 2 Major parts: Blood Vascular system and
Lymphatic system
• Blood Vascular system: Blood, Heart, Veins, Arteries,
Capillaries
• Lymphatic system : (i) Lymph is the tissue fluid, lying
between and bathing body cell. It is similar to blood
plasma but lacks the red blood corpuscles and some
proteins. (ii) Lymphs capillaries,

4. Evolution of Heart in Vertebrates
Types of Hearts Vertebrates
(i) Single chambered heart cephalochordata
(ii) 2-chembered, single circuit
venous heart
Cyclostomes, Elasmobranchs,
Teleosts
(iii) 3-chembered transitional
hearts
Dipnoi, Amphibians, Reptiles
(iv) 4-chembered double circuit
pulmonary hearts
Birds and Mammals

5. Single chambered heart
• Available in primitive chordate and a true heart is lacking
• A part of ventral aeorta becomes muscular and contractile
and this structure known as single chambered heart.
• Closed blood system; Main ventral and paired dorsal aorta
and no central heart. No hemoglobin
• The following progression of heart formation is identified
from lower to higher vertebrates:
(i) Cardiac tubes forms chamber due to constrictions.
(ii) Due to formation of partition that one chamber forms two separate
chambers
(iii) The location of heart shifts from just behind the head (fish,
amphibians) near gills into thoracic cavity (amniotes) with
elongation of neck and development of lungs.

6. 2-chembered, single circuit venous heart
• Cyclostomes: A linear series of four chembers:
sinus venosous, atrium, ventricle and a small
conus arteriosus, through which blood flows in
that sequence.
• Elasmobranchs: Only auricle and ventricle are
true chambers that’s why known as two-
chambered. All the valves prevent backflow.
• Teleosts: In Ventral aeorta, the conus becomes
thick muscular walls called “bulbus arteriosus”.
• Fish heart is known as venous heart because
the heart is small, 2-chembered, and with a
single circuit of blood circulation. All blood
passing only once through heart is non
oxygenated.

7. (iii) 3-chembered, single transitional heart
• Dipnoi or Lung fish: A partial partition divides the ventricle and Atrium.
Oxygenated blood from lungs or swim bladder returns directly to the heart
without making a detour of the whole body.
• Amphibians: Oxygenated and deoxygenated blood is separated by complete
inter-auricular septum. In Urodele Amphibians Conus is reduced and replaced
by Bulbus Arteriosus. In anuran amphibians there is spiral valve to separate
blood.
• Reptiles: Heart becomes more strongly muscular. Two auricles and two
incomplete ventricles. Sinus venous modified in reptiles and crocodiles.

8. 4-chembered double circuit pulmonary hearts
• Birds and Mammals have
completely divided 4 chambered,
double circuit heart
• No mixing of oxygenated and
deoxygenated blood
• Sinus venous is absent and conus
aerteriosus is replaced by
pulmonary aorta

9. Quick questions !
• Why fish heart is known as venous heart?
• Why amphibian and reptile heart is known as
transitional heart?
• Make a comparison between fish and human
blood circulation.

10. Comparison between Fish and Human Blood Circulation
• In mammals (like us) there is a double
circulation. The right side of the heart receives
blood returning back from the body; this blood
is low in oxygen (blue in the figure to the right).
This “deoxygenated” blood enters the right
atrium and then the right ventricle to be
pumped to the lungs were the blood will be
oxygenated. The oxygenated blood (red in the
figure) from the lungs enters the left ventricle
via the left atrium and is then pumped out into
the larger body circulation. Note that the
volume pumped by the right and left side of
the heart is the same: approximately 5 liters
per minute in a 70 kg human.
• In fish, the heart only has one atrium and one
ventricle. The oxygen-depleted blood that
returns from the body enters the atrium, and
then the ventricle, and is then pumped out to
the gills where the blood is oxygenated, and
then it continues through the rest of the body.