The Circulatory System

The Circulatory System

Vertebrates have the most highly evolved circulatory system in the animal kingdom

The circulatory system performs a variety of functions including:

• transport of respiratory gases, nutrients, metabolic wastes, hormones and antibodies
• maintain internal environment (homeostasis) in conjunction with the kidneys
• responds quickly to the changes in the body depending on the needs of the moment The circulatory system is made up of two primary components: Blood-vascular system: a closed system composed of the heart, arteries (which distribute blood from the heart to the tissues), veins (return blood from the tissues to the heart) and capillaries (small thin-walled vessels at which physiological exchange occurs) and the blood
- although the system itself consists of a continuum of ducts, all are interconnected and allows for little or no loss of contents

Lymphatic system: drains fluids that accumulate in the tissues (tissue fluids), which are first collected by lymphatic capillaries, which pass into lymphatic vessels and then empty into the venous system

The circulatory system has more individual variation than any other system and is the first of all organ systems to become functional during development

The system is also highly adaptable - you can graft veins to other locations (such as in a heart bypass) or tie off a vessel without seriously inconveniencing the system

Blood and blood vessels

Blood is a fluid tissue containing cellular elements that are derived from mesoderm. There are two primary components to blood:

Plasma: constitutes approximately 2/3 of the blood and is composed of about 90% water • the non-water component contains fibrinogen, which contributes to blood clotting, and globulins, which respond to the entry of foreign materials into the body. Cellular: consists of two types of cells • erythrocytes - the red blood cells which carry hemoglobin that binds oxygen for transport to the tissues
• leukocytes - the white blood cells that destroy foreign bodies through phagocytosis and are also involved in the immune response Blood cells are produced by hemopoietic tissues • in embryos, hemopoietic tissue is found distributed throughout the body
• in adults, hemopoiesis occurs primarily in the red bone marrow (which contains stem cells, the primordia of blood cells) and the spleen Blood vessels are the first indicator of the formation of the circulatory system. Blood islands first form in the yolk and then become contiguous to form a network of vessels. Endothelium lines the blood vessels, and the rest is composed of muscular fibers, collagen and elastic fibers.

Blood vessels have three layers of tissues (Fig. 12.1, p. 423)

• the tunica intima is the inner layer of blood vessel that includes epithelium and elastic fibers
• the tunica media is the middle layer of blood vessel that contains primarily smooth muscle fibers
• the tunica externa is the extreme outer layer of the blood vessel that contains collagen fibers Veins are usually larger in diameter and thinner-walled than arteries

Veins channel blood to the heart, while arteries channel blood away from the heart

Capillaries, the intermediaries between the arteries and the veins, are generally composed only of endothelium because they are where most of the diffusion in the circulatory system occurs - the junction at the capillaries marks an anastomosis, or a peripheral union between the blood vessels

The Heart

The embryonic heart is formed from the splanchnic layer of the mesoderm. When first developed, the heart is composed of two layers (Fig. 12.8, p. 429):

• the endothelium forms the lining of the heart

• the myocardium forms the muscular part of the heart, containing cardiac muscle fibers.

The primitive heart is a nearly straight tube having four parts which pumps a single stream of deoxygenated blood throughout the body • blood flows from the sinus venosus, passes through the sinoatrial valve into the atrium, and then from the atrium into the ventricle through the atrioventricular valve

• from the ventricle, the blood passes along a series of semilunar valves into the muscular conus arteriosus, and finally into the arterial system

Fishes
The heart of fishes differs very little from the ancestral vertebrate form (Fig. 12.24, p. 445; Fig. 12.27, p. 447): • the heart is position very far forward in the body and lies adjacent to the gills
• Teleosts have lost the conus arteriosus and have developed the bulbus arteriosus, which is elastic and not muscular like the conus arteriosus
• blood is moved through contraction of the respiratory hypobranchial muscles, which allows the sinus venosus to suck blood from the venous sinuses and propels blood into the ventricle Amphibians and Reptiles
The amphibian heart is an intermediate three-chambered heart that allows for some separation of oxygenated and deoxygenated blood (Fig. 12.30, p. 450): • the amphibian and reptile heart has two atria - one atrium receives blood returning from the body and one receives blood returning from the lungs
• the ventricle is undivided, so mixing of oxygenated and deoxygenated blood still can occur
• the only separation of blood is in the timing of when blood enters the ventricle
• after passing into the conus arteriosus blood flows into the truncus arteriosus, which bifurcates and travels through the rest of the body Reptiles also have a three-chambered heart (Fig. 12.32, p. 451), but the circulation divides into three channels after the conus arteriosus - pulmonary trunk, right and left systemic trunks

