Circulatory System General Aspects In small invertebrate organisms

Circulatory System – General Aspects • In small invertebrate organisms, no circulatory system is necessary, as diffusion distances are short. • With greater size, diffusion from the external environment becomes insufficient and a circulatory system is required to transport materials within the body.

Circulatory System – General Aspects • Two types of circulatory systems are present among invertebrates: – Open System = more primitive, heart pumps blood out through vessels to various parts of the body. Return vessels are absent so blood oozes back to heart via interstitial spaces. – Closed System = advanced condition, also present in all vertebrates; blood never leaves vessels. Delivery of nutrients occurs via diffusion across thin-walled capillaries. • Amphioxus shows intermediate condition = both outgoing and return vessels present, but no capillaries connecting them.

Fig 12. 4 – Microcirculation in a closed vascular system

Functions of Circulatory System • • • Facilitate gas exchange Delivery of nutrients Removal of wastes Transport of hormones 3 Subcomponents (all mesodermal in origin): – Blood Vascular System = includes heart, arteries (carry blood away from heart), capillaries (gas and nutrient exchange), and veins (carry blood back to heart) – Hemopoietic Organs = sites of blood cell production – Lymphatic System = thin-walled vessels that carry fluid from interstitial spaces back to blood stream

Fig 12. 2 – Blood pressure and crosssectional areas of the different vessels of the blood vascular system.

Fig 12. 11 – Basic vertebrate blood vascular system circulation, as illustrated in a shark

Single and Double Circulation • Fishes = possess single circulation pattern – Blood passes through heart only once during each complete cycle – Sequence: Heart Gills Body Heart • Amniotes = possess double circulation pattern – Blood passes through heart twice during each complete cycle – Sequence: Heart Lungs Heart Body Heart • Intermediate condition with characteristics of both present in lungfish, amphibians and some reptiles

Fig 12. 6 – Single and double circulation circuits in vertebrates

Developmental Anatomy • General Principles: • Main vessels develop in association with major centers for metabolic activity. • Primitive adult conditions are demonstrated in transitory embryonic stages in advanced vertebrates. – “Ontogeny recapitulates phylogeny” works well for the circulatory system.

Developmental Anatomy • Developmental Stages 1. Primitive Condition = heart pumps blood anteriorly through ventral aorta passes through paired aortic arches into paired dorsal aortae to capillary network of yolk sac drain to vessels leading back to heart. — Yolk sac is site of formation of first blood vessels and first blood cells in the embryo. 2. Systemic Arteries branch off dorsal aortae to supply developing organs and gut. — Each organ develops a capillary bed that merges into systemic veins leading back to heart. 3. Yolk Sac is supplied and drained by systemic veins (Portal System) in Osteichthyes and Amphibians. Yolk sac supplied via Vitelline Arteries and drained by vitelline veins in Chondrichthyes and Amniotes.

Developmental Anatomy • Developmental Stages 4. As the Liver develops, it is supplied by veins leaving the yolk sac (= Hepatic Portal System) which pass through the liver capillary network then coalesce into veins passing back to the heart. 5. In Amniotes the Allantois (=extraembryonic membrane functioning in waste storage) develops and is supplied by allantoic artery and drained by allantoic vein. – Allantoic vein initially passes directly to back of heart, but later becomes incorporated into capillary bed of liver. Some anatomical bypass of liver capillary network in birds and reptiles, but not in mammals. – Associated with nutritional role of chorioallantoic membrane of the placenta in mammals. Functions solely in respiration in birds and reptiles.

Developmental Anatomy • Accessory Circuits • Embryonic stages of all vertebrates except most primitive (cyclostomes) possess a Renal Portal System supplying capillary beds of kidney. – Renal Portal System also present in all adult vertebrates, except for cyclostomes and mammals, although in birds and reptiles much of the portal supply bypasses the kidney capillary network. • Aortic Arches shift from gill circulation positioned on main aortic trunk (gill-breathers) to side circuit associated with other structures (lung-breathers).

