EVOLUTIONARY TRENDS IN THE EXCRETORY SYSTEM

Palaeontological evidence has been interpreted as indicating that the earliest vertebrates lived in fresh water and that the early stages of evolution of fishes took place in that medium. Animals that are submerged in fresh water inevitably acquire excess water by absorbing it through the skin or by swallowing it with food. Therefore, a mechanism for eliminating the excess ware is necessary. On the other hand, salt is scarce in fresh water, the only source being food. Freshwater organisms must therefore prevent any wasting of salt from the body.



The elimination of water and the reclamation of salts may have been the earliest functions of vertebrate kidneys. Tufts of blood vessels - glomeruli, filtered water out of the blood stream into the body cavity and convoluted tubules with openings into the coelom collected the filtrate, retrieved any salts from it and emptied the final filtrate into a longitudinal duct that passed to the cloaca.



Archinephros

Glomeruli arise as localised modifications of blood vessels, which can be traced from segmental branches of the dorsal aorta - supplying the glomerulus is an afferent glomerular arteriole and emerging from the glomerulus is an efferent glomerular arteriole. The latter leads to capillary beds that surround the kidney tubules.



The most anterior embryonic/larval glomeruli may be suspended in the coelomic cavity. They are sometimes called "external" glomeruli to differentiate them from "internal" glomeruli, which are encapsulated by the kidney tubule.



In internal glomeruli, each tubule typically commences as a Bowmann's capsule. This is a blind end of the tubule that surrounds an internal glomerulus and receives the glomerular filtrate. The more anterior tubules may exhibit a ciliated funnel-shaped nephrostome, which is an opening into the coelom. Nephrostomes are usually confined to the embryo and larvae. If the embryonic tubules that exhibit a nephrostome are not altogether closed, the nephrostome may close at a later stage of development.



Kidney tubules arise from the intermediate mesoderm. This is a ribbon of nephrogenic tissue extending uninterrupted from the level of the heart to the cloaca. It lies just lateral to the segmental/dorsal mesoderm. Almost the entire ribbon produces kidney tubules. The anterior-most tubules are always metameric, since one tubule develops a the level of each mesodermal somite. Farther back, numerous tubules develop in each segment and the metamerism is lost.



The longitudinal ducts of the basic patter appear first at the anterior end of the nephrogenic mesoderm as posteriorly directed extensions of the first tubule. Each duct grows caudal until it achieves an opening into the cloaca. At this time, it is known as the pronephric duct. The kidneys of myxinoid cyclostomes closely resemble an archinephros.



Pronephros

The first embryonic kidney tubules in all vertebrates arise from the anterior end of the intermediate mesoderm. They are called pronephric tubules because they are the first of what will become a series of tubules extending the length of the coelom.



Each pronephric tubule arises in the intermediate mesoderm as a solid bud of cells, which later organises a lumen and in most anamniotes, a nephrostome. Associated with each tubule, typically, is a glomerulus. The number of pronephric tubulesis never large. The tubules lengthen and become coiled.



The pronephros in most vertebrates is functional only until such a time as the tubules farther back are prepared to supersede them. This is at the end of the larval stage in amphibians or at an equivalent development stage in fish. Occasionally, in larvae, several glomeruli unite to form a single glomus. The glomus and the pronephric tubules are generally enclosed in a pronephric chamber. The pronephros is retained in the adult only in cyclostomes and a few teleosts. The pronephros is often called head kidney because it lies immeadiately behind the head.



Mesonephros

Under the stimulus of the pronephric duct, acting as an inductor, additional tubules develop sequentially in the mesoderm behind the pronephric region. The new tubules establish contact with the pronephric duct. For at least several segments, these tubules too, may be segmentally disposed. They exhibit the same convolutions as the ones anterior to them and often have open nephrostomes. Later, the pronephros is obliterated. With the disappearance of the pronephric region, the erstwhile pronephric duct is now called the mesonephric duct. The mesonephros is the functional adult kidney of fish and amphibians. It is sometimes called opisthonephros. The mesonephros is also the functional kidney in the embryonic amniotes.