Homeotherms
Homeotherms are characterized by having a four-chambered heart with a double circuit pump

• the pulmonary circuit is located on the right side of the heart while the systemic circuit is found on the left side of the heart
• homeotherms lack the sinus venosus that is found in more primitive hearts Control of heartbeat
The control of heartbeat in amniotes is influenced by the autonomic nervous system • the cardiac muscle itself has an inherent rhythm which assists in controlling the heartbeat
• the sinoatrial node serves as the pacemaker of the heart and sets the initial rhythm of the heartbeat
• the signal from the SA node is then conducted through the heart muscle by the Purkinje fibers, and the atrioventricular node transmits the signal through the cardiac muscle in the ventricle In lower vertebrates, the sinoatrial node is present, but the Purkinje fibers and the atrioventricular node are lacking

Coronary circulation is necessary to supply the metabolic needs of the cardiac muscle of mammals because it is larger in size than the two or three-chambered heart

• the coronary circuit is composed of a pair of coronary arteries that leave the base of the aortic arch
• coronary veins return blood to the right atrium Arterial channels and their modifications

The arterial channels are vessels that comprise the initial functioning system of the embryo, and is basically the same for all vertebrates (Figure 19-4, p. 676 in text).

The heart

pumps blood into the ventral aorta (truncus arteriosus)
the ventral aorta then distributes blood into the aortic arches that run upward into the visceral arches
blood then enters the dorsal aorta from the aortic arches (Embryos of jawed animals usually have six aortic arches.)
blood from the anterior aortic arches runs forward into the head to the internal carotid arteries
blood from the posterior aortic arches runs into the dorsal aorta and posteriorly, and may branch into the vitelline or umbilical artery or any of the other intersegmental arteries

The first aortic arch (in the mandibular visceral arch) is always lost in the adult, along with the second aortic arch.

Aortic arches of fishes

Afferent branchial arteries lead into the gills from the aortic arches.

Blood then flows through the gills through collector loops, and the oxygenated blood travels into the efferent branchial arteries, which continue into the dorsal aorta.

Rostrally the dorsal aorta branches into the internal carotid arteries that supply oxygenated blood the head.

Caudally the dorsal aorta continues into the caudal artery, from which the following arteries offshoot:

Coeliac and mesenteric (supply the abdominal viscera)

Gonadal (supply the gonads)

Renal (supply the kidneys)

Intersegmental (associated with the myomeres)

Subclavian (leads to the branchial arteries)

Iliac (leads to the femoral arteries)

Aortic arches of tetrapods

Tetrapods also lack the first and second aortic arches.

The carotid system of tetrapods carries blood to the head and is derived from the third aortic arches. It is composed of the common carotid arteries, which branch into

external carotid arteries, which supply the throat and the ventral part of the head

internal carotid arteries, which supply the brain and the rest of the head.

The left branch of the fourth aortic arch becomes the arch of the aorta in mammals, while the right one becomes the subclavian arteries.

The sixth aortic arch becomes the pulmonary arteries, which are derived from a common pulmonary trunk on the aorta.

Posterior arteries

The dorsal aorta is the large median longitudinal artery that extends posteriorly and eventually branches into the caudal artery.

Other branches of the dorsal aorta include:

Ventral visceral branches: the celiac artery, which leads to the stomach, duodenum, liver and pancreas, and the mesenteric artery, which serves the remainder of the liver and the gut.
Lateral visceral branches: these branches serve the urogenital organs (renal, ovarian, spermatic)
Dorsal somatic branches: these branches serve the spinal cord, muscles and skin.

Venous channels and their modifications

The initial pattern of the venous channels comprises three systems:

Subintestinal-vitelline system

Drains the tail, digestive tract and yolk sac. Also includes the caudal vein, which runs to the cloacal area. The subintestinal veins continue drainage forward (after receiving blood from the vitelline veins) and eventually drains into the common cardinal vein.

