The Circulatory Adaptations of the Amniotes: Reptiles, Birds and Mammals

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Describe the changes in circulation patterns of chordates from the most primitive to the most advanced.

Circulation of blood in chordates changes when the chordates transform from their primitive state to the most advanced state. This is a result of development of new structures in the more advanced form. To demonstrate a change in blood flow change as a primitive chordate develops into a more advanced form, this paper examines the case of amphioxus (Branchiostoma lanceolatum). Branchiostoma lanceolatum is a cephalochordate which exhibits similar features of a normal vertebrate and only lacks the vertebral column and the cranial cavity. Blood flow in the lancelet involves passage of blood in the forward side of the ventral to backward side in the dorsal vessels. The sinus venosus draws blood from different body parts, after which the blood flows to branchial hearts through subpharyngeal ventral aorta. The branchial hearts then pump the blood upwards through gill arches. At this point, the oxygenated blood is diverted into two dorsal aortas moving both backwards and forward later uniting behind pharynx. Meanwhile, the blood flowing from the gut collects in subintestinal median vein and flows to the liver. The blood then passes through capillary vessels and then collects in hepatic vein before passing to the sinus venosus. The blood from the muscle and the body walls are then collected and directed to the duct of cuvier then to sinus venosus by the paired anterior and posterior cardinal veins. Since the lancelet does not have a heart, the blood is pumped through independent contractions of the subintestinal vein, sinus venosus and other vessels to all body parts. Oxygen transport is through simple solution in the blood since the blood in lancelet lacks pigmentation and cells.

Another example of blood flow in chordates involves the case of fishes. A fish has a two chambered heart, which are an atrium and a ventricle. The atrium receives blood from body parts while the ventricle pumps blood to gaseous exchange site (gills). After re-oxygenation, the blood is then pumped to body parts and later flows back to the atrium through systematic circulation process. Most fishes have a swim bladder found between the gut and the kidney, it is homologous to the lungs due to the position it occupies as higher vertebrates lungs. The swim bladder and lungs differs in that the swim bladder gets blood supply from dorsal aorta and arises from dorsal wall of the gut while the lungs in vertebrates receives blood from the sixth aortic arch and originates from ventral wall of the pharynx.  During countercurrent process the blood moves through the gills in the opposite direction as water flows over the gills. This hence ensures the blood moves through the gills and picks oxygen from the water; it then meets water of high oxygen concentration which empties oxygen into blood. The blood with little oxygen concentration enters the gills and encounters water of high oxygen concentration, oxygen then flows from water into the blood.  To efficiently ensure there is high oxygen intake in blood, a lot of time is required by blood as it is exposed to water with high oxygen concentration.

Another case that demonstrates the change in blood flow when a chordate transforms into a more advanced form is that of tadpole and frog. A tadpole has one atrium and one ventricle, the blood from the body enters through atrium, it then moves to the ventricle. The ventricle then contracts releasing blood into the gills where gaseous exchange takes place where oxygen diffuses into blood vessels. Oxygen is then distributed to all body parts while carbon dioxide is released. When a tadpole grows into an adult frog it develops a three chambered heart with a double loop. The first loop carries blood from lungs to heart for gaseous exchange while the second loop carries oxygenated blood to body parts while at the same time collecting deoxygenated blood. The ventricle is divided into chambers to prevent  blood from mixing, the ventricle contracts after receiving blood from atria and oxygenated blood is distributed to all body parts while deoxygenated blood is passed through pulmocutaneous arteries which takes blood to gaseous exchange site.

The circulatory adaptations of the amniotes: reptiles, birds and mammals

Amniotes are animals that use extensive membranes to protect their embryo. Amniotes include mammals, birds and reptiles.  During their early developmental stage, mammals and birds have sinus venosus. However, the sinus venosus disappears into the walls of the right atrium as the atrium grows at a faster than the sinus venosus. Another important change in amniotes is the closing of the interatrial foramen after the birth of the fetus. The interatrial septum completely separates the hearts of amniotes. The sinus venosus empties the blood into the interatril septum. The pulmonary veins empty the blood into the left atrium.

