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What are two main perspectives when understanding circulation?
Mechanically, it involves the pressure-driven bulk flow of blood through vessels or passages, delivering it to all parts of the body
It is also defined by it’s function, transporting O2, CO2, nutrients, waste, hormones, immune agents and heat
What is the circulatory system?
It’s the system compiling the vessels and blood
Why is hydraulic pressure provided by the circulation necessary?
It’s important for certain organ functions
What is one important feature of circulation?
It’s speed compared to for example diffusion, which is too slow to effectively transport substances over distances greater than 1 mm
What is a another type of transportation?
Convective transport, which is the transportation by bulk flow of body fluids. This transport is significantly faster and essential for larger animals
What is the circulatory systems most critical function?
The transport of oxygen, and the capacity to transport O₂ is closely linked to metabolic intensity.
This means that as an organism’s need for O₂ increases, the blood flow rate adjusts accordingly.
This relationship is evolutionary, with the development of oxygen transport and metabolic capacity evolving together over time.
What is a key component in the circulatory system?
The heart, responsible for pumping blood. A heart can vary in complexity.
Arthropods have a single-chambered heart, vertebrates have multichambered hearts. Some animals have accessory hearts, helps pump blood in specific body regions.
What is the myocardium?
It is the heart muscle and consists of cardiac muscle, which has distinct properties compared to other muscle types. Vertebrate cardiac muscle has unique structural and physiological features.
In vertebrates, the ventricular myocardium is classified as compact.
In teleost fish, amphibians, and non-avian reptiles, the ventricular myocardium is typically spongy.
Myocardium is dependent on a continuous supply of O₂.
How is blood supplied to the myocardium in different animals?
In vertebrates, blood is supplied by the coronary circulation, a network of coronary arteries that branch from the systemic aorta and deliver oxygen-rich blood to it. After passing through capillary beds, blood is drained by coronary veins into the right atrium.
In teleost fish, amphibians, and non-avian reptiles, the myocardium features an anastomosing network of spaces, allowing luminal blood to flow through and oxygenate the myocardial cells.
However, this is less efficient because luminal blood isn't always well-oxygenated.
How is the heart of mammals and birds structured and how does the blood flow through it?
Two sides, each with two chambers, an atrium and a ventricle.
The left side receives oxygenated blood from the lungs via the pulmonary veins and pumps it to the rest of the body through the systemic aorta.
→ The blood flows through the systemic circuit to supply tissues and then returns to the right side of the heart via the venae cavae.
→ The right side pumps deoxygenated blood to the lungs through the pulmonary arteries.
Both sides of the heart contain passive valves that ensure blood flows in the correct direction and prevent backflow.
How does the heart function as a pump and what are the two phases?
By contracting and relaxing in a cycle.
The contraction phase is called systole.
The relaxation phase is called diastole.
What happens during systole?
The ventricle contracts, increasing the pressure on the blood within.
When the ventricular pressure surpasses the pressure in the aorta, the aortic valve opens, allowing blood into the aorta.
This is phase of ventricular ejection. Toward the end of this phase, although aortic pressure slightly exceeds ventricular pressure, blood continues to flow into the aorta due to momentum
What happens during diastole?
As the ventricle relaxes, pressure in the ventricle falls, and the aortic valve closes, marking the start of isovolumetric relaxation, during which both the inflow and outflow valves are closed.
When ventricular pressure falls below atrial pressure, the atrioventricular valve opens, allowing the ventricle to fill with blood.
Most of this filling occurs before the atrial systole, driven by the pressure of blood in the atrium.
Atrial contraction then pushes some additional blood into the ventricle just before the next ventricular systole.
What is cardiac output?
The volume of blood the heart pumps per unit of time. It is determined by the heart rate and stroke volume.
What makes the heart contract?
Electrical impulses that may originate in muscle cells or neurons.
There are two main types of hearts based on where the contraction impulse originates:
Myogenic hearts
Neurogenic hearts
What is a myogenic heart?
A heart that generates its own electrical impulses without needing input from nerves.
How does the myogenic heart work?
