Cardiovascular System

The cardiovascular system is made up pumps, a distribution system and a return system.

Branchial Heart

Teleost

Elasmobranch

Cuverian ducts, hepatic, anterior jugular veins

sinus venosus (with pacemaker region)

atrium

ventricle

depulsing to create continuous aortic flow

bulbus arteriosus

 

 

contractile conus arteriosus,

2 to 6 sets of valves

atrium

Myocardium and mode of life

Type I

Only spongosia (spongy layer). No capillaries in myocardium. Trabeculae of spongiosa compromise between allowing diffusion of oxygen from lumen blood to muscles and a large muscle cross-sectional area.

hagfish,

Hb-free icefish,

Hb icefish,

flounder.

sluggish

Type II

Spongosia and compacta. Coronary vessels in compacta only, with veins draining venous blood into the atria.

Salmonids etc.

Active fish.

Type III

Spongosia and compacta. Coronary vessels reach spongosia. (c.f. mammalian Thebesian system)

Most elasmobranchs.

  

Type IV

Spongosia and compacta. Coronary vessels reach spongosia. No veins, few capillaries, and terminal arterioles open directly into the ventricle.

Tuna.

Very active and endothermic elasmobranchs and teleosts.

Cardiac output (Q) varies 15-fold among species, with more active species usually having higher values.

Blood Distribution

All cardiac output of water breathers passes through the gills and from thence to the systemic circulation. The gill circulation includes various pathways that can act as shunts, contributing to control of functional gill area.

Dorsal aorta is the main systemic circulation distribution conduit, with a general pattern similar in most fishes. Venous return from the trunk muscle returns via the kidneys and liver. Elasmobranchs and some teleosts have caudal accessory hearts to facilitate venous return. The greatest proportion of the cardiac output goes to the myotomal muscles - 10 to 36% to red and 28 to 49% to white. Gut gets 3 to 9% of the cardiac output, which may double after feeding.

Secondary Circulation

Fish circulation is unique in having a secondary circulation but no lymphatics. The primary role of the secondary circulation is nutritive.

Air-breathers

Many air-breathing fishes have a double circulation, and in addition, lungfishes have partially divided blood streams in the heart.

Blood Oxygen Transport

Oxygen is transported in physical solution (<5% of total) and chemically bound to hemoglobin. Ability of blood to load and deliver oxygen is described through blood-oxygen dissociation curves. In general, more active fish have a higher hematocrit than less active fish. Except for monomeric hagfish hemoglobin, fish have tetrameric hemoglobin exhibiting binding cooperativity. The blood-oxygen dissociation curve is fundamentally sigmoid. Dynamic binding properties are affected by several intra-cellular allosteric modulators.
In general there are two major strategies for meeting tissue oxygen requirements:

More active species

Less active species

blood has low oxygen affinity blood

blood has high oxygen affinity

high arterial pO2

low arterial pO2

blood 90-100% saturated in both

blood 90-100% saturated in both

higher venous pO2

lower venous pO2

high hematocrit

low hematocrit

high blood oxygen capacity

low blood oxygen capacity

low resting Q, but high expansibility

high resting Q limiting expansibility

trout - higher resting D [O2]A-V

flounder - lower resting D [O2]A-V

trout - lower resting Q

higher resting Q

trout - greater ability to meet elevated MO2 by increasing Q

flounder - reduced ability to increase MO2

trout- lower extraction efficiency and higher ventilation energy cost

flounder - advantage maximizes oxygen extraction from water by maintaining large D pO2, with reduced ventilation costs

trout - high cooperativity typical of low affinity hemoglobins offsets to some extent the low D pO2

  

trout - high tissue D pO2 for high unloading capability

flounder - disadvantage is reduced tissue D pO2 and reduced unloading capability

trout - active fish in well aerated water

flounder strategy - adequate for a sluggish fish - good strategy overall as long as tissue oxygen delivery is not compromised.

Carbon Dioxide Transport

Carbon dioxide is transported in solution and bicarbonate ions and on the Hb of red blood cells. 95% of the CO2 in venous blood is in the form of plasma HCO3-.

Icefish

Antarctic icefish lack hemoglobin so that all oxygen is transported in physical solution. Oxygen is especially soluble in the plasma at low temperatures. Cardiac output is high, achieved at low heart rate (14/min) and high stroke volume. Low peripheral resistance ensures the work done by the heart is kept low. The life-style of this fish is sluggish, so that MO2 is low.