Question

1.(a)When a person exercises, one of the physiological responses is an increase in blood flow. In terms of the nervous system and various blood vessels, describe what’s happening.

(b)How did dorsal vessels work in arthropods?

(c)Mollusk hearts compared to fish hearts?

(d)True or false, all vertebrate hearts start off as two chambered hearts in the embryo.

Answer 1

1.(a) During exercise, the cardiac output can increase upto five folds- from about 5 L per minute to about 25 L per minute. This is primarily due to an increase in heart rate which increases the blood flow to skeletal muscles. The cardiac output is mathematically related as the product of stroke volume and the heart rate.The normal cardiac output is 7270 or 5040 ml per minute, i.e about 5 L per minute. The increased blood flow to the heart and skin during cardiac output is the result of local anterior vasodilation. In skeletal and cardiac muscles vasodilation is mediated by local metabolic factors and in skins it is done by decrease in sympathetic neurons firing supplying the skin vessels. This successive vasodilation is followed by vasoconstriction in the kidneys and GIT organs due to an increase in sympathetic neuron activity. Vasodilation of arterioles is not compensated by arteriolar vasoconstriction, increasing the peripheral resistance to blood flow. During exercise, cardiac output increases more than the total resistance decreases, so the mean arterial pressure increases by small amount. Pulse pressure increases markedly due to the increase in both stroke volume and the speed of ejection of stroke volume. The cardiac output increases by contrasting variation in heart rate as compared to stroke volume. The heart rate increases due to decrease in parasympathetic (vagus) activity of SA node combined with simultaneous increase in sympathetic activity. The stroke volume increases due to increased ventricular contraction, mediated by sympathetic nerves. The increased output can be elevated to more high level if the peripheral processes favoring venous return to the heart are activated simultaneously.

A discrete control centers in the brain is activated by output from the cerebral cortex. During exercise, local chemical changes initiated in the muscles due to imperfect matching between blood flow and metabolic demands activate chemoreceptors in the muscle. Afferent signals from these receptors reach the medullary cardiovascular centers, consequently there is an increase in heart rate, myocardial contractility and vasoconstriction. This also stimulate the mechanoreceptors which provide excitatory input to the cardiovascular center.

(b) Arthropods hava an open circulatory system comprising of the blood or haemolymph, dorsal blood vessels heart, haemocoel and anterior aorta. In arthropods, the dorsal blood vessel run along the dorsal, or top, interior of the body and consists of two major regions. The abdominal region or the pericardial sinus containing the heart. It consists of a number of chambers that pump haemolymph collected from the haemocoel in the abdomen towards the head region. The heart receives the haemolymph from the sinuses through ostia, located on each chamber. The portions of the dorsal blood vessel that reach thorax is not muscular, and is called aorta which lead into the sinuses. Haemolymph emerges from the aorta in the head region and re-enter into the haemocoel and ultimately diffuse throughout the body to reach all tissues.

(c) Mollusca have open circulatory system in which blood pumped by the heart passes through large vessels into open spaces or body cavities called as sinuses. But fishes have closed circulatory system where the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Mollusca have a three-chambered heart. Two auricles collect oxygenated blood from the gills and the ventricle forces it from the aorta into the small vessels which finally transport to the tissues directly. But fishes have two chambered heart with only single atrium and single ventricle. Fishes heart is called venous heart as deoxygenated blood flows in the single circuit circulation but in mollusca, the deoxygenated blood is oxygenated in the gills and then return to heart and from there, it is pumped through the blood vessels to the bodies organs.

(d) The development of heart in vertebrates embryo starts basically from a hollow tube of special muscle (cardiac muscle) and gradually it evolves. Deoxygenated blood from the body is brought by veins into sinus venosus. From there, it pases to the atrium, ventricle and the conus arteriosus and eventually to the arterial system. The atrium, ventricle in the vertebrate embryos are not form as a distinct chambers but rather as different parts. As the embryo heart develops, each part of the tube develops as chambers, separated from others by valves to prevent backward flow of blood. So it is unjustified to say that all vertebrate hearts start off as two chambered hearts in the embryo.