Muscle cells in the heart are electrically coupled through gap junctions. When one cell depolarizes, it quickly triggers neighboring cells, causing the entire heart region to contract as a unit.
The pacemaker is a group of specialized muscle cells that control the heartbeat rhythm.
In mammals and birds, the pacemaker is located in the right atrium and is known as the sinoatrial (S-A) node.
These pacemaker cells have the fastest spontaneous depolarization rate and thus set the heart's rhythm.
Depolarization spreads through the heart in a process called conduction.
In mammals, this occurs through a specialized system, starting from the atrioventricular (A-V) node and spreading down the atrioventricular bundle, dividing into bundle branches and ending in Purkinje fibers.
This system ensures rapid, synchronized contraction of the ventricles.
What is a neurogenic heart?
A heart that contracts with the help of nerve impulses
How does a neurogenic heart work?
In for example crustaceans, each heart muscle cell contracts only when stimulated by nerve impulses, known as neuronal action potentials.
The heart is controlled by a cardiac ganglion located on the heart’s dorsal wall, made up of nine neurons.
The five anterior neurons directly innervate and stimulate the heart muscle to contract.
The four posterior neurons do not contact the muscle but connect to the anterior neurons. If the ganglion is separated from the heart muscle, the ganglion continues to produce impulses, but the muscle stops contracting.
What is vascular endothelium?
The epithelium that lines the heart and the lumen of blood vessels in vertebrates. The cells of the endothelium are exceedingly important: They perform many functions, which are only gradually being understood.
How does arteries differ size?
Small arteries:
The arteries become smaller as they branch outward toward the periphery of the circulatory system.
The walls of the arteries simultaneously become thinner.
Great arteries:
have thick walls that are heavily invested with smooth muscle and with elastic and collagenous connective tissue.
The elasticity of the great arteries enables them to perform important hydrodynamic functions.
What is Laplace’s law?
The elasticity of the great arteries enables them to perform important hydrodynamic functions. T = r∆P
Where T is wall tension, r is the radius of the lumen, and ΔP here represents the pressure difference across the walls.
What is microcirculatory beds?
Microscopically fine blood vessels which consist of three types of vessels: arterioles, capillaries, and venules.
What is arterioles?
A microscopically fine blood vessel with muscular walls that carries blood from arteries to capillaries in a microcirculatory bed of a vertebrate.
Contraction and relaxation of the muscular walls controls the rate of blood flow to the capillaries supplied by the arteriole.
What is capillaries?
A microscopically fine blood vessel, the wall of which consists of only a single layer of epithelial cells. Capillaries are the principal sites of exchange between blood and other tissues in a closed circulatory system.
What is a capillary bed?
Many capillaries that branch and connect among the cells of a tissue.
What is angiogenesis?
The formation of new blood vessels (e.g., new capillaries) by sprouting of branches from existing vessels.
What is venules?
Small vessels with thin walls (2–5 μm in humans) containing connective tissue and muscle cells which the capillary beds drain into.
What is veins?
Blood vessels that are thinner than arteries, only go in one direction and always towards the heart
Why does mammals and birds require high rates of blood flow?
Because of their high O2-transport demands. The resistance to flow through their systemic circuits is also high, however. To maintain high rates of flow in the face of this high resistance, mammals and birds require exceptionally high pressures in their systemic arteries.
What does ultrafiltration mean?
Pressure-driven bulk flow of fluid out of the blood plasma across the walls of blood capillaries, considered a form of filtration because solutes of high molecular weight are left behind while ones of low molecular weight travel with the fluid.
In ordinary tissues, fluid that leaves the blood plasma by ultrafiltration enters the tissue interstitial fluids. In kidneys that form primary urine by ultrafiltration, the fluid that leaves the blood plasma enters the kidney tubules (e.g., nephrons).
What is colloid osmotic pressure?
The difference in osmotic pressure that arises between two solutions on either side of a cell membrane or epithelium because the two solutions differ in their concentrations of nonpermeating protein solutes.
What is the Starling-Landis hypothesis?