Cardinal system

Drains the head, dorsal body wall and kidneys. Includes the anterior cardinal vein (lateral to the carotid arteries), posterior cardinal veins (lie adjacent to the kidneys) and the common cardinal veins (into which the posterior and anterior cardinal veins drain)

Abdominal system

Drains the ventral body wall and appendages. Consists of the lateral abdominal veins, which receive blood from the iliac and subclavian veins.

Anterior veins

The anterior veins are derived from the cardinal system. In tetrapods, the anterior veins are composed by the internal and external jugular veins
The jugular veins unite with the subclavian vein and lead to the superior (cranial) vena cava, which also receives blood from the coronary veins (that first drain into the coronary sinus).

Hepatic portal system

The hepatic portal system is derived from the subintestinal system. The hepatic portal vein receives blood from the gut region.

Inside the liver the vein breaks up into hepatic sinusoids, where the blood comes into contact with hepatic cells and phagocytic cells. Harmful materials are detoxified and removed from the blood in the liver.

Posterior to the liver the blood is collected into the hepatic vein, which joins with the caudal vena cava.

Renal portal system

The renal portal system is derived from the posterior cardinal veins.

Blood from the posterior part of the body flows into the renal portal veins, which passes into the caudal vena cava.

The renal portal system is found only in fishes, amphibians, reptiles and birds. Thus, mammals have no renal portal system. All that remains in mammals is the azygous vein, which is an unpaired vein that drains most of the intercostal space on both sides of the mammalian thorax.

Posterior veins

The posterior veins contain veins that are derived from one or all three initial systems (subintestinal, cardinal, abdominal). Adult birds and mammals lack the abdominal system, but as fetuses possessed two parts of the system (allantoic or umbilical veins).

Circulation in the mammalian fetus

Fetal mammals possess shunts between the pulmonary and systemic circuits because the placenta, not the lungs, is the site for gas exchange

Blood returns to the fetus from the placenta via the umbilical vein, enters the ductus venosus in the liver, and then passes into the caudal vena cava. The blood then passes through the foramen ovale (located in the septum between the right and left halves of the heart) and into the left atrium (thus bypassing the pulmonary circuit).

Since fetal lungs are not inflated, the pulmonary circuit is bypassed by the ductus arteriosus (a remnant of the sixth aortic arch) which joins to the aorta.

At birth, lungs are inflated and the pulmonary circuit becomes more important in gas exchange. The pressure due to the flow of the blood from the lungs causes closure of the foramen ovale, leaving a grown-over region called the fossa ovalis. Due to lack of use as a shunt the ductus arteriosus closes and is filled with connective tissue to become the ligamentum arteriosum.

Lymphatic system

The purpose of the lymphatic system is to drain fluids that accumulate in the tissues and empty into the venous system.

Although Chondricthyes and other primitive fishes lack a true lymphatic system, they do have some vessels that help to drain the tissues (called the hemolymphatic system) that seems to be a precursor of the true lymphatic system.

Components of the tetrapod lymphatic system include lymphatic capillaries to drain the tissues.

Across the vertebrate classes there is significant variation in the drainage of lymphatic capillaries into common larger ducts (Figure 19-20, p. 705 in text).

Amphibians and reptiles have three primary lymph vessels (subcutaneous, subvertebral, visceral) as well as lymph hearts, which are segmentally arranged masses containing smooth muscles that help propel lymph through the lymphatic system.

In birds and mammals the subvertebral ducts called thoracic ducts. Mammals also have the cisterna chyli, which is a sac that receives lymph from the abdominal viscera and caudal parts of the body. Mammals and birds also have lymph nodes, found in the neck, armpits and groin. Lymph nodes are the site of convergence for lymphatic vessels.

Other parts of the lymphatic system include the tonsils (lingual, pharyngeal and palatine), Peyer's patches (patches of lymphatic tissue in the small intestine of mammals), the vermiform appendix, and the bursa of Fabricius (a pouch in the cloaca of birds that contains lymphatic tissue.

The thymus is a lymphatic organ that produces T lymphocytes, which are involved in the humoral immune response.

Definitions

Anastomosis - peripheral union between the blood vessels

Bursa of Fabricius - pouch in the cloaca of birds that contains lymphatic tissue Cisterna chyli - a sac that receives lymph from the abdominal viscera and caudal parts of the body in mammals

Hemopoietic tissue - a tissue in which blood cells are formed

Homeostasis - the condition in which a constant internal environment is maintained despite factors that tend to destabilize it

Peyer's patches - patches of lymphatic tissue in the small intestine of mammals