Animal Physiology 2. Blood, circulation and gas exchange

All blood vessels have similar architecture, consisting of concentric layers of cells, fibrous tissue and smooth muscle.

KW: circulation; blood vessel; endothelium; tunica intima; tunica media; tunica adventitia; collagen; elastin; smooth muscle; tight junction; gap junction;

Blood vessel diameter is actively regulated through sensing of shear forces by endothelial cells.

KW: circulation; endothelial cells; shear stress; shear rate; nitric oxide; endothelin; vasodilation; vasoconstriction;

Blood vessels can remodel, growing larger or shrinking through linking of blood vessel development with shear. This, combined with angiogenesis to growing organs, produces the circulatory system’s basic architecture.

KW: circulation; endothelial cells; shear stress; angiogenesis; fibroblast; angioblast; vascular endothelial growth factor; VEGF; reticulum;

Vessel networks conform to Murray’s Law because shear stresses are constant throughout such a network. This connects shear regulation by endothelial cells with the emergence of large-scale vessel network design.

KW: circulation; shear stresses; shear regulation; Murray’s law; dimensional analysis;

Some general principles for how hearts work, using the fish heart as an example.

KW: circulation; fish heart; atrium; ventricle; sinus venosus; bulbus arteriosus; valves; regurgitation; pressure; diastole; systole; cardiac cycle;

The heart is an intermittent pump that has to supply a steady flow of blood to the tissues. The elastic properties of the main arteries leading from the heart help do this.

KW: circulation; ventricle; intermittent; duty cycle; elastic storage; bulbus arteriosus; pressure; energy balance; work; conservation of energy;

The heart is made up of cardiac muscle, one of three distinctive muscle types within the body. Cardiac muscle as a number of important properties, including the ability to contract rhythmically.

KW: circulation; cardiac muscle; smooth muscle; skeletal muscle; striated muscle; pacemaker cell; contractile cell; pacemaker potential; action potential; auto rhythmic; heart rate;

The heart has a special a set of cells that coordinate the heartbeat within and between the chambers. These are pacemaker cells, and there are specific rules that govern how they interact and how they coordinate the heartbeat.

KW: circulation; pacemaker cell; gap junction; functional syncytium; autonomic nervous system; sympathetic nervous system; parasympathetic nervous system; adrenergic; cholinergic; autonomic tone;

Starling’s law of the heart is one of the fundamental mechanisms that match the venous return with cardiac output. Failure of the heart to obey Starling’s law is at the heart of many pathological conditions of the heart, including congestive failure. Understanding Starlings law ultimately boils down to understanding the mechanical properties of of contracting cardiac muscle fibers.

KW: circulation; Starling’s law of the heart; venous return; cardiac output; stroke volume; and diastolic volume; striated muscle; sliding filament theory; actin; myosin; length tension relationship;

Starling’s law of the heart is an empirical observation of the relationship between stroke volume and end diastolic volume. To understand Starling’s Law, it must be recast in a new form that makes energetic sense. When we do so, we see that Starling’s law is derivable from the sliding filament theory of muscle contraction.

KW: circulation; Starling’s law of the heart; venous return; cardiac output; stroke volume; and diastolic volume; striated muscle; sliding filament theory; actin; myosin; length tension relationship;

Return of the blood to the heart takes place without the pressure the heart imparts to the blood in the arterial system. Venous return is driven by numerous external sources that impart small amounts of potential energy to the venous blood.

KW: circulation; venous return; skeletal muscular pump; elasticity; blood storage; pericardium; elastic recoil; gravity;

Bernoulli’s principle is a restatement of the principle of conservation of energy as applied to fluids. This principle is important in analyzing the low-pressure flows in the veins.

KW: circulation; venous return; Bernoulli’s principle; gravitational potential; pressure; energy density; kinetic energy; conservation of energy;

Variations of posture impart considerable variation in the gravitational potential that impedes venous blood returning to the heart. The skeletal muscular pump is very important in helping move blood against this gravity potential.

KW: circulation; venous return; posture; skeletal muscular pump; gravity potential; undulate; hoof; snake;

The action of breathing can impart transient low pressures that can help move blood toward the heart. This is known as the thoracic suction pump. Locomotion also imparts differentials of momentum of blood with six of the body that can aid in venous return.

KW: circulation; venous return; thoracic suction pump; inspiration; exhalation; locomotion; momentum;

The distribution of blood flow in the body has to be controlled to match varying demands of the different tissues with the ability to supply nutrients and oxygen through blood flow. Control of the distribution of blood flow is centered around the regulation of the systemic arterial pressure. This is controlled through a combination of demand driven control by the cells and tissues, and top-down control by the autonomic nervous system.