The concept that the ventricle of the heart tends to contract with greater force when it is stretched to a great extent at the start of contraction rather than stretched to a small extent.
What is the pulmonary circulation?
The system of transportation that shunts de-oxygenated blood from the heart to the lungs to be re-saturated with oxygen before being dispersed into the systemic circulation.
What is the form for O2-delivery in the systemic circuit?
Rate of O2 delivery = cardiac output × (arterial O2 concentration − venous O2 concentration)
When comparing mammal species of various body sizes, there is a apparent allometric relation between resting cardiac output and body size:
Cardiac output per unit of body weight tends to increase as body size decreases.
What is the ventral aorta?
In fish, the vessel that carries blood from the heart to the gills.
What is the dorsal aorta?
In the circulatory system of a fish, the major vessel that conveys blood from the gills to the systemic tissues.
What is the atrium?
In the study of hearts, a heart chamber; in a mammalian heart, one of the two weakly muscular chambers into which blood flows as it enters the heart. In the book lungs of arachnids, the chamber into which the multiple page-like lamellae project.
What is a ventricle?
A relatively muscular heart chamber
What is conus arteriosus?
Includes cardiac muscle and contracts in sequence with the ventricle, helping pump the blood.
What is bulbus arteriosus?
Consists of vascular smooth muscle and elastic tissue and does not contract in sequence with the other heart chambers; it seems to act primarily as an elastic chamber that smooths pressure oscillations and serves as a pressure reservoir between heart contractions.
What are some main differences between mammals and fish?
Compared with mammals of similar body size, fish generally have far smaller hearts and far lower cardiac outputs. Their lower cardiac outputs correlate with their lower O2 demands: Fish have far lower metabolic rates than mammals and thus can satisfy their O2 needs with lower rates of blood flow. Fish also maintain lower arterial pressures than mammals.
What is trabelculae?
In many fish the ventricular myocardium is entirely spongy, consisting of a mesh of relatively narrow, long sheets of myocardial cells. It is attached at both ends to the ventricular walls, crisscross parts of the ventricular lumen, being bathed on either side by the luminal blood (usually systemic venous blood) that is passing through (and being pumped by) the ventricle. Some fish have, in addition, a layer of compact myocardium that surrounds the spongy myocardium.
What does ABO mean?
Air-breathing organs.
A few hundred species of fish are able to breathe air. In most cases, the air-breathing organs (ABOs) of these species are derived from structures, such as the mouth membranes, gut, or swim bladder, which are primitively served by the systemic circulation. Accordingly, the oxygenated venous blood leaving the ABO typically flows into the systemic venous vasculature, not the systemic arterial vasculature.
What is present in lung-breathing amphibians and nonavian reptiles?
Two completely separate atrial chambers
How does blood flow through lung-breathing amphibians and nonavian reptiles?
Oxygenated blood from the lungs flows to the left atrium, and from systemic venous blood to the right atrium. Oxygenated and non-oxygenated blood is separated until it enters the ventricle where it mixes and flows through the systemic tissues
In amphibians, what does and doesn’t a ventricle have?
The ventricle lacks a septum, but its lumen is filled with spongy myocardium.
How does blood flow through amphibians?
Blood is discharged into the conus arteriosus, and is directed to lungs and systemic circuit through three pairs of arteries. Amphibians can also oxygenate blood by taking in O2 through their skin.
How does blood flow in nonavian reptiles?
Principal arteries arise directly from the hearts ventricle, which is separated by muscular ridges. Pulmonary and systemic blood can potentially mix but studies show selective blood distribution.
What are invertebrates?
Animals that lack a vertebral column (backbone), including groups like mollusks, annelids, and arthropods.
What are muscular arteries and veins?
Blood vessels with muscular walls that help regulate blood flow and pressure by contracting or relaxing.
What is a principal (systemic) heart?
The main heart in cephalopods responsible for pumping oxygenated blood from the gills to the rest of the body.
What is a auxiliary branchial heart?
Secondary hearts in cephalopods that pump deoxygenated blood to the gills for oxygenation.
What is respiratory pumps?