In sharks and apoda (amphibians), the adult kidney begins far forward and extends the lenth of the coelom, lying against the dorsal body wall. In other fish and other amphibian, the kidney is much shorter. The mesonephric duct may lie along the lateral edge of the kidney, or on the ventral surface or embedded in it. The posterior ends may enlarge to form the seminal vesicle in males.

The mesonephros of amniotic embryos has essentially the same structure as the adult kidneys of fish and amphibians, except that nephrostomes are rudimentary in most birds and seldom appears in mammals. Although the mesonephros is basically an embryonic kidney in amniotes, it functions for a short time after birth in reptiles, prototherians and metatherians.

During the time that the mesonephros is functionin, a new kidney, the metanephros, is in the process of developing. When the metanephros takes over the functions of a kidney, the mesonephros involutes and only remnants remain after birth.

Metanephros

The metanephros, or adult amniotic kidney, organises from the caudal end of the nephrogenic mesoderm, which is displaced anteriorly and laterally during development. The number of tubules that form this caudal section is extremely large (several million) and the tubules are highly convoluted. This has a duct of its own, the metanephric duct/ureter.

The proximal portion of the metanephric duct becomed the ureter. The distal tip becomes the pelvis (of the kidney). Many finger-like outgrowths of the pelvis invade the nephrogenic mass to become collecting tubules. Meanwhile, the metanephric tubules get organised. They commence as 'S'-shaped tubules. The upper arm opens into a collecting tubule. The lower arm becomes invaginated by developing a glomerulus to become a Bowmann's capsule.

The mammalian metanephros exhibits a greater organisation than that of reptiles and birds. The organisation is the result of the formation of a long, thin, 'U'-shaped loop of Henle, positioned between the proximal and distal convolution of the metanephric tubule. As the loop of Henle elongates, they grow away from the surface of the kidney and towards the renal pelvis. The kidney therefore comprises of a cortex, in which the renal corpuscles are concentrated and a medulla, consisting of loops of Henle and collecting tubules.

The metanephric tubules of reptiles have no loop of Henle and those of birds have only a very short equivalent segment. Small glomeruli result in the conservation of water. Many metanephric kidneys are lobulated, each lobe consisting of clusters of tubules. In snakes and legless lizards, the kidneys are elongated to conform to the slender body. The kidneys of birds are flattened against the sacrum, iliun and ribs and fit snugly against the contours of these bones.

TRENDS IN THE EVOLUTION OF THE MIDDLE EAR AND EAR OSSICLES (Cyclostoma to Mammals)

It is important for animals (incidentally all chordates are free moving) to maintain the correct posture and to detect signals in the form of changes in the pattern of sound waves. The appearance of the ear in chordates actually begins from the incorporation of the membranous labryinth in cyclostomes. Further development concerns the addition of ear ossicles in the middle ear by the modification of visceral arches and the progressive development of the membranous labyrinth. The ear is best developed in mammals where all the divisions - external, middle and inner are present with their respective components.

Development of the ear

A small patch of ectoderm on each side of the head in the region of the hind brain thickens to form an auditory placode. This sinks in and thereafter invaginates to form an auditory vesicle/autocyst. It lies embedded in the mesenchyme of the head. The invagination canal connets each auditory vesicle to the outside. In elasmobranchs, this remains open. In other vertebrates, the auditory vesicle remains as a closed sac having no invagination canal. Ductus endolymphaticus / endolymphatic duct arises as a new outgrowth from the auditory vesicle. It has no external opening. In amphibians and some reptiles, the end of this duct expands to form an endolymphatic sac. The auditory vesicle now constricts to form a dorsal utriculus and a ventral sacculus. One/two/three semicircular canals also appear and open into the utriculus at both ends. One end of each canal expands to form an ampulla. Two parallel grooves arise and these two meet and ___ to form a tube. An evagination called lagena arises from the lower part of the sacculus. This appears first in fish. In successive groups, it begins to grow increasingly. The auditory capsule is formed by the mesenchyme. It is first made of cartilage and is later replaced by bone. It encloses the membranous labyrinth.