KW: circulation; total hydraulic resistance; organs; tissues; brain; viscera; muscles; skin; demand driven control; top-down control; cardiac output; systemic arterial pressure;

Demand driven control, or bottom-up control, matches the varying levels of demand for oxygen and nutrients by local cells and tissues with local delivery of blood flow. This is known as metabolic autoregulation. This comes about through paracrine control by the cells of the capillary sphincters that control blood flow through capillary beds. This is centered around the ability to sense and regulate local oxygen concentration.

KW: circulation; capillary bed; arteriole; venule; precapillary sphincter; brain; body dysmorphic disorder; nitric oxide; metabolic autoregulation; selfish regulation; oxygen concentration; oxygen delivery rate; oxygen consumption rate;

Top-down control of the distribution of blood flow operates through the autonomic nervous system, and is mediated by the ability to sense arterial pressure through baroreceptors located in the aorta and carotid sinuses. These pressure sensors send information about arterial pressure to a neural circuit in the medulla oblongata, which mediates the level of autonomic tone.

KW: circulation; baroreceptors; aortic arch baroreceptor; carotid sinus baroreceptor; medulla oblongata; nucleus of the solitary tract; caudoventral lateral medull; rostroventrolateral medulla; nucleus ambiguous; intermediolateral nucleus of the spinal cord; autonomic nervous system; sympathetic tone; parasympathetic tone; systemic arterial pressure; arterial manifold; baroreceptor reflex;

Regulation of the systemic arterial pressure operates through a so-called baroreceptor reflex arc. The normal operation of the baroreceptor reflex arc is illustrated with three examples: sinus arrhythmia, in which heart rate varies slightly in synchrony with the breathing cycle; variations of posture, which affect venous return to the heart, and therefore cardiac output; and the response to mild hemorrhage.

KW: circulation; baroreceptor reflex; sympathetic tone; parasympathetic tone; respiratory sinus arrhythmia; posture; venous return to the heart; hemorrhage; total body water; interstitial water; intracellular water; systemic arterial pressure; plasma volume;

Circulatory shock is a dangerous and potentially fatal syndrome that often arises with a large reduction of plasma volume, as can occur suddenly in severe hemorrhage, or gradually in severe dehydration. Circulatory shock is not a pathology, but the results of a perfectly functioning regulatory system for systemic arterial pressure that is pushed beyond its normal regulatory range. This has interesting implications for understanding the evolution of physiological function.

KW: circulation; circulatory shock; coronary arteries; cardiac work; coronary blood supply; compensatory shock; progressive shock; dehydration; diarrhea; cholera; total body water; interstitial water; intracellular water; venous return to the heart;

The evolution of the heart has to be understood in the context of the evolution of the vertebrates themselves. Here, we outline the basic evolutionary history of the vertebrates from their origin in the Precambrian to the present.

KW: circulation; evolution; vertebrate evolution; Agnatha; Chondrichthyes; Actinopterygii; labyrinthodonts; Therapsida; Archosauria; crocodilians; birds; dinosaurs; amniotic egg; pharyngeal lung; alveolar lung; pneumatic lung;

The evolution of the vertebrate circulatory system starts with one pump for fusing gills and systemic circulation in series, and culminates in hearts that are two pumps for fusing pulmonary and systemic circulations, also in series. In between these two endpoints were several stages in which the lungs were perfused in parallel with the systemic circulation.

KW: circulation; fish circulatory system; mammalian circulatory system; amphibian circulatory system; series circulation; parallel circulation; amphibian heart; pulmonary artery; pulmocutaneous artery;

The hearts of adult amphibians represent the most primitive instance of a perfused long. In these creatures, the lungs and systemic circulation are perfused in parallel from a single ventricle. On the face of it, this would seem to be an inferior design, because it allows mixing of oxygenated and the oxygenated blood in the single ventricle. A proper understanding of the frog heart, however, shows it to be a remarkably sophisticated design.

KW: circulation; amphibian heart; pulmonary artery; pulmocutaneous artery; cutaneous artery; parallel circulation; ventricle; right atrium; left atrium; arteriovenous shunt; lung breathing; skin breathing; conus arteriosus; spiral valve; momentum; viscosity; mixing;

Most reptiles also have a single undivided ventricle, and this also leads to the misconception that the reptilian heart is in an efficient design. However, the reptilian heart accomplishes a considerable degree of separation of the pulmonary and systemic circulation through a combination of low mixing in the single ventricle, and complicated dynamic changes in shape that make the supposedly three chambered heart actually a five-chambered heart.

KW: circulation; squamate heart; lizard; snake; turtle; horizontal septum; mid-ventricular ridge; cavum venosum; cavum arteriosum; cavum pulmonale; systemic arch; pulmonary artery; five chambered heart; viscosity; momentum; mixing; arteriovenous shunt;

Among the reptiles, the crocodilians stand out as having a completely divided four chambered heart. Despite this, crocodilians are capable of arteriovenous shunt similar to those in amphibians and squamate reptiles that have single ventricles.

