CIRCULATION CHAPTER 23 OPEN AND CLOSED CIRCULATORY SYSTEMS • Among the unicellular protists, oxygen and nutrients are obtained directly by simple diffusion. • Cnidarians and flatworms have cells that are directly exposed to either the external environment or to a body cavity that functions in digestion, the gastrovascular cavity. Gastrovascular cavity Pharynx Mouth Planaria: gastrovascular cavity OPEN AND CLOSED CIRCULATORY SYSTEMS • Large animals have tissues that are several cell layers thick so that many cells are too far away for surface exchange. • Instead, oxygen and nutrients are transported from the environment and digestive cavity to the body cells by an internal fluid within a circulatory system. • There are two main types of circulatory systems: • Open circulatory system • Closed circulatory system OPEN AND CLOSED CIRCULATORY SYSTEMS • In open circulatory systems, there is no distinction between the circulating fluid (blood) and the extracellular fluid of the body tissues (interstitial fluid or lymph). • This fluid is called hemolymph. • Insects have a muscular tube that serves as a heart to pump the hemolymph through a network of open-ended channels. Tubular heart Insect: open circulation OPEN AND CLOSED CIRCULATORY SYSTEMS • In a closed circulatory system, the circulating fluid (blood) is always enclosed within blood vessels that transport blood away from and back to a heart. • Annelids and all vertebrates have a closed circulatory system. Dorsal blood vessel Lateral hearts Ventral blood vessel Earthworm: closed circulation OPEN AND CLOSED CIRCULATORY SYSTEMS • Arteries carry blood away from the heart, and veins return blood to the heart; blood passes from the arterial system to the venous system in capillaries. • The pressure of the blood forces some fluid out of the capillary walls. OPEN AND CLOSED CIRCULATORY SYSTEMS • The pressure of the blood forces some fluid out of the capillary walls. • This fluid is called interstitial fluid. • Some of it will return to the blood but some becomes lymph and travels through the lymph vessels. OPEN AND CLOSED CIRCULATORY SYSTEMS • The functions of the circulatory system can be divided into three areas: • Transportation • Substances essential for cellular functions are transported by the circulatory system. OPEN AND CLOSED CIRCULATORY SYSTEMS • Regulation • The cardiovascular system participates in temperature regulation, such as by countercurrent heat exchange. Core body temperature Warm blood 36º C Veins Artery 5º C Temperature of environment Capillary bed Artery Cold blood Veins OPEN AND CLOSED CIRCULATORY SYSTEMS • Protection • The circulatory system protects against injury and foreign microbes or toxins introduced into the body. ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • The vertebrate circulatory system (also known as the cardiovascular system) is made up of three elements. • Heart—a muscular pump that pushes blood through the body. • Blood vessels—a network of tubes through which the blood moves. • Blood—fluid that circulates through the vessels. ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Blood moves through the body in a cycle, from the heart, through a system of vessels. • Blood leaves the heart in arteries. • From the arteries, blood passes into smaller arterioles. • Tiny vessels called capillaries connect arterioles to venules, or small veins. • Venules and then veins carry blood back to the heart. Heart Arteries Veins Arterioles Venules Capillaries ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Although each capillary is very narrow, there are so many of them that the capillaries have the greatest total cross-sectional area of any other type of blood vessel. ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Capillary beds can be opened or closed based on the physiological needs of the tissues. • Precapillary sphincters can contract or relax and affect whether blood flows into a capillary bed for exchange of3 gases and metabolites. Precapillary sphincters open Through-flow channel Precapillary sphincters closed 1 2 Arteriole Capillaries (a) Blood flows through capillary network Venule (b) Blood flow in capillary network is limited ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • An artery is more than a simple pipe. • It needs to be able to expand with and be strong against the pressure caused by contraction of the heart. • For this reason, arteries have both elastic and smooth muscle layers. Connective tissue Smooth muscle Elastic layer Endothelial cells (a) Artery ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Arterioles differ from arteries in that they are smaller in diameter and respond to nervous and hormonal stimulation. • They can constrict or expand to affect blood flow during periods of stress or body activity. ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Capillaries are where O2 and food molecules are transferred from the blood to the body’s cells and waste CO2 is picked up. • Capillaries are narrow and have thin walls for exchange. • Almost all cells of the vertebrate body are no more than 100 micrometers from a capillary. • The blood pressure is actually far lower in the capillaries than in the arteries. Endothelium Endothelial cells ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Veins are vessels that return blood to the heart. • The walls of veins are thinner because the blood pressure is not great. Connective tissue Smooth muscle Elastic layer Endothelium (c) Vein ARCHITECTURE OF THE VERTEBRATE CIRCULATORY SYSTEM • Veins have unidirectional valves that prevent the flow of blood backwards. Blood flows toward heart Open valve Vein Contracting skeletal muscles Valve closed THE LYMPHATIC SYSTEM: RECOVERING LOST FLUID • The cardiovascular system is Lymph nodes very leaky. • From capillary exchange, the body loses about 4 liters of fluid each day. • To collect and recycle this fluid, the body uses a second circulatory system called the lymphatic system. • The lymphatic system is also a network of vessels filled with a fluid called lymph. • Ultimately the lymph reenters the bloodstream through veins in the neck . Thymus Spleen Lymphatic vesse LYMPHATIC CAPILLARIES RECLAIM FLUID FROM INTERSTITIAL FLUID Lymphatic capillary Excess interstitial fluid becomes lymph Osmosis due to plasma proteins causes net absorption Blood pressure causes net filtration Interstitial fluid Blood flow Capillary Arteriole Venule THE LYMPHATIC SYSTEM: RECOVERING LOST FLUID • The lymphatic system has 3 important functions: • It returns proteins to circulation. • If this protein remains in the tissues, it would cause swelling or edema. • It transports fats absorbed from the intestine. • It aids in the body’s defense. • Swellings along lymph vessels called lymph nodes and a lymph organ called the spleen are where bacteria and dead blood cells are destroyed. • The thymus produces white blood cells. BLOOD • Blood plasma is a complex solution of water with three kind of substances dissolved in it: • Metabolites and wastes • Glucose, vitamins, hormones, wastes. • Salts and ions • Sodium, chloride, and bicarbonate. • Proteins • Proteins help keep water in the plasma. • Serum albumin functions in maintaining osmotic balance. THREADS OF FIBRIN • Other proteins found in blood include: antibodies, globulins, and fibrinogen. • Fibrinogen (which converts into fibrin) is required for blood clotting. BLOOD • Nearly half the volume of blood is occupied by cells. • The three principal cell types are: • Erythrocytes (red blood cells) • The blood’s hematocrit is the fraction of the total volume of the blood that is occupied by red blood cells. • In humans, the hematocrit is usually about 45%. • Leukocytes (white blood cells) • Platelets (cell fragments) BLOOD • Erythrocytes resemble flat disks with a central depression on both sides. • Almost the entire interior is packed with hemoglobin, which carries oxygen. • Because these cells have no nucleus they are short-lived and must be replaced by new cells synthesized in the bone marrow. BLOOD • Leukocytes contain no hemoglobin and are essentially colorless. • There are several different kinds, all of which help defend the body against invading microorganisms and other foreign substances. BLOOD • Platelets are cell fragments, pinched from large cells in the bone marrow, called megakaryocytes, that play a key role in clotting. FISH CIRCULATION • The chordates that were ancestral to the vertebrates have simple tubular hearts. • The evolution of gills by fishes required a more efficient pump, a true chamber-pump heart. FISH CIRCULATION • The fish heart is essentially a tube with four chambers arrayed one after another. • The sinus venosus (SV) and atrium (A) are collecting chambers, and the ventricle (V) and conus arteriosus (CA) are pumping chambers. • The SV and CA chambers are reduced in higher vertebrates. • The chambers contract in a peristaltic sequence. • The blood that is pumped to the body is fully oxygenated because it passes through the gills first, but it has less pressure. Sinus venosus SV Atrium Ventricle A V Conus arteriosus CA (a) Systemic capillaries Respiratory capillaries Gills Body SV A V CA AMPHIBIAN AND REPTILE CIRCULATION • The advent of lungs involved a major change in the pattern of circulation. • After blood is pumped by the heart to the lungs, it does not go directly to the tissues of the body but instead returns to the heart. • Pulmonary circulation goes to and from the heart and lungs. • Systemic circulation goes to and from the heart and the rest of the body. AMPHIBIAN AND REPTILE CIRCULATION • The amphibian heart has structural features to prevent the mixing of deoxygenated blood from the body with oxygenated blood from the lungs. AMPHIBIAN AND REPTILE CIRCULATION • The