Structures or organs that assist in moving blood through the respiratory system (such as gills in cephalopods) for gas exchange.
What is lacunae?
In an open circulatory system, small spaces between nonvascular cells that serve as passageways for blood flow.
What is sinuses?
In an open circulatory system, large spaces, between nonvascular structures, which serve as passageways for blood flow.
What is hemolymph?
A synonym for blood in an animal that has an open circulatory system. The term emphasizes that the blood in such animals includes all extracellular fluids, and thus that there is no distinction between the fluid that is in the blood vessels at any one time and the interstitial fluid between tissue cells.
What is ostia?
In the heart of an arthropod, valved passages through the heart muscle through which blood enters the heart chamber.
What is the pericardial sinus?
A fluid-filled space or cavity surrounding the heart.
What is the Infrabranchial sinus?
In a crustacean, a ventral sinus where blood collects after perfusing the systemic tissues, just prior to return to the gills.
What are some differences between open and closed systems?
Closed systems found in vertebrates and cephalopods, maintain blood flow within vessels allowing precise control of circulation through arterioles.
Open systems which are less understood in terms of spatial control, can be as efficient as closed systems in oxygen delivery in organisms like crustaceans.
How does different animals maintain high blood flow?
Cephalopods maintain high blood flow rates through a high-resistance systemic circuit by keeping systemic arterial pressure high, similar to fish.
Open circulatory systems typically operate at lower pressure gradients compared to closed systems. However, this does not equate to sluggish flow; crustaceans have rapid circulation due to low resistance in the lacunae and sinuses.
How is cephalopods oxygen supplied?
Squid gills can obtain oxygen efficiently, but their circulatory system has limitations. Squids rely on hemocyanin to carry oxygen in their blood, which is less effective than hemoglobin used by fish.
What are some challenges for cephalopods oxygen transport?
Squids have almost no venous oxygen reserve at rest, meaning they cannot store extra oxygen. During activity, their hearts must work harder to increase oxygen delivery, unlike fish that can tap into their venous oxygen reserves.
Which animals breath continuously?
Mammals and birds
How is the efficiency of O2 promoted in continuously breathing vertebrates?
By anatomically separating the pulmonary and systemic circuits and connecting them in series
Are extant amphibians and nonavian reptiles modifying their pulmonary and systemic circuits over time to become more like mammals and birds?
No they are not
What is marine mammal?
Mammals that rely on marine ecosystems for their existence (informal group, unified solely by this reliance in marine ecosystems)
When a mammal dives, does it dive its maximum capability of the species?
No, most dives by marine mammals are both shorter timewise and shallower than the maximum capability.
What are some factors affecting the physiology of a mammals dive?
If the dive is
Forced or voluntary
Short or long
Relative to the maximum for the species
Quiet or active during the dive
What are the major internal O2 stores i a diving mammal?
Bound to blood hemoglobin.
Bound to muscle myoglobin.
Contained in air in the animal’s lungs.
How is the catabolism affected by the length of the dive?
Because of the storage of oxygen, aerobic catabolism is able to continue, and in theory, all tissue is able to function aerobically throughout relatively short dives.
And this seems to be true for relatively short, voluntary, dives in many species.
The O2 stores are however inadequate to permit fully aerobic function during lengthy dives.
Dives of maximum length last several times longer than would be predicted if the animal was to function aerobically at the rate seen during rest in air.
Do all tissues need oxygen to produce ATP?
No, Certain tissues are predominantly, or exclusively, dependent on aerobic catabolism for production of ATP; they need oxygen on a steady basis and are quickly damaged by oxygen deprivation.
E.g. The central nervous system and the heart
On the other hand, some tissues have a well-developed ability to meet their ATP demands anaerobically.
E.g. skeletal muscle
They are therefore relatively tolerant of oxygen deprivation.
How is the oxygen storage of help in diving mammal?
Due to many species’ ability to ‘reserve’ some oxygen for the oxygen dependent tissues, those tissues can continue to have adequate oxygen levels, even while other parts of the body exhaust their supplies and turn to anaerobic catabolism.