KW: circulation; crocodilian heart; divided ventricle; Foramen of Panizza; arteriovenous shunt; systemic arches; Archosaurs; dinosaurs; birds; Therapsida;

Even though mammals have a completely divided ventricle, and complete separation of the pulmonary and systemic circuits, during fetal life, the heart is capable of arteriovenous shunts through the atrium, and between the pulmonary artery and aorta. This pattern must change abruptly at birth.

KW: circulation; mammalian heart; fetal heart; arteriovenous shunt; foramen ovale; ductus arteriosus; birth; left atrial pressure; right atrial pressure; pulmonary circulation; pulmonary resistance; birth; placenta; umbilicus;

The evolution of the vertebrate heart culminates in the pattern of circulation seen in the mammals and birds, that is a complete separation of pulmonary and systemic circulations, and the evolution of an alternate heart to perfuse them. These are convergent endpoints of two entirely separate evolutionary lineages of the vertebrates. What holds them in common is a high-energy endothermic metabolic lifestyle.

KW: circulation; evolution; pulmonary pressure; systemic pressure; pressure overlap; parallel circulation; series circulation; mammal; bird; endothermy; low metabolic rate; high metabolic rate; convergence;

The lungs of vertebrates originate as extensions of the pharyngeal region. Lungs take a variety of forms among the vertebrates, ranging from the sack like lungs of amphibians and lungfish to the high exchange capacity lungs of mammals and birds. Lung design of mammals is radically different from lung design in birds.

KW: respiration; lungs; pharynx; lungfish; amphibians; mammals; birds; ventilation; alveolar lung; fractal lung; pneumatic lung; surfactant; pneumocyte; air sac; pneumatization; branching morphogenesis;

Lungs must be ventilated, like any gas exchange organ. The primitive lung of amphibians is ventilated by positive pressure, through mechanism similar to ventilation of gills. The advanced lungs of mammals, in contrast, are ventilated by suction pressure. This was made possible by the evolution of a stiffening rib cage.

KW: Lung ventilation; buccal force pump; buccal cavity; diaphragm; ribs; amphibian; suction pump; crocodilian lung; glottis; nostril; rib cage; 4-link chain; intercostal muscle; diaphragmaticus;

The alveolar lungs of mammals are continuously exposed to a collapsing force due to surface tension at the interface between air and liquid in the alveolus. At equilibrium, these collapsing forces are held in check by negative pressures generated by solute transport in the pleural cavity. These collapsing forces also contribute to the work of breathing, because expansion of the lung requires an expansion of alveolar surface against surface tension. These collapsing forces are minimized by the secretion of surfactant by type II pneumocytes.

KW: surface tension; law of Laplace; surfactant; pleural space; solute transport; suction pressure; work of ventilation; type II pneumocyte; premature birth;

The lungs of birds are radically different in design from the lungs of mammals. The lungs are stiff, and are ventilated by a kind of bellows action involving the system of air sacs within the body. Airflow through the lung is unidirectional, and gas exchange occurs at numerous fine tubular extensions from the parabronchi known as air capillaries. The pattern of ventilation and perfusion in the bird lung makes it a so-called cross current gas exchanger.

KW: bird lung; avian lung; air sac; dorsobronchus; ventrobronchus; parabronchi; air capillaries; ventilation; perfusion; cross current gas exchanger; diffusion;

Efficiency of extraction of respiratory gasses from the ventilated fluid is an important part of gas exchanger design. All gas exchangers face structural and functional limitations on how efficient they can be. The lungs of mammals are limited in their efficiency by the extensive mixing between convection flux in the upper airways, and diffusion flux in the alveoli. The lungs of birds, with their flow-through ventilation, do not face this limitation. Comparing the relative efficiencies of gills, and the lungs of mammals and birds, we see that gills are the most efficient gas exchangers, and lungs are relatively inefficient.

KW: dead space; three phase gas exchange; convection; diffusion; mixing region; tidal ventilation; flow-through ventilation; uniform pool gas exchanger; cross current gas exchanger; countercurrent gas exchanger; efficiency;

Capacity for gas exchange is another important aspect of gas exchange design. Increasing exchange capacity is tantamount to increasing the surface area for gas exchange within the lung. Geometric and physical constraints on doing so force lung structure to develop from being a sac-like gas exchanger, as in amphibians, to being a complex fractal surface, as in the lungs of mammals. Over the evolutionary history of the vertebrates, we’ve seen a transition from high-efficiency gas exchangers, two gas exchangers that sacrifice efficiency for high-capacity.

KW: amphibian lung; mammalian lung; scaling; isometric scaling; fractal scaling; fractal geometry; exchange capacity; Fick’s law; dead space; surface area; volume; self similarity; branching morphogenesis;