Circulatory System notes 1-4

Annelids

• Annelids have open and closed circulatory systems
• Tube worms have an open circulatory system
• Earthworms have a closed circulatory system
• Earthworms use peristalsis (muscle contractions) to move blood through vessels

Molluscs

• Molluscs have open and closed circulatory systems
• All molluscs have hearts and some blood vessels
• Most molluscs have open circulatory systems
• Only cephalopods (squids, octopuses, and cuttlefish) have closed circulatory systems

Arthropods - Crustaceans

• Crustaceans have open circulatory systems
• Circulatory systems become more complex in larger animals
• Small sinuses function as vessels in crustaceans
• Some crustaceans have control over the distribution of hemolymph flow in the body

Arthropods - Insects

• Insects have a relatively simple open circulatory system
• Insects have multiple, contractile "hearts" along the dorsal vessel
• Insects use a tracheal system for the majority of gas transport

Vertebrates

• All vertebrates have a closed circulatory system
• The heart contracts to increase pressure in the chambers
• Blood flows away from the heart in arteries
• Arteries branch into smaller arteries, which branch into arterioles
• Blood flows from arterioles into capillaries
• Capillaries are where diffusion of molecules between blood and interstitial fluid occurs
• Capillaries coalesce to form venules
• Venules coalesce to form veins
• Veins carry blood to the heart

Vertebrate Blood Vessels

• A complex wall surrounds a central lumen within blood vessels
• The wall is composed of up to three layers: tunica intima, tunica media, and tunica externa
• The thickness of the wall varies among vessels
• Veins have one-way valves and arteries do not

Capillaries

• Capillaries lack tunica media and tunica externa
• There are three types of capillaries: continuous, fenestrated, and sinusoidal
• Continuous capillaries have cells held together by tight junctions, found in skin and muscle, and the CNS (blood brain barrier)
• Fenestrated capillaries have cells that contain pores, specialized for exchange, found in kidneys, endocrine organs, and the intestine
• Sinusoidal capillaries have few tight junctions which makes them the most porous type for exchange of large proteins, found in liver and bone marrow

Circulatory Patterns of Vertebrates

• All vertebrates have a closed circulatory system
• Water-breathing fish have a single circuit
• Air-breathing tetrapods (amphibians, reptiles, birds, and mammals) have two circuits: pulmonary and systemic
• The pulmonary circuit is on the right side of the heart and pumps blood to the lungs
• The systemic circuit is on the left side of the heart and pumps blood to the body
• Water-breathing fish have a systolic pressure of 30-45 mm Hg, resulting in lower metabolic rates and oxygen consumption, resulting in lower blood pressure
• Mammals have a systolic pressure of 120-180 mm Hg, resulting in higher metabolic rates and oxygen consumption, resulting in higher blood pressure

Amphibians and Reptiles

• Amphibians and reptiles have hearts that are only partially divided
• Amphibians and reptiles have two atria and one ventricle
• Amphibians have a three-chambered heart (frog), and lizards have a five-chambered heart
• Blood from both atria flow into the ventricle in amphibians and reptiles
• Oxygenated and deoxygenated blood can mix in amphibians and reptiles, except for crocodiles which have a four-chambered heart
• Oxygenated and deoxygenated blood are kept fairly separate by a mechanism that is not completely understood in amphibians and reptiles
• The ventricle pumps blood into the pulmonary and systemic circuits in amphibians and reptiles
• Blood can be diverted between the pulmonary and systemic circuits in amphibians and reptiles

Summary

• The circulatory system is composed of three components: a pump, tubes, and fluid
• There are two types of circulatory systems: open and closed
• Blood contains three main components: plasma, red blood cells, and white blood cells
• There are three types of blood vessels: arteries, veins, and capillaries
• There are two circuits in the circulatory system: pulmonary and systemic

Arthropod Heart

• The arthropod heart pumps hemolymph out via arteries.
• Hemolymph returns to the heart through ostia (holes) during diastole.
• Valves in the ostia regulate the flow of hemolymph.
• Ligaments suspend the heart.
• The heart is neurogenic, meaning it contracts in response to signals from the nervous system.

Cardiac Cycle in Arthropods

• Neurons in the cardiac ganglion undergo spontaneous rhythmic depolarization.
• Cardiomyocytes contract, decreasing the heart's volume and increasing pressure.
• Valves in the ostia close as pressure increases, forcing hemolymph out via arteries.
• Stretched ligaments pull apart the heart walls, increasing volume and decreasing pressure.
• Valves in the ostia open, allowing hemolymph to be sucked into the heart.

Vertebrate Hearts

• Vertebrate hearts have complex walls with different parts:
  • Pericardium: A sac of connective tissue surrounding the heart.
    • It has outer (parietal) and inner (visceral) layers, with lubricating fluid between them.
  • Epicardium: The inner layer of the pericardium, continuous with connective tissue on the heart.
    • Contains nerves regulating the heart and coronary arteries.
  • Myocardium: The layer of heart muscle cells (cardiomyocytes).
  • Endocardium: The innermost layer of connective tissue covered by endothelial cells (endothelium).

Myocardium

• Myocardium exists in two forms:
  • Compact: Tightly packed cells arranged in a regular pattern.
  • Spongy: A meshwork of loosely connected cells.
• The relative proportions of compact and spongy myocardium vary between species.
  • Mammals, birds, and reptiles have mostly compact myocardium.
  • Fish and amphibians have mostly spongy myocardium, arranged as trabeculae that extend into the chambers.