Blood • A specialized connective tissue in which the intercellular material is entirely fluid (plasma) • Plasma components include … – Stable components = inorganic salts and blood proteins (e. g. , albumin = increases osmotic pressure of blood, fibrinogen = clot formation, globulins = immunological role). – Fluctuating components = glucose, fats, amino acids, waste products (e. g. , urea, uric acid), hormones

Blood Cells • No blood cells present in Amphioxus – Restricted to low energy lifestyle • Two general categories in all vertebrates: 1. Erythrocytes (RBCs) = involved in oxygen and carbon dioxide transport — Hemoglobin serves as respiratory pigment to dramatically increase O 2 carrying capacity of blood 2. Leukocytes (WBCs) = many actions – immunological, phagocytic, etc. — Most of these roles accomplished outside of circulatory system — Use circulatory system for transport to sites of action

Leukocytes (WBCs) • Unlike RBCs, leukocytes contain nucleus and all other subcellular organelles • Have capacity for ameboid movement, many of WBC functions are carried out after movement across vessel wall = diapedesis

Other Blood Cell Types • Platelets = disc-shaped cell fragments derived from megakaryocytes in bone marrow – Function in clotting – Found only in mammals • Thrombocytes = spindle-shaped cells developmentally related to WBCs – Function in clotting – Found only in non-mammalian vertebrates

Human Blood Smear Anucleate erythrocyte Neutrophil (a WBC) Platelet

Frog Blood Smear Nucleated erythrocyte Thrombocyte WBC

Hemopoietic (blood-forming) Tissues • Formation of blood cells continues throughout lifetime (from stem cell precursors) to replace dying blood cells. • Embryonic Blood-forming Sites: – Blood Islands in yolk sac in large-yolked eggs – Chorion of placenta in mammals • Becomes more localized later in embryonic development: – kidney (sharks to reptiles and birds) – liver – spleen – thymus (not in hagfish ? ) – diffuse sites in intestinal walls

Hemopoietic (blood-forming) Tissues • Adult Blood-forming Sites – Spleen (except for mammals and birds) – Thymus (T-lymphocytes) – Kidney (lampreys, elasmobranchs, teleosts) – Gonads (elasmobranchs leukocytes) – Liver (teleosts, amphibians, turtles) – Intestinal Walls lymphoid elements (or walls in other regions of digestive tract – esophagus, pharynx, appendix) – Bone Marrow (amphibians [frogs] – mammals) – Bursa of Fabricius lymphoid elements (birds) = dorsal pouch off of cloaca (combined urinaryreproductive-anal opening)

Hemopoietic Tissues – Myeloid Tissue Arrows = adipose cells Bone Marrow Arrows = developing blood cells Embryonic Liver

Lymphatic System • Functions to return interstitial fluid to blood stream • Lymphatics not connected with arteries; have blind capillaries and vessels. • In Cyclostomes and Sharks there is no typical lymphatic system. – Instead possess an open system of sinusoids (low b. p. , so low interstitial fluid production in these groups). • Teleosts possess paired lymph hearts (muscular structures along lymphatics and at junctions with veins that help propel lymph along vessels) and 2 major sets of ducts: – Paired subvertebral ducts (deeper) – Paired lateral ducts – A similar condition occurs in Tetrapod embryos.

Lymphatic System • Amphibians with well-developed lymphatic system, many with numerous lymph hearts • Reptiles show complex system; reduction in the number of lymph hearts and development of valves in vessels to prevent backflow • Birds & Mammals generally have no lymph hearts in adults (although present in bird embryos and in a few adult birds). – Series of valves present to prevent backflow. – Movement of lymph in conjunction with skeletal muscle contraction.

Lymphatic System • Intestinal region lymphatics particularly well developed in advanced vertebrates. • Most of the fats entering circulation from the gut do so by way of the lymphatics (due to the large size of the fat molecules and the incomplete endothelial lining of lymphatic vessels, fatty lymph = chyle). • Lacteals = lymphatic vessels leaving intestine carrying chyle. • Thoracic Ducts = lymphatics running along back of body cavity, serve as collectors for lacteals. – Major duct in mammals (single, paired in many primitive vertebrates). • Cisternae chyli = area where many tributaries feed into thoracic duct (occurs in lumbar region)

Fig 12. 46 – Formation of lymph

Fig 12. 49 – Lymphatic system in a salamander. Note the prominence of the lymph hearts for moving lymph within the lymphatic system.

Fig 12. 48 – Lymphatic system in turtles

Figure 12. 47 Fig 12. 47 – Lymphatic circulation and lymph nodes

Valve in Lymph Vessel in mammal Adipose tissue Valve Flow Lymph vessel