Question 2: Fish Sensory Systems

Describe the structure and function of chemical, electrical and pressure sensory systems of fishes. Include in your answer:

The structures of fishes enable them to adapt to their environment besides allowing them to perform special functions such as sensing. This paper begins its analysis by examining the sensory system of the shark. Sharks have strong eyesight facilitated by the presence of a large number of rods with a nictitating membrane which prevents damage from UV light. This also enables them to see clearly in murky waters. Sharks have their ears under their skin which detects sounds for prey which is far away. They also have high smelling power and taste enabling them to reject or accept prey which is far away. A sensitive lateral line which is in a sharks body enables it to detect water movements caused by the prey hence it locates the position and prey size. Ampullae of lorenzini enable the shark to detect small electric field which enables it to easily detect and catch the prey as well as aiding its movement in water.

One of the specialized sensory organs of the fishes is the lateral line. Lateral line system is tactile sense organ which consists of mechanoreceptors (neuromasts) arranged along the body and head of the fish. The neuromast also lie deep in lateral line canals which are filled with mucus. The neuromasts consists of support and sensory cells covered by cupula (jellylike sheath structure) the sensory cell has a lot of small cilia which gets stimulated by water movements and pressure. The lateral line system enables the fish to determine the direction of water movement; the fish can then sense the movement of nearby prey. Some fishes have ampullae of lorenzini an electroreceptor modified from neuromasts which detects electrical potentials arising from prey`s muscle contractions.

Lateral lines in weakly fishes have specialized functions that enables them to see even in murky waters. The lateral-line organ has neutromast which is flexible, flag-like and gelatinous in nature, they are found around the eye and mouth of the fish. Their location underneath the skin enables water to easily pass through the pores to reach sensitive channels which detects water movements. Hair cells are also present and acts as acoustic pressure sensors, the nerves delivers  signals from the hair cell to a fish`s brain for processing which then interprets and locate the changes in water movement.

Question 3: From Amniote Egg to Mammals

Describe amniote eggs and the transition from animals that lay eggs to placental mammals.

 Amniotic egg has membranes filled with fluid including the amnion which has amniotic fluid which nourishes the embryo. The allantois permits gaseous exchange and excretion of waste products while the yolk sac provides nutrients to the embryo. The chorion encloses the other inner membranes and the embryo, the egg outer shell prevents desiccation and allows easy passage of air and prevents physical damage to egg.

The production of bird egg shells at the molecular level and in the reproductive tracts of birds and reptiles.

In birds, as in other reptiles, the oocyte moves into the isthmus (a specialized area of the reproductive tract). The isthmus then produces two soft membranes which covers the egg. The egg then moves into the shell gland that has cells that produces shell materials. The excreted shell material covers the egg. Here, first a protein meshwork is made after which calcium phosphate is deposited in the meshwork by the cells resulting in a harder eggshell. The eggshell then hardens in a process known as the calcification, and subsequently passes through the reproductive tract ready for laying.

Question 4: Bird Flight: Mechanics, Weight, Balance and Power

Describe flight mechanics and adaptations for flight in birds

With evolution of flight, birds have acquired many features to compliment the wings and feathers. Power and light weight structures are primary requirement for all heavier-than-air flying machines. Through the pneumatization process, birds have been able to eliminate and fuse bones and in the process reduce their weight. The development of the avian skeleton in birds, through evolution, enables birds to have requisite distribution of weight and strength in birds facilitating flight. While birds have different skeletal pneumatization, some birds have solid bones that reduce the buoyancy making diving easier. Also, birds have developed mechanisms of reducing their body weight. For example, keeping their reproductive organs tiny only enlarging them during breeding.

