circulation

Chapter 42a: Circulation

Overview of Circulation

• Every organism must exchange substances with its environment.

  • Exchanges occur at the cellular level through the plasma membrane.

  • Unicellular organisms: direct exchanges with the environment.

  • Multicellular organisms: direct exchange is not feasible; relies on circulatory systems.

Specialized Exchange Systems

• Gills: a specialized system for gas exchange in animals.

  • Oxygen (O₂) diffuses from water into blood vessels.

  • Carbon dioxide (CO₂) diffuses from blood into water.

• Internal transport and gas exchange are functionally related in most animals

• Small molecules move via diffusion, efficient over short distances:

  • Diffusion time is proportional to the square of distance.

• Some animals, many or all cells are in direct contact with the environment

• In most animal, cells exchange materials with environment via fluid filled circulatory system

Gastrovascular Cavities

• Certain animals lack a circulatory system.

  • Example: Cnidarians (e.g., moon jelly) have gastrovascular cavities for digestion and substance distribution.

  • Body walls are typically two cells thick.

  • Flatworms also possess gastrovascular cavities to minimize diffusion distances.

Circulatory System Components

• A circulatory system consists of:

  • A circulatory fluid (blood or hemolymph).

  • Interconnecting vessels.

  • Muscular pump (heart).

• Function: connects surrounding fluid with organs for gas exchange, nutrient absorption, and waste disposal.

Types of Circulatory Systems

Open Circulatory Systems

• Found in insects, arthropods, and some mollusks.

  • Circulatory fluid (hemolymph) bathes organs directly.

Closed Circulatory Systems

• Found in annelids, cephalopods, and vertebrates.

  • Blood is confined to vessels; distinct from interstitial fluid.

Organization of Vertebrate Circulatory Systems

• Vertebrates, including humans, possess a closed cardiovascular system.

  • Main blood vessels: Arteries, veins, and capillaries.

• Blood flow is unidirectional:

• Starts at heart, then capillaries then back to heart

  • Arteries branch into arterioles, (carrying blood away from heart) leading to capillary beds for chemical exchange.

  • Venules converge into veins returning blood from capillaries to the heart.

• Arteries and veins are categorized by blood flow direction, not O₂ content.

• Vertebrate heart contains two or more chambers

• Blood enters through an atria and Is pumped out through ventricles

Types of Circulation

Single Circulation

• Present in bony fishes, rays, and sharks:

  • Two-chambered heart (atrium and ventricle).

  • Blood passes through two capillary beds before returning.

  • Blood goes from atrium, ventricle, artery to gill capillaries, goes down to body capillaries then goes up to the heart by the vein back to the heart

Double Circulation

• Found in amphibians, reptiles, and mammals:

  • Oxygen-poor and oxygen-rich blood are pumped separately from two sides of the heart.

  • Reptiles and mammals send oxygen-poor blood to lungs (pulmonary circuit) for oxygenation.

  