This preferential delivery of oxygen is achieved by vasoconstriction of the arterial vessels.
The heart pumps blood primarily between itself, the lungs and the brain, meaning that any oxygen extracted from the air in the lungs is delivered preferentially to those tissues.
The tissues that are deprived of active blood flow can make use of the hemoglobin-bound oxygen in the small volume of blood that passed through their capillaries.
They can also use the myoglobin-bound oxygen.
How is blood flow redistributed during voluntary vs forced dives?
Voluntary dives:
During shorter dives relatively little redistribution of blood flow occurs
When voluntarily diving for longer periods of time profound redistribution of blood flow occurs
Forced dives:
During forced dives the responses of the circulatory system are a lot less flexible
Blood flow redistribution tends to occur rapidly, consistently, and to a profound extent.
When forced under water the animal senses that they have no control over the length of time they will be submerged, and therefore exhibit stereotypic and ‘reflexive’ circulatory responses?
The same responses they would employ for a prolonged voluntary dive.
How is the sized of the oxygen store of an animal determined?
It is determined by how long the animal can stay submerged.
Since the dive only can last as long as the brain, and other oxygen dependent tissues, is supplied with oxygen.
Which features affect the blood oxygen store?
The oxygen-carrying capacity of the blood
The total volume of blood
The degree to which the blood is saturated with oxygen at the time of submergence.
How is maximum possible oxygen store calculated?
By multiplying the oxygen-carrying capacity of the blood, by its blood volume.
How is the size difference of myoglobin storage between diving and terrestrial mammals?
Diving mammals almost always have really high myoglobin concentrations in their skeletal muscles, compared to terrestrial species, and large myoglobin-bound oxygen stores. The amount of oxygen stored as oxymyoglobin at the time of submergence also depends on how much myoglobin is present in each unit of muscle tissue.
Where is myoglobin used?
Oxymyoglobin could be viewed as a private store of oxygen, just for the muscles.
Myoglobin typically draws oxygen from the blood hemoglobin, rather than donate oxygen to the blood, due to its high affinity for oxygen.
This means that even if the muscle does have some blood flow during the dive, the oxymyoglobin within the muscles does not yield muck oxygen to the blood for use elsewhere in the body.
The oxygen instead remains bound to the myoglobin until the oxygen partial pressure in the muscles falls to a low enough level, and the oxygen is donated to the muscle mitochondria to permit continued aerobic ATP production.
Is a large air store an advantage for diving mammals?
No, three considerations argue against it:
A large amount of air in the lungs can strongly buoy the animal upward, forcing it to work hard to remain submerged.
It is believed that the alveoli usually are the first parts of the lungs to collapse as the lung air compresses, leading to most of the air being contained in parts of the lungs where the oxygen is unavailable for transfer to the blood.
While diving mammals are able to make effective use of their pulmonary oxygen stores during shallow dives, this isn’t as useful during the deeper ones.
A large pulmonary air store can increase the likelihood of decompression sickness.
What is diving bradycardia?
Decreased heart rate when diving. A single part of an integrated, body-wide, reorganization of cardiovascular function.
Less parts of the active cardiovascular system leads to less output of blood from the heart is required.
Output declines roughly in proportion to the decline in heartrate.
What two factors are systemic arterial blood dependent on?
Rate of blood flow out of the heart into the vascular system.
The resistance to blood flow posed by the vascular system.
Since overall resistance to blood flow does not seem to affect the blood pressure, we can see that the cardiac output is reduced during dives matching the increase in peripheral vascular resistance.
How does the heart rate decrease in voluntary vs forced dives?
Heart rates decrease in graded manner as length of dive increases
In forced dives heart rate more of “on-off” than gradual.
Important distinction between these types of dives is graded vs stereotyped nature of heart rate response.
How does vasoconstriction happen in dives ?
Vasoconstriction occurs in important arteries in at least some diving species, constrict shut and deny body parts of blood.