atrium is divided by a septum that separates the blood coming from the body and from the lungs. • There is a single, common ventricle, but little mixing of blood occurs because • Some species of amphibians have folds in the ventricle that direct the flow of blood from the atria • The conus arteriosus is branched AMPHIBIAN AND REPTILE CIRCULATION • Amphibians in water supplement the oxygenation of their blood by obtaining additional oxygen by diffusion across their skin. • This is called cutaneous respiration. AMPHIBIAN AND REPTILE CIRCULATION • The reptilian heart is additionally specialized. • There is a partial septum in the ventricle. • The conus arteriosus has become incorporated into the large arteries leaving the heart. MAMMALIAN AND BIRD CIRCULATION • Mammals, birds, and crocodiles have a fourchambered heart with two complete pumping circuits. • This increased efficiency of the double circulation in mammals and birds may have been important in the evolution of endothermy. • More efficient circulation is necessary to support the high metabolic rate required. MAMMALIAN AND BIRD CIRCULATION • In the mammalian heart, • Oxygen-rich blood returns from the lungs through pulmonary veins to the left atrium of the heart and flows through the mitral valve into the left ventricle. • The thick-walled left ventricle contracts, sending oxygenated blood through a large artery called the aorta and out to the body. • Backflow of blood from the aorta is prevented by the aortic semilunar valve. Aorta Superior vena cava Aortic semilunar valve Pulmonary semilunar valve Right atrium Pulmonary artery Pulmonary veins Left atrium Bicuspid (mitral) valve Tricuspid valve Left ventricle Inferior vena cava Right ventricle MAMMALIAN AND BIRD CIRCULATION • Blood travels through the body and returns to the heart via the vena cavae, which drain into the right atrium. • Blood flows from the right atrium through the tricuspid valve to the right ventricle. • The right ventricle contracts, pushing blood through the pulmonary valve into pulmonary arteries that lead to the lungs. Aorta Superior vena cava Pulmonary artery Aortic semilunar valve Pulmonary veins Pulmonary semilunar valve Left atrium Right atrium Bicuspid (mitral) valve Tricuspid valve Left ventricle Inferior vena cava Right ventricle http://youtu.be/gn6QmETEm8s Carotid artery Jugular vein Superior vena cava Brachial artery Aorta Pulmonary artery Heart Hepatic portal system THE HEART AND CIRCULATION OF MAMMALS AND BIRDS Radial artery Inferior vena cava Aorta Femoral artery andvein Femoral artery Femoral vein Great saphenous vein MAMMALIAN AND BIRD CIRCULATION • The simplest way to monitor heartbeat is to listen using a stethoscope. – “Lub” is the sound made by the closing of the bicuspid and tricuspid valves at the start of ventricular contraction. – “Dub” is the sound made by the closing of the pulmonary and aortic valves at the end of ventricular contraction. • A heart murmur is heard due to turbulence created by the valves not closing fully. MAMMALIAN AND BIRD CIRCULATION • Another way to examine the events of the heartbeat is to monitor the blood pressure. – A device called a sphygmomanometer is used to measure the blood pressure in the brachial artery of the arm. – Diastolic pressure is the low pressure when the atria are filling. – Systolic pressure is the high pressure associated with the ventricles contracting. MEASURING BLOOD PRESSURE Blood 2 pressure gauge 1 100 50 150 100 200 0 3 50 250 150 200 0 250 100 50 150 200 0 250 Cuff Stethoscope Cuff pressure: 150 No sound: artery closed Cuff pressure: 120 Pulse sound: Systolic pressure Cuff pressure: 75 Sound stops: Diastolic pressure MAMMALIAN AND BIRD CIRCULATION • The contraction of the heart consists of a carefully orchestrated series of muscle contractions. • First the atria contract, followed by the ventricles. • The sinoatrial (SA) node is the pacemaker of the heart and determines the rhythm of the heart’s beating. MAMMALIAN AND BIRD CIRCULATION • Contraction of the atria is initiated by the SA node. • The wave of depolarization does not immediately spread to the ventricles because it must pass first through cardiac muscle called the atrioventricular (AV) node. • This delays the signal for about 0.1 sec until the atria have finished contracting. MAMMALIAN AND BIRD CIRCULATION • The ventricles finally contract after the signal passes from the AV node to an atrioventricular bundle of muscle called the bundle of His. • Bundle branches divide into fast-conducting Purkinje fibers which initiate the almost simultaneous contraction of the right and left ventricles. • The electrical activity of the heart can be measured by a recording called an electrocardiogram (ECG or EKG). HOW THE MAMMALIAN HEART CONTRACTS SA node AV node LA RA LV RV 1 2 3 4 Bundle of His Purkinje fibers QRS wave in ECG P wave in ECG R 1 sec R ECG P T QS QRS wave