Lesson 30

Lesson Outline: • Circulatory System Embryonic Origins • Heart • Blood Vessels Heart • Basic Form • Basic Function Circulation • Basic Form o General Circulation o Branchial Circulation • Basic Function o Branchial Circulation Countercurrent gas exchange

◊ Objectives: At the end of this lesson you should be able to:

♦ Describe the embryonic origins of the heart and blood vessels in vertebrates ♦ Describe the form of the basic vertebrate heart and how it functions ♦ Describe the branchial circulation in fishes and how it functions

◊ References: Chapter 14: 314-350

◊ Reading for Next Lesson: Chapter 14: 314-350 Circulatory System

The role of the circulatory system is to transport substances between sites in the body.

As organisms grew in size, specialized exchange sites developed for nutrient uptake, gas exchange and waste removal, amongst other things. This meant that many key functions now took place at localized sites far removed from many of the cells of the body that required the benefits of those exchange processes. The circulatory system took on the role of transporting materials between all cells of the body.

The circulatory system also took on the role of distributing the hormones that play a key role in regulatory processes, and of transporting the chemicals of the immune system that defend the body from foreign organisms.

Embryonic origins

The circulatory system consists of the heart and the cardiovascular system.

Heart Both the heart and the blood vessels arise within the embryonic mesoderm (splanchnic hypomere).

The embryonic heart forms from splanchnic mesoderm. It begins as a pair of endocardial tubes which become surrounded by a thick layer of cells called the epimyocardium. These grow toward the midline and fuse into a single centrally located tubular heart. The original cells of the endocardial tube become the endocardium that lines the interior of the heart while the epimyocardium becomes the muscular myocardium of the heart.

When first formed the muscle is already contractile, the tube is subdivided into four parts and it already is autonomous and rhythmically beating - even before there are blood vessels connected to it!!

Blood Vessels Small clusters of mesodermal cells called blood islands yield both blood vessels and blood cells (i.e. both angiogenesis and hematopoeisis). The blood islands eventually merge to form tubes that eventually link all tissues in the embryo together to the heart.

Heart

Basic Form While the specific arrangement of all the blood vessels of the body is tremendously varied from animal to animal, these variations are based on modifications of a basic vertebrate plan (which you have gone over in the lab).

The arrangement and names of all the major arteries and veins are similar in all vertebrates and are described in your text.

The two things that I do want to cover in lecture are: - the evolutionary trends that we see in the heart - the evolutionary trends that we see in the aortic arches.

While these are considered separately in your textbook I want to consider the trends in these two sets of structures together rather than separately in the lectures.

Phylogenetically, the heart probably began as a contractile vessel with no distinct chambers or valves - as is still the case in amphioxus. While this may seem inefficient, at this point in evolution, organisms were sessile and most exchange still took place across the general body surface. Under these conditions, such a form of circulation was more than adequate to meet their needs.

As protochordates evolved, we see the development of a true heart.

In early vertebrates, the first chamber of the heart that receives all returning venous blood is the sinus venosus. This leads through into the atrium that in turn leads into the ventricle that finally leads into the conus arteriosus. Each chamber is separated from the former by one-way valves.

All chambers are muscular and contractile and all are capable of generating an autonomous rhythm (i.e. each one has pacemaker like properties).

Flexion and expansion of the tubular heart twists the heart into different configurations in different animals but the internal path of blood flow is always the same. The heart goes from being a relatively straight tube to having a distinctive “S” shape in sharks and fish which brings the thin-walled sinus venosus and atrium to lie above the ventricle with the atrium at the anterior of the heart.

The heart lies within the pericardial cavity, part of the coelom. In many fishes, including sharks, the pericardial cavity lies within cartilage or bone, forming a semi rigid compartment that holds and protects the heart. It is also believed to aid the filling of the heart (below).

Many hearts have a bulbous chamber just outside the ventricle but not all of these chambers are the same and, hence, several names have been given to them. These distinguish different origins and functions of this chamber.

Bulbus cordis – name applied to the anterior chamber in embryos.

Conus arteriosus – a contractile chamber made of cardiac muscle and containing internal valves (chondricthyans, holosteans and dipnoans).

Bulbus arteriosus – a thinner walled chamber made of smooth muscle and elastic fibres - derived from the same embryonic source (and hence part of the heart) but without the cardiac cells. It may incorporate part of the ventral aorta (other fishes).

Truncus arteriosus – a chamber of the ventral aorta or its derivatives but not a part of the heart.

Basic Function All vertebrate hearts are myogenic. Contraction is an intrinsic property of the cardiac muscle. While all chambers of the heart have pacemaker potential, the cells of the different chambers have different intrinsic rhythms. The chamber with the fastest rhythm dominates and sets the heart beat and that chamber is the sinus venosus. Cardiac cells are electrically coupled hence once the dominant pacemaker fires, a wave of excitation will spread along the length of the heart from the sinus venosus to the atrium, ventricle and conus arteriosus.