Another adaptation is seen in the respiratory system of birds. Birds have a much more efficient and larger respiratory system than mammals. Through evolution, the birds have developed a one-way air flow to the lungs. During the first inhalation, the air first passes through the posterior air sacs and into the lungs on exhalation. As the inhalation and exhalation of air is one-directional, the oxygenated and deoxygenated bloods do not mix. This explains the partial pressure of oxygen in the lungs of birds.

The other structure of birds that helps in handling rigor of the flight is the heart. Birds have a powerful and large heart with four chambers which are effectively separated. The presence of cytochrome and myoglobin accounts for the red color.  The abundant supply of blood in the heart of birds facilitates the sustained flight.

Question 5: Kidneys, Rats and Otters

Describe the specialized mechanisms for conserving water on land and at sea while ridding the body of nitrogenous metabolic wastes

The excretory system of flatworms is composed of two tubules. These tubules contain cells known as flame cells which have a cluster of cilia which drive metabolic materials down the tubules from where they are removed out of the body. The presence of tubule in annelid, which has cilia, facilitates the excretion process. Excretion in annelids takes place via a pore known as the nephridiopore. Some insects have Malpighian tubules, mostly found in some arthropods species and are often in pairs. The convolutions of Malpighian tubules increase their surface area while the microvilli alignment facilitates maintenance of the osmotic balance the reabsorption of water and minerals. The potassium iron pumps lining of the tubules through active transport, facilitates the removal of potassium ions and water as urine. When these organisms are exposed to low-water environments, they excrete uric acid as thick powder and reabsorbs electrolytes and water.

The special adaptations for water conservation of Sea Otters in ocean waters and Kangaroo Rats in deserts

Sea otters has a button nose with turbinals which supports a labyrinth of membrane in its naval passage which conserves water and heat during cold seasons. The warm and moist air exhaled from sea otters lungs condenses back as it passes along nose turbinals which are cold thus conserves heat and water. Kangaroo, on the other hand conserves water through reabsorbing water as it passes through intestine hence producing dry feces. Kangaroos are mostly active during cool hours that are in the morning and evening, when it becomes hot at day time they spend time under tree shade. Kangaroo hops over long distances searching for food and water, this conserves water and energy as hopping is fast and efficient. Both rats and kangaroo has a long loop of henle and a thick medulla this enables them to reabsorb water back to the body as a result concentrated urine which is hyperosmotic to the blood plasma is produced this hence conserves a lot of water.

The mechanisms and structural adaptations of the vertebrate kidney for eliminating urea and controlling water loss

The kidney is responsible for the excretion of urine and reabsorption of essential nutrients back to the body. Kidney is not one large filter but a combination of millions of filters known as nephrons. Each nephron is composed of a glomerulus and a tubule. The functioning of thje nephron is a two-stepped process. The initial step involves the glomerulus allowing fluids and wastes to pass through it while preventing the large molecules (most of which are proteins) and blood cells from passing. The second and final stage involves the passing of the filtered fluid through the loop of henle (tubule) sending back the required minerals to bloodstream and removing the waste substances. The resultant product becomes the urine.

Works Cited

Elinson, Richard P. "Change in developmental patterns: embryos of amphibians with large eggs." Development as an evolutionary process 8 (1987): 1-21.

Maina, J. N. "What it takes to fly: the structural and functional respiratory refinements in birds and bats." Journal of Experimental Biology

203.20 (2000): 3045-3064.

Moller, Peter C., and Charles W. Philpott. "The circulatory system of amphioxus (Branchiostoma floridae) I. Morphology of the major vessels of the pharyngeal area." Journal of morphology 139.4 (1973): 389-406.

Nelson, Mark E., and Malcolm A. Maciver. "Prey capture in the weakly electric fish Apteronotus albifrons: sensory acquisition strategies and electrosensory consequences." Journal of Experimental Biology 202.10 (1999): 1195-1203.

Norberg, Ulla M. Vertebrate flight: mechanics, physiology, morphology, ecology and evolution. Vol. 27. Springer Science & Business Media, 2012.

August 04, 2023



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