The middle ear and eustachian tube are derived from the spiracles of fish. The upper part of the hyomandibular arch gives rise to the columella auris of lower tetrapods and the stapes of higher tetrapods. The external opening of the spiracle becomes closed and it fuses with with the skin to form the tympanic membrane. The cavity of the spiracle expands to form the cavity of the middle ear. The middle ear establishes a connection with the pharynx by the Eustachian tube. During the transition of aquatic to terrestrial mode of life, the spiracle becomes a sound conducting device. In mammals, the stapes is joined by two more ear ossciles. The quadrate gets modifies into the incus and the articular gets modified into the malleus. These are joined to each other and also connect the tympanic membrane with the internal ear. The external ear / pinna, in mammals, is formed mainly as an outgrowth of the skin. It is reinforced by an elastic cartilage.

COMPARATIVE ACCOUNT

Cyclostomes and Fish
The middle ear and external ear are absent.

Amphibia (e.g. frog)
The middle ear is visible from outside due to the absence of pinna. It encloses an air-filled tympanic cavity. It is limited internally by the auditory capsule and externally by the tympanic membrane / tympanum. This is visible as a dark tightly stretched patch of skin. The cavity of the middle ear communicates with the pharynx by a slender, narrow passage called the eustachian tube running downward and opening into the buccopharyngeal cavity just near the angle of the mouth. The tympanic membrane is tightly stretched over a ring of cartilage called the annulus tympanicus. It is modified skin. A club-shaped rod, the columella auris, touches the centre of the tympanum and is extended across the tympanic cavity to a cartilagenous, small nodule known as the stapedial plate, which is fised with a hole in the auditory capsule called the fenestra ovalis. Structurally, the columella auris is made up of bone as well as cartilage. A ring-like bone, the operculum is present in the fenestra ovalis.

Reptiles (e.g. lizard)
The middle ear is represented by an air-filled cavity called the tympanic cavity. It is derived from the firdt pharyngeal pouch. The external limit of the tympanic cavity is formed by the tympanic membrane, whereas, its internal boundary is limited by the auditory capsule. A narrow passage called the eustachean tube / pharyngo-tympanic tube extends downward and inward to open into the posterior part of the pharynx. The eustachian tube of reptiles is narrower and longer than that of amphibians. A single, rod-like ear ossicle, the columella auris stretches across the tympanic cavity. It is derived from the hyomandibular of the hyoid arch. This is made of
(i) An inner body - stapes / stapedial bone
(ii) An outer cartilagenous extrastapedial / extracolumellar cartilage.
The stapes fits into a small membrane covered aperture called the fenestra ovalis located in the outer wall of the auditory capsule. The extracolumellar cartilage is attached to the inner surface of the tympanum. One more membrane covered aperture is present in the outer wall of the auditory capsule. It is called the fenestra rotunda.

Variations

The tympanum, eustachian tube and the tympanic cavity are totally absent in snakes. Yet, snakes recieve sound vibrations with the help of the columella auris which is attacched at its outer end to the quadrate.

In crocodiles, the two eustachian tubes open into the ___________

In turtles, the tympanic membrane is thin and delicate, while it is thick and covered with skin in the terrestrial form. In crocodiles, a movable integumentary fold covers and protects the depressed tympanic membrane.

Aves (e.g. pigeon)
The middle ear is an air-filled cavity. The tympanic cavity is limited externally by th tympanum and internally by the auditory capsule. A narrow eustachian / pharyngo-tympanic tube arises from the lower medial part of the tympanic cavity and extends downward and inward. The two eustachian tubes join to finally open into the roof of the pharynx by a common aperture. The columella auris extends across the tympanic cavity and can be easily distinguished into four parts - an inner, disc-like bony stapes and an outer three-layered cartilagenous extracolumella. The bony stapes fits into a membrane- covered aperture in the outer wall of the auditory capsule called the fenestra ovalis. The extracolumella is attached to the inner surface of the tympanum. There is present an additional circular aperture below the fenestra ovalis ____________. It is also membrane covered.