Fish Hearts

• Fish hearts have two chambers and two other compartments arranged in series:
  • Sinus venosus
  • Atrium
  • Ventricle
  • Bulbus arteriosus (non-contractile)

Cardiac Cycle in Fish

• The atrium and ventricle contract serially.
• Valves are passive, opening and closing due to pressure differences.
• The non-contractile bulbus arteriosus acts as a volume and pressure reservoir.

Amphibian Hearts

• Amphibian hearts have three chambers:
  • Two atria
  • One ventricle
• Trabeculae in the ventricle help prevent mixing of oxygenated and deoxygenated blood.
• A spiral fold in the conus arteriosus directs deoxygenated blood to the pulmocutaneous circuit and oxygenated blood to the systemic circuit.

Non-Crocodilian Reptile Hearts

• Non-crocodilian reptiles have five-chambered hearts with two atria and three interconnected ventricular compartments:
  • Cavum venosum: Leads to systemic aortas.
  • Cavum pulmonale: Leads to the pulmonary artery.
  • Cavum arteriosum.

Shunting in Reptile Hearts

• Reptiles can shunt blood to bypass either the pulmonary or systemic circuits.
  • Right-to-left shunt: Deoxygenated blood bypasses the pulmonary circuit and enters the systemic circuit, occurring during breath-holding.
  • Left-to-right shunt: Oxygenated blood reenters the pulmonary circuit, aiding oxygen delivery to the myocardium.

Crocodilian Reptile Hearts

• Crocodilian reptiles have four-chambered hearts with two atria and two ventricles, with complete separation of the chambers.

Birds and Mammals

• Birds and mammals have four chambers:
  • Two atria: Thin-walled.
  • Two ventricles: Thick-walled, with the left ventricle thicker than the right.
  • The ventricles are separated by the intraventricular septum.

Valves in Bird and Mammalian Hearts

• Atrioventricular (AV) valves: Located between the atria and ventricles.
  • Tricuspid valve: On the right side.
  • Bicuspid or mitral valve: On the left side.
• Semilunar valves: Located between the ventricles and arteries.
  • Aortic valve: Between the left ventricle and aorta.
  • Pulmonary valve: Between the right ventricle and pulmonary artery.

Cardiac Cycle

• The cardiac cycle is the pumping action of the heart, with two phases:
  • Systole: Contraction, forcing blood into or out of a chamber.
  • Diastole: Relaxation, allowing blood to enter the heart.

Mammalian Cardiac Cycle

• The atria and ventricles alternate systole and diastole.
  • The two atria contract simultaneously, followed by a slight pause.
  • The two ventricles contract simultaneously.
  • Atria and ventricles relax while the heart fills with blood.

Ventricular Pressure

• The left ventricle contracts more forcefully, developing higher pressure.
• The resistance in the pulmonary circuit is low due to high capillary density, resulting in lower pressure required to pump blood through it.
• This low pressure protects the delicate blood vessels of the lungs.

Mean Arterial Pressure (MAP)

• The average arterial pressure in the aorta over time.
  • MAP = 2/3 diastolic pressure + 1/3 systolic pressure

Blood Velocity

• Flow (Q): The volume of fluid transferred per unit time.
• Velocity: The distance traveled per unit time.
• Blood velocity = Q/A (A = cross-sectional area of the vessels).
• Velocity is inversely proportional to total cross-sectional area.
  • The large cross-sectional area of capillaries leads to slow velocity, allowing for more diffusion.

Vasoconstriction and Flow Resistance

• The Law of Bulk Flow: Q = ΔP/R (Q = flow, ΔP = pressure drop, R = resistance).
• Poiseuille's equation: Q = ΔPπr⁴ / (8Lη) (L = length of tube, η = viscosity, r = radius).
• Small changes in radius significantly impact resistance and flow.

Factors Enhancing Diffusion Across Capillaries

• Fick's Law of Diffusion: Diffusion Rate = ΔC * A * D / ΔX (ΔC = concentration gradient, A = surface area, D = diffusion coefficient, ΔX = distance).
• Capillaries facilitate diffusion due to:
  • Thin capillary walls.
  • The capillary diameter being slightly larger than a RBC.
  • Slow blood velocity.

Increasing Heart Rate

• Norepinephrine (from sympathetic neurons) and epinephrine (from the adrenal medulla) increase heart rate.
• They open more pacemaker (funny), Ca²⁺, and Kv channels, increasing the rate of depolarization, repolarization, and action potential frequency.

Decreasing Heart Rate

• Acetylcholine (from parasympathetic neurons in the vagus nerve) decreases heart rate.
• It activates GIRK (Kir3) K⁺ channels, slowing the heart rate.
• It also inhibits cAMP and PKA activity, reducing If current and Cav/Kv channel activity.

Summary

• The text explores the structure of the heart, focusing on the differences between arthropod, fish, amphibian, reptile, bird, and mammal hearts.
• The cardiac cycle is detailed for each type, explaining how blood flow is directed and regulated.
• The text discusses blood pressure, the factors influencing it, and how it is affected by vasoconstriction and vasodilation.
• The importance of capillaries in facilitating diffusion is highlighted.
• Finally, the text explains how the nervous system regulates heart rate through the sympathetic and parasympathetic nervous systems.

Animal Physiology Midterm 2 prep