(a) Define double circulation. Give a comparative account of heart in reptiles, birds and mammals. (20 marks) (b) Describe…

(a) Double circulation and the heart in reptiles, birds and mammals

In double circulation, blood passes through the heart twice in one complete circuit: through the pulmonary circuit (right side of the heart to the lungs and back to the left side) and the systemic circuit (left side to the body and back to the right side). Oxygenated and deoxygenated blood stay separate, so tissues receive blood at high pressure and high oxygen content, which supports a high metabolic rate and warm-bloodedness.

• Reptiles (lizards, snakes, turtles). The heart is three-chambered, with two atria and one ventricle partly divided by an incomplete septum into cavum venosum, cavum pulmonale and cavum arteriosum. A sinus venosus is present. The circuit is double but with some mixing of blood in the ventricle, which is kept low by the timing of contraction and the muscular ridge. The truncus arteriosus is split into a pulmonary trunk and two systemic arches.
• Crocodiles. The ventricles are completely separated, so the heart is four-chambered, but the two systemic arches are connected through the foramen of Panizza, which allows some bypass of blood.
• Birds. The heart is four-chambered with two atria and two completely separated ventricles, so there is no mixing. The sinus venosus is reduced to the sinu-atrial node. Only the right systemic arch persists. The heart is relatively large, with a thick-walled left ventricle, and beats fast.
• Mammals. The heart is also four-chambered with complete separation; the sinus venosus forms part of the right atrium (sinu-atrial node) and the conus is absorbed into the aorta and pulmonary trunk. Only the left systemic arch (aortic arch) persists. The right atrioventricular valve is tricuspid and the left bicuspid (mitral).

Birds and mammals evolved this complete septum independently.

(b) Habitat, habits and life history of Branchiostoma

Branchiostoma (amphioxus, lancelet) is a marine cephalochordate living in shallow, clean coastal waters of tropical and temperate seas, buried in coarse sand. It lies obliquely with only the anterior end, bearing the oral cirri, projecting from the sand.

Habits: it is a ciliary filter-feeder. Cilia of the wheel organ and the gill bars draw a current of water into the mouth and pharynx; the endostyle secretes mucus that traps food particles, which are carried to the intestine, while water leaves through the atrium and atriopore. It is a poor swimmer, though it can swim by quick lateral undulations of the body and re-burrow, mainly at night.

Life history: the sexes are separate, with no sexual dimorphism, and there are about 26 pairs of gonads without ducts. The gametes are shed into the atrium (by rupture of the gonad walls) and leave by the atriopore, and fertilisation occurs in sea water. Cleavage is holoblastic, nearly equal, and gives a blastula which forms a gastrula by invagination. The embryo hatches as a free-swimming, ciliated larva, which becomes an asymmetrical, feeding planktonic larva. After some months it undergoes metamorphosis, settles in the sand and becomes the adult with the notochord, dorsal hollow nerve cord and pharyngeal gill slits retained.

(c) Life cycle of Leishmania donovani

Leishmania donovani causes visceral leishmaniasis (kala-azar). It is digenetic, with a human (or other mammal) host and the female sandfly (Phlebotomus argentipes in India) as vector. It has two forms: the amastigote (Leishman–Donovan body), a small oval, non-flagellated form of 2–4 µm living inside macrophages of the spleen, liver and bone marrow, and the promastigote, an elongated, flagellated form of 15–20 µm found in the sandfly gut.

In man: an infected sandfly injects metacyclic promastigotes while feeding. Macrophages engulf them, and inside the phagolysosome they change into amastigotes, which multiply by binary fission until the cell bursts and the amastigotes infect fresh macrophages.

In the sandfly: the fly ingests infected macrophages or free amastigotes with the blood meal. In the midgut they change into promastigotes and multiply by longitudinal binary fission. Over about 6–9 days they pass forward to the pharynx and proboscis as infective metacyclic promastigotes, ready for the next bite.