Mammal (e.g. rabbit)
The middle ear is represented by an irregular air-filled space called the tympanic cavity, enclosed in the tympanic bulla. It is lined by a mucus membrane. It communicates with the pharynx by a passage called the eustachian tube which extends downward and inward. The periotic bone forms the inner wall of the tympanic cavity. This wall bears two apertures - fenestra ovalis / oval window and the fenestra rotunda / round window, both covered with a thin membrane of connective tissue.

An important feature of the middle ear is the presencce of three ear ossicles - malleus, incus and stapes. These are movably articulated with each other extending from the point of the fenestra ovalis to the periotic bone. The malleus is the outermost, hammer-shaped ossicle and is attached to the inner surface of the tympanum. The middle ossicle, incus, is anvil-shaped. It articulates with the malleus by a synovial joint and with the stapes by a ball-and-sockect joint. The inner moer is the stapes. These three ossicles function as a system of levers.

Human Reproductive System

THE MALE REPRODUCTIVE SYSTEM

The male reproductive system consists of a pair of testes lying in the scrotal sacs (scrotum) which are attached to the lower anterior wall of the abdomen. During the early embryonic development of a male, the testes are located in the body cavity. But, before birth, the testes descend into the cavities of the scrotum through a tube called the inguinal canal which is a connection between the scrotum and the body cavity. The canal gets blocked by connective tissue after the testes descend. The reason for the testes’ descent is that the sperms produced by the testes survive and mature in the cooler temperature of the scrotum and not with the internal body temperature in the body cavity.

Each testis contains 1,000 coiled tubules called the seminiferous tubules which actually produce the sperms. Cells called the sertoli cells (nurse cells) nourish the developing sperm.

The seminiferous tubules open into fine tubes called the vasa efferentia (or efferent ductules or ductus efferentes). The vasa efferentia join to form a long, highly coiled tube called the epididymis where the sperms are stored. (The epididymis is derived from the embryonic kidney.) The epididymis looses its convolution and continues as a long tube called the vas deferens (ductus deferens). The vas deferens passes from the scrotum through the inguinal canal, into the abdominal cavity and over the urinary bladder to a point where it joins the duct of an accessory sex gland called the seminal vesicle to form a short ejaculatory duct. The seminal vesicle secretes a fluid which forms a major part of the semen. The ejaculatory duct opens into the urethra which is a tube that continues from the urinary bladder to the exterior. The urethra in males passes through the male copulatory organ called the penis.

The urethra is differentiated as the prostatic urethra, membranous urethra and the cavernous urethra. The prostatic urethra is a part of the urethra surrounded by an accessory sex gland called the prostate found below the bladder. The prostate secrets a thin, milky, alkaline fluid that forms a part of the semen. The membranous urethra is a part of the urethra surrounded by a membrane called the urogenital membrane. Lying on either side of the membranous urethra are a pair of pea sized glands called the bulbo-urethral glands (Cowper’s glands). They open into the cavernous urethra and secrete a fluid which forms a little part of the semen. The cavernous urethra is a part of the urethra that passes through the corpus cavernosum and the corpus spongiosum of the penis.

THE FEMALE REPRODUCTIVE SYSTEM
The female reproductive system consists of a tube-like muscular passage called the vagina which is surrounded by the labia majora and the labia minora. The labia majora is a fatty tissue of two folds covered by skin with hair while the labia minora is a thin fold of tissue devoid of hair. The labia majora conceals the labia minora. At the ventral junction of these two is a sensitive, erectile organ called the clitoris, which is a major site of female sexual excitement.

The vagina is lined by mucosa which is moistened by secretions from the uterus. The vagina extends from the exterior to a hollow, thin-walled muscular structure called the uterus. The muscular ring of the uterus which projects into the vagina is called the cervix.

The uterus continues into the fallopian tubes (or oviducts or utrine tubes) which are about 10 cm in length and differentiated as the isthmus and the ampulla. The ampulla ends in a funnel-like structure called the infundibulum formed by branched processes called fimbriae. The fimbriae spread over the ovary and help in picking up the ovum at ovulation.

The ovaries are the main reproductive organs found attached to the posterior side of the broad ligament of the uterus by a short fold called the mesovarium.