As the wave of contraction spreads along the heart, blood is pushed through the system. The one-way valves between the chambers prevent reverse flow from occurring as successive chambers contract.

After a contraction, as the heart relaxes, venous blood returning to the heart will push open the sinoatrial valves and allow blood to fill both chambers. Filling of the sinus venosus and atrium occur both due to venous pressure (which is very small) and the expansion of the sinus venosus and atrium as they relax after each contraction.

Contraction of the sinus venosus will then push its contents into the atrium and subsequent contraction of the atrium will close the sinoatrial valves and push the atrial contents into the ventricle. The S-shaped structure of the heart places the thin walled sinus venosus and atrium dorsal to the ventricle, so that gravity assists the flow of blood into the ventricle.

The ventricle then contracts driving blood forward both into the conus arteriosus as well as into the ventral aorta. Once the ventricle has contracted fully and begins to relax, the conus contracts pushing its contents into the ventral aorta.

With this system, blood is driven into the ventral aorta even after the ventricle has stopped contracting and begun to relax. This extends the period of positive flow in the ventral aorta.

Note the relative thickness of chamber walls. The sinus venosus and atria only push blood into successive chambers and are not very muscular. The ventricle pumps blood throughout the body and is very thick walled and muscular. The conus in sharks is also very muscular.

Finally, note that because the heart in the shark is contained within a semi-rigid cartilaginous chamber, as the ventricle contracts, and ejects blood out into the body, the volume of the heart will fall while the volume of the chamber does not. This will create a negative pressure within the pericardial sac, which will suck on the sinus venosus and atria and assist them in their filling (the aspiration effect).

Circulation

Basic Form With the development of the heart in protochordates, we also see the development of the pharyngeal slits with their supporting branchial arches.

The number of primitive branchial arches, and the aortic arches that ran through them, are still debated. In present day fishes we can see as many as 15 pair (hagfishes can have 15, sharks can have 10-12, lampreys can have 8 pair).

In most gnathostomes and tetrapods, only 6 pairs are present embryonically and thus, this is considered the basic pattern and they are numbered I to VI in your text (but not in all texts).

General Circulation In fish, all blood leaving the heart first enters into the ventral aorta and goes to the gills. After passing through the gills, where it is oxygenated and where CO2 is excreted to the water, it then goes to all the tissues of the body. Within spiracle each tissue it passes through smaller and smaller vessels until it reaches the capillaries where the exchange of nutrients, head gases and wastes take place. Blood then returns to the heart through a series of veins that connect together into larger and larger vessels until all veins return to the sinus venosus.

In all vertebrates, there are two special cases where blood leaving a capillary bed does not return directly to the heart but subsequently passes through a second capillary bed. These are referred to as portal systems.

The hepatic portal system delivers blood directly from the heart to the liver (hepatic) where nutrients are further processed and/or stored.

The renal portal system delivers venous blood to the kidneys where nitrogenous wastes can be removed before the blood returns to the heart. The arrangement of this system changes dramatically in vertebrates as the structure of the kidney changes.

Branchial Circulation The ventral aorta gives rise to the arteries supplying the gill arches, the afferent branchial arteries. Each of these arteries runs along the arch near the skeletal element.

As it runs along the arch from bottom to top, it gives off branches that run out into the septum at the level of each primary lamella. These arteries in turn give rise to branches that run out into the primary lamella on each side and ultimately give rise to the capillary beds that lie within the secondary lamella.

Blood leaving the capillary beds is collected in efferent vessels running along the outside of the primary lamella. These efferent vessels return to the main stem of the gill arch, where they flow into the efferent arteries that also run along the arch, near the skeletal elements from ventral to dorsal. There will be two major efferent vessels in each gill arch, one associated with each hemibranch.

The two efferent vessels in each gill arch do not join together. Rather, the vessels from the pre-trematic sides of each gill slit join together with the vessels from the pos-trematic sides of the gill slits to form the efferent arteries that leave the gills.

Basic Function Branchial Circulation Counter current gas exchange The significance of this arrangement of blood vessels and the design of the lamella of the gill is to: Provide an extremely large surface area for gas exchange. Establish a counter current flow system between blood in the capillaries and the water flowing past them.

This arrangement provides extremely efficient gas transfer. As a result, blood leaving the gills is in total equilibrium with the water; it has very high levels of oxygen and very low levels of CO2. Mammalian lungs are nowhere near as efficient (think about why not and what the implications are) although bird lungs can be (think about how this has evolved and what the selection pressures might have been).