Invertebrate Transport Systems

Transport in Invertebrates

• A circulatory system moves fluids between body parts.
• Small aquatic animals lack a circulatory system
  • Their cells are exposed to water
  • They can independently exchange gasses and eliminate wastes
• Sponges, cnidarians, and flatworms are examples with no circulatory system.
• Pseudocoelomates use coelomic fluid of their body cavity to transport substances
  • Nematodes are an example
• Coelomate echinoderms rely on movement of coelomic fluid within a body cavity as a circulatory system and use body fluids for locomotion.

Invertebrate Circulatory Fluids

• There are two main types:
  • Blood is contained within blood vessels
  • Hemolymph is a mix of blood and tissue fluid that fills the body cavity around internal organs
• Open circulatory systems evolved first
  • The heart pumps hemolymph via vessels
  • Vessels empty into tissue spaces
  • Hemolymph drains back into the heart
• Closed circulatory systems have capillaries close to body cells
  • Blood is pumped to capillaries
  • Gasses and materials then diffuse to and from nearby cells
  • Vessels return blood to the heart without blood/tissue contact

Transport in Vertebrates

• All vertebrates have a closed cardiovascular system

Vertebrate Heart

• The atria receive blood from general circulation
• The ventricles pump blood out through blood vessels.

Vertebrate Vessels

• Arteries carry blood away from the heart
• Arterioles are small arteries leading to capillaries
  • Nervous and endocrine systems regulate their diameter
• Capillaries exchange materials with tissue fluid (interstitial)
• Venules join to form a vein
• Veins return blood to the heart
  • Both venules and veins collect blood from capillary beds

Circulatory Pathways

• Fish exhibit a single-loop blood flow
  • They have a single atrium and ventricle
• Amphibians exhibit a double loop
  • This includes both systemic and pulmonary circuits
  • They have two atria with a single ventricle
• Most reptiles have a septum partially dividing the ventricle
  • Mixing of oxygen-rich and oxygen-poor blood is minimized, completely separated in crocodilians
• Birds and mammals exhibit double-loop flow (two circuits)
  • The heart is divided by a septum into separate sides
  • Right ventricle pumps blood to the lungs and the left ventricle pumps it to the rest of the body
  • Blood pressure is adequate for pulmonary and systemic circuits

Human Cardiovascular System

• The heart is fist-sized and cone-shaped, and is located between the lungs behind the sternum (breastbone)
• It is a muscular organ of cardiac fibers within a membranous sac called the pericardium
• Valves control flow through the heart

Heart Structure

• The septum separates the left and right sides of the heart.
• Each side has two chambers
  • Upper chambers are the atria with thin walls that receive blood from circulation.
  • Lower chambers are the ventricles, which are thick-walled, and pump blood away from the heart

Valves

• Atrioventricular valves:
  • The tricuspid valve sits between the right atrium and ventricle
  • The bicuspid valve sits between the left atrium and ventricle
• Semilunar valves:
  • The pulmonary semilunar valve sits between the right ventricle and pulmonary trunk
  • The aortic semilunar valve sits between the left ventricle and aorta

Blood Flow

• Blood returning from systemic circuit flows through the venue cavae to the right atrium
• The right atrium pumps blood through the tricuspid valve to the right ventricle, and it passes to the pulmonary circuit via the pulmonary semilunar valve
• Blood from the pulmonary circuit enters the left atrium
• The blood then passes to the left ventricle via the bicuspid valve, then moves to the systemic circuit from the aortic semilunar valve
• Oxygen-poor and oxygen-rich blood don't mix in humans
• Blood must pass through the lungs to move from the right to the left side of the heart.

Heartbeat

• Systole is the contraction of heart chambers.
• Diastole is the relaxation of heart chambers.
• The cardiac cycle is a two-part pumping action that takes about a second.
  • Blood collects in the atria, which then contract, pushing it through the tricuspid and mitral valves into the resting lower ventricles
  • This diastole phase is the longer of the two
  • After the ventricles fill (systole) they contract
  • Ventricles then relax following the moving of blood to the pulmonary artery and aorta

Pulse and Conduction System

• The pulse refers to the wave effect caused by the aorta expanding as the ventricles undergo systole
• Rhythmic contraction of atria and ventricles relies on the internal conduction system
  • The sinoatrial (SA) node maintains a regular heartbeat, serves as the pacemaker
  • The atrioventricular (AV) node signals ventricles to contract
• The brain can increase or decrease heart contraction rate/strength and the adrenal glands secrete epinephrine and norepinephrine to stimulate the heart

Electrocardiogram (ECG)

• An ECG records electrical changes in the myocardium during the cardiac cycle
• When the SA node triggers an impulse, the atrial fibers produce an electrical charge (P wave).
• The P wave indicates that the atria are about to contract
• The QRS complex signals that the ventricles are about to contract, and the atria are relaxing
• The T wave is due to electrical changes as the ventricular muscle fibers recover