Human Circulatory System

THE HUMAN HEART

STRUCTURE OF THE HUMAN HEART

The human heart is a hollow, conical and muscular organ that is placed obliquely between the two lungs in front of the 5^th, 6^th, 7^th and 8^th thoracic vertebrae, just above the diaphragm. In adults it weighs about 300 g in males and 250 g in females; the size of the heart is almost the same as the size of a closed fist. The heart is enclosed in a protective, fibrous sac called the pericardium. The pericardium is differentiated as parietal pericardium and visceral pericardium (epicardium). Between the two layers is a space called the pericardial space filled with pericardial fluid. The pericardium protects the heart from shocks and injuries and also keeps the heart moist and prevents friction during functioning.

The human heart has four chambers – the upper right and left thin-walled auricles (atria) and the lower right thick-walled ventricles. The auricles are the receiving chambers while the ventricles are referred to as pumping chambers. The auricles are separated by a vertical wall called the inter-auricular septum. Two large veins called the superior and inferior vena cava convey deoxygenated blood to the right auricle. The oxygenated blood from the lungs enters the left auricle by way of the pulmonary veins, two from each lung.

The ventricles are separated by a vertical wall called the inter-ventricular septum. The inner walls of the ventricles have irregular projections called trabaculae formed by the papillary muscles of the ventricular wall. The right ventricle communicated with the right auricle by an opening called the right aurico-ventricular opening which is guarded by a valve having three cusps (flaps) called the tricuspid valve. It allows the blood to flow only from the right auricle to the right ventricle. The left ventricle communicates with the left auricle by an opening called the left aurico-ventricular opening which is guarded by a valve called the bicuspid valve or mitral valve. It allows the blood to flow only from the left auricle to the left ventricle. These two valves are connected to the trabaculae by thin, elastic cords called chordate tendinae. These cords regulated the opening and closing of the two valves.

The right ventricle opens into a large blood vessel called the pulmonary artery, which bifurcates; each branch supplies a lung. The opening of the right ventricle into the pulmonary artery is guarded by a semi-lunar valve called the pulmonary valve. The left ventricle leads into a large blood vessel called the aorta which bends or arches to the left side of the heart (in man and all mammals) to form to form the left aortic arch. The opening of the left ventricle into the aorta is guarded by another semi-lunar valve called the aortic valve. These two valves regulate the flow of blood from the ventricles to their respective vessels.

The wall of the heart consists of three layers namely

(i) The inner endocardium – made of the endothelium which lines the chambers, covers the valve surfaces and continues lining the blood vessels that enter and leave the heart.

(ii) The middle myocardium - a thick layer consisting of the cardiac muscles. It is thinner on the atrial walls and thicker on the ventricular walls. It forms the irregular trabaculae on the inner surfaces of the ventricles.

(iii) The out epicardium or visceral pericardium is a thin and transparent layer composed of fibrous tissue with an outer covering of mesothelium. Above the visceral pericardium is the parietal pericardium and in between these layers is a space called the pericardial cavity which is filled with pericardial fluid.

THE HEART AS A DOUBLE PUMP

The hearts of all mammals including human beings are called double pumps. Since, during one complete circulation, one side of the heat pumps deoxygenated blood and the other side of the heart pups oxygenated blood. This type of circulation is referred to as double circulation.

Double circulation involves two separate pathways, the pulmonary circulation and the systemic circulation. Each pathway is constituted by both arterial and venous systems. The arterial system consists of arteries, arterioles and capillaries while the venous system consists of capillaries, venules and veins. The arterial system carries blood away from the heart while the venous system brings blood back to the heart.

The arteries have thick, elastic, muscular walls with narrow lumen; veins have thin, non-elastic fibrous walls with wide lumens. Arteries don’t have valves, while veins have valves to prevent backflow of blood. Arteries are deep seated (located) while veins are superficial. In arteries, blood flows with jerks under high pressure, while in veins the blood flows smoothly under low pressure. All arteries convey oxygenated blood except the pulmonary artery while all veins convey deoxygenated blood except pulmonary veins.