Vasoconstriction is selective, but always free flow of blood to lungs, brain and myocardium
Heart rate and vasoconstriction seem to be integrated
During short dives vasoconstriction is low, O2 shared in all body parts. This means dive cannot be long, but the animal avoids stressors of anaerobic catabolism.
What happens with red blood cell count in diving mammals with large O2?
Species of diving mammals with large O2 stores typically have large amount of red blood cells in body while diving.
Allows to store a lot of O2 but increases viscosity (tjockhet) of the blood -> heart must work harder.
Some seals store part of red blood cells in spleen while not diving -> lower viscosity -> heart does not have to work as hard when resting.
What happens with metabolization and lactic acid production during dives?
Tissue denied blood flow during forced protracted dives initially continue metabolize aerobically – using local O2 stores
After that, lactic acid starts to produce in the skeletal muscles but release of lactic acid from muscles only when animal resurfaces
Leads to concentration of lactic acid increase in blood circulation . Concentration from muscles and blood converge and fall together as the acid metabolizes.
Can take tens of minutes for lactic acid concentration to return to normal after diving.
Does mammals dive until their metabolic limits are reached?
No, Mammals choose to end dive before metabolic limits are reached. Protracted dives can be terminated by exhaustion of O2 in body parts that receive active circulation, by excessive accumulation of lactic acid in anaerobic tissue or by other metabolic limitations.
What are the three factors that determine limits of endurance of O2 dependent tissue that receive active circulation during dive?
Magnitude of O2 store available to those body parts.
Rate of use of O2 store
Extent to which the partial pressure of O2 can fall before impairing function.
How is brain and heart affected by the dive in mammals?
Myocardium (heart muscle) may start anaerobic glycolysis to some extent after several minutes of diving. Reduce hearts O2 needs -> longer time before O2 supplies become inadequate.
Brain believed to be aerobic even while animal is diving. Some seals seem to have their brain remain functional at lower O2 partial pressures than terrestrial mammals
What is end products?
Products that accumulate in diving mammals: CO2 and lactic acid. How the diver responds to build-up of these determine possible length of dive.
What is acidification?
Both CO2 and lactic acid tend to cause pH to decline. Diving species have particularly high blood buffering capacities to counteract this.
How is the stimulation of urge to breathe in diving mammals?
Diving species have reduced ventilatory sensitivity to changes in blood CO2 and pH than terrestrial species = blunted drive to breathe.
What are the behavioral consequences for accumulation of lactic acid during long dives?
Ridding the body of lactic acid requires a lot of time.
O2 is required for the process, meaning that a diving mammal typically must stay at the water's surface where it can breathe, or return often to the surface, while it metabolizes lactic acid.
If a diving mammal has a lactic acid burden to metabolize following a protracted(långvarig) dive, it cannot engage immediately in a second highly protracted dive. This is so because the lactic acid of a second dive simply adds to the preexisting lactic acid, and there is a cap on the total lactic acid that can be accumulated and tolerated
What is aerobic dive limit(ADL)?
The longest dive that can be undertaken without a net accumulation of lactic acid above the resting level. In adult Weddell seals, the ADL is 20–25 min.
Why does ADL matter?
A central hypothesis of modern diving physiology is that it is adaptive for diving mammals to keep most of their dives shorter than their species-specific ADL.
By keeping their dives fully aerobic, they avoid lengthy recovery times to metabolize lactic acid. Accordingly, they can minimize their time at the surface and maximize their time underwater.
How exactly do fully aerobic diving patterns translate into increased underwater time?
When a dive is fully aerobic, replenishing O2 stores is the only task an animal must carry out to recover. Vigorous breathing can replenish body O2 stores rapidly.
How might animals reduce their metabolic costs while diving?
Three different aspects of diving physiology:
Tissue cooling
When an diving animal let their body temperature decline during diving
Delay in food processing
Postpone of processing food until breathing air again
Limiting the costs of swimming
By gliding and other high-efficiency modes of locomotion
What is decompression sickness
A pathological state that arises after diving when bubbles are formed within body fluids because the reduction in pressure during surfacing allows gases (especially N2) present at high dissolved partial pressures to come out of solution
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