Animal Circulatory Systems Chapter 44: pp. 944-1003 Types of circulatory systems: gastrovascular, open, closed. Vascular system: arteries, veins, capillaries. Capillary - tissue fluid exchange.

Absence of a Circulatory System Very small animals may not need a circulatory system. Small size may permit nutrients and other substances to reach all the body parts by simple diffusion

Larger Animals Without a Separate Circulatory System figure 49-01.jpg Larger Animals Without a Separate Circulatory System Cnidarian Gastrovascular Systems Some larger animals such as sea anemones, jellyfish, and flatworms lack a true circulatory system. The gastrovascular cavity extends to most areas of the body in these animals and serves as a circulatory system as well as a digestive cavity.

Flatworm Gastrovascular System

Circulatory Systems For larger or more active animals, some form of more efficient circulatory system is necessary for internal transport. Two types of circulatory system are found: Open Circulatory Systems Closed Circulatory Systems

Open Circulatory System Hemolymph leaves the heart in short, branched arteries that open up into large spaces called sinuses. Hemolymph percolates around organs, directly bathing the cells. Hemolymph then returns to the heart directly or through short veins.

Open Circulatory System Advantage - Exchange of materials is direct between the hemolymph and tissues. There is no diffusion barrier. Disadvantage - Little fine control over distribution of the hemolymph to body regions. No mechanism for reducing flow to a specific part of an organ.

Open Circulatory System Open circulatory systems tend to be found in more inactive animals. Most molluscs have an open system, but the highly active cephalopods (squid and octopus) have evolved a closed system. Insects have circumvented limitation of their open system by their tracheal system for oxygen supply.

Closed Circulatory System The blood is contained within a completely closed system of vessels. Vessels form a closed loop, usually with some sort of pumping organ like a heart or contractile vessels. Vessels branch into smaller and smaller tubes that penetrate among the cells of tissues.

Closed Circulatory System Advantages: Fine-scale control over the distribution of blood to different body regions is possible. Muscular walls of vessels can constrict and dilate to vary the amount of flow through specific vessels. Blood pressures are fairly high and the circulation can be vigorous.

Earthworm Circulation Extensive capillary beds: Body wall Gut wall Excretory tubules

Coelomic Cavities - Circulatory Function Coelomic cavities are filled with fluid that can transport materials around the body. Nematode worms have an extensive body cavity, the pseudocoel, but lack a separate circulatory system.

Ascaris Cross-Section Pseudocoel (fluid-filled space)

The Vertebrate Vascular System: Arteries, Veins, and Capillaries Arteries and arterioles have a layer of smooth muscle tissue which allows them to contract (vasoconstrict) and expand (vasodilate), altering their diameter and thus blood flow. Walls of arteries and arterioles have many elastic fibers enabling them to withstand high pressures.

Artery and Vein Artery Vein Note the much thinner walls in veins.

Blood Pressure and Flow Velocity

Capillaries Capillaries are very small, about the diameter of a red blood cell (8µm or less). Capillary walls are a single layer of very thin endothelial cells, attached at their edges and surrounded by a basement membrane (extracellular matrix). Endothelial cells

Vesicles; large, lipid-insoluble (proteins) Filtration; fluid and small, lipid-insoluble molecules (water, amino acids, NaCl, glucose, urea) Blood cells, most proteins. Diffusion; lipid-soluble molecules (O2, CO2, lipids)

Capillary - Tissue Fluid Exchange Blood hydrostatic pressure exceeds the opposing negative colloidal osmotic potential of the blood plasma. Water, containing small dissolved molecules, is forced out of the capillary through small pores in the capillary wall by the excess hydrostatic pressure.

Capillary Fluid Exchanges Blood pressure (hydrostatic) 32 mm Hg Plasma colloidal osmotic potential -22 mm Hg Net pressure 10 mm Hg

Capillary Fluid Exchanges Frictional Blood pressure (hydrostatic) 32 mm Hg resistance Blood pressure (hydrostatic) 15 mm Hg Plasma colloidal osmotic potential -22 mm Hg Plasma colloidal osmotic potential -22 mm Hg Net pressure 10 mm Hg Net pressure -7 mm Hg Less water re-enters the capillary than originally left at the arterial end.

The Lymphatic System The lymphatic system, returns excess tissue fluid to the blood.

Capillary - Tissue Fluid Exchange The bulk flow of fluid out of the capillary exchanges material much faster than would be possible by simple diffusion alone. Nutrients and O2 are released to the tissues rapidly. Wastes from cell metabolism are more rapidly cleared away by the circulatory system.

Control of Capillary Circulation Arteries and arterioles that feed blood to the capillaries contain a circular layer of smooth muscle in their walls. Contraction of these smooth muscles (vasoconstriction) is important in controlling the blood flow through capillary beds. Relaxation of smooth muscles results in vasodilation, an expansion of the vessel diameter that increases blood flow.

figure 49-18.jpg

Circulatory Patterns in Vertebrates The circulatory pattern has been modified during evolution of the major groups of vertebrates.

(and capillaries)

Cardiac cycle

Blood flow in veins One-way flow of blood (toward heart) is determined by valves.

Human blood components