Circulatory Circuits

• The human cardiovascular system has two major circular pathways
  • Pulmonary Circuit: transports -poor blood to the lungs and returns -rich blood to the heart
  • Systemic Circuit: transports -rich blood to the body, returning -poor blood to the heart through the venae cavae
• In a portal system, blood from capillaries goes through veins to another set of capillaries without traveling directly via the heart
  • Hepatic portal system takes blood from intestines directly to the liver

Blood Pressure

• Contraction of the heart supplies pressure that keeps blood moving in the arteries
• Systolic pressure results from blood forced into the arteries during ventricular systole
• Diastolic pressure is the pressure in the arteries during ventricular diastole
• It is normally measured with a sphygmomanometer on the brachial artery on the upper arm
• It is expressed in the form systolic "over" diastolic, measured in millimeters (mm) of mercury

Mechanics of Blood Pressure

• In arteries, the pressure of the blood forces it to move forward.

• Blood pressure falls as blood flows from the aorta into arteries and arterioles.

• Blood flow in the capillaries is slow.

• Blood pressure in the veins is too low to move blood back to the heart.

  • Skeletal muscle contraction pushes blood in the veins toward the heart
  • Veins have valves that prevent backward flow of blood
  • However, varicose veins develop when valves become ineffective
  • A respiratory pump reduces pressure in the thoracic cavity, causing blood to move from the abdominal cavity (higher pressure) into the thoracic cavity (lower pressure) during each inhalation

Velocity and Blood Pressure

• Blood is under minimal pressure and has the least velocity in capillaries
• Blood pressure and velocity drop off due to greater total cross-sectional area in capillaries compared to arterioles

Cardiovascular Disease (CVD)

• CVD is the leading cause of death in most Western countries
  • Hypertension (high blood pressure) has afflicted 30% of people in America
  • Atherosclerosis is caused by narrowing of arteries due to accumulation of fatty materials, and a clot called a thrombus may form on an arterial wall
  • Stroke can disrupt the blood supply to the brain and occurs when a cranial arteriole bursts or is blocked by an embolus
  • Angina pectoris leads to squeezing from myocardial oxygen insufficiency from partial blockage of a coronary artery
  • Heart attack (myocardial infarction)is when a coronary artery becomes completely blocked
    • The vessel can be re-opened using stents (self-expanding wire mesh tubes). If stents are unsuccessful, a coronary bypass may be required in which a surgeon replaces it with a healthy artery from elsewhere in the body

Blood

• Blood transports gasses, nutrients, waste, antibodies, and hormones, combats pathogenic microorganisms, maintains water balance and pH, regulates body temperature and carry platelets and factors that ensure clotting to prevent blood loss.

Red Blood Cells (RBCs)

• RBCs are small biconcave disks without a nucleus that contain hemoglobin.
  • Hemoglobin contains four globin protein chains and is associated with heme, an iron-containing group
  • It is manufactured in bone marrow of the skull, ribs, vertebrae, and long bone ends
• Insufficient RBC number or hemoglobin leads to anemia.

Blood types

• They are determined by the presence or absence of a surface antigen
  • This includes the ABO and Rh systems
• Antibodies in the plasma cause agglutination (cross-reaction) when antigens meet antibodies
  • This can lead to organ damage

White Blood Cells (WBCs)

• WBCs are, for the most part, larger than red blood cells
• They contain a nucleus, lack hemoglobin, and are important in inflammatory responses
• Five main types exist including:
  • Granular leukocytes (neutrophils, eosinophils, and basophils): contain granules composed of proteins and enzymes used to help defend the body against invading organisms
  • Agranular leukocytes (monocytes and lymphocytes): monocytes migrating to tissues to affect chronic and ongoing infections. T cells and B cells are lymphocytes which trigger immune responses and form antibodies

Platelets

• Platelets result from fragmentation of megakaryocytes in red bone marrow
• They are non-cellular, formed elements present at 150,000 to 300,000 per cubic millimeter of blood
• They are involved in blood clotting along with a blood clot that consists of platelets, red blood cells, and fibrin threads
• Clotting is triggered by Thrombin that is activated by prothrombin enzymes that converts fibrinogen to fibrin
• Clotting is ended by Plasmin that destroys the fibrin network

Capillary Exchange

• Capillaries are have RBCs traveling through them in single file since they are both very narrow
• Fluid passes through a capillary wall is controlled by osmotic pressure and blood pressure
  • Since walls are thin, thus facilitate diffusion of nutrients, gases, and wastes
• Water exits near the arterial end and enters near the venous end
• Solutes diffuse based on concentration gradient:
  • Oxygen and nutrients diffuse out of capillaries.
  • Carbon dioxide and wastes diffuse into the capillary

Capillary Exchange and Lymph

• Substances leaving capillaries contribute to interstitial fluid
• Excess interstitial fluid is collected by lymphatic capillaries and becomes lymph
• Lymph is returned to systemic venous blood when the major lymphatic vessels enter the subclavian veins in the shoulder region