Pulmonary Circulation

It is the pathway of the blood taken between the heart and the lungs, hence it is referred to as pulmonary circulation.

Deoxygenated blood from the heart goes to the lungs via the pulmonary artery. After gaseous exchange, oxygenated blood returns to the heart via the pulmonary veins.

Pulmonary Circuit

The pulmonary artery arises from the conus arteriosus (front upper portion of the right ventricle) of the right ventricle. It lies in front of the ascending aorta and then passes to its left and under the aortic arch, where it divides as the left and right pulmonary arteries. The right pulmonary artery is larger and longer than the left pulmonary artery. The right pulmonary artery passes horizontally in front of the right bronchus to the root of the right lung, where it divides into two branches – the upper and the lower branch.

The upper branch is smaller than the lower branch and supplies the upper lobe of the right lung. The lower branch divides further and supplies the middle and lower lobes of the right lung.

The left pulmonary artery passes horizontally in front of the descending aorta and the left bronchus to the root of the left lung where it divides into two branches, each one supplying the upper lobe and lower lobe of the left lung.

The arterioles of the pulmonary arteries open into the capillary beds in the lungs. These capillary beds are found surrounding the alveoli. The capillaries join the vennules which in turn lead into the pulmonary veins. Four pulmonary veins, two from each lung enters the posterior upper side of the left auricle. These vessels are without valves.

Systemic Circulation
It is the pathway of the blood taken between the heart and the different parts of the body.

Oxygenated blood from the heart goes to different parts of the body via the aorta and its branches. After gaseous exchange at the tissue, the deoxygenated blood returns to the heart via the superior and inferior vena cava.

Systemic Circuit

The starting point of the arterial system of the systemic circuit is the large aorta (25 mm in diameter). It is the largest artery in the body which supplies oxygenated blood to all the arteries of the systemic circuit.

The aorta which originates from the aortic valves extends upwards as the ascending aorta, then arches or bends to the left side to form the aortic arch and passes in front of the root of the left lung and runs down as the descending aorta along the left side of the thoracic vertebrae. The descending aorta is differentiated into the thoracic aorta and the abdominal aorta.

The ascending aorta is found within the pericardial sac. It gives off two branches, namely the left and the right coronary arteries, each of which profusely branch to supply the muscles of the heart (myocardium) and brings back deoxygenated blood by the coronary veins. This part of the systemic circulation constitutes the coronary circulation.

The right and left coronary arteries are the only branches of the ascending aorta. They originate from bulb-like swellings called the aortic sinuses, located behind the cusps of the aortic valve; hence blood can enter these arteries only when the left ventricle is relaxed. The right coronary artery arises from the right aortic sinus, passes between the right auricle and the conus arteriosus into the coronary sulcus (a groove between the right atrium and ventricle) and turns around the inferior margin of the heart and divides into a right posterior inter-ventricular descending artery, a right marginal artery and a transverse artery. The left coronary artery, which is larger than the right one, divides into a left anterior inter-ventricular descending artery and a circumflex artery.

The branches of the left and right coronary arteries completely encircle the heart forming an upside-down crown around it, hence called coronary arteries.

The left coronary artery supplies the anterior part of the inter-ventricular septum and the adjacent part of the right ventricle as well as the anterior surface of the left ventricle, a small part of the inferior surface of the left ventricle and the left margin of the left ventricle.

The right coronary artery supplies a major portion of the right ventricle, the posterior part of the inter-ventricular septum and many parts of the left ventricle.

The venous system of the coronary circuit is represented by the large coronary sinus, lying in the coronary sulcus on the posterior side of the heart.

The tributaries of the coronary sinus are the greater cardiac vein, the small cardiac vein and the middle cardiac vein. All these veins enter the coronary sinus which in turn empties deoxygenated blood into the right atrium (except the anterior cardiac vein which directly empties into the right auricle).

The greater cardiac vein runs with the left anterior inter-ventricular and circumflex branches of the left coronary artery. The small cardiac vein runs with the marginal branch of the right coronary artery. The middle cardiac vein runs with the posterior inter-ventricular branch of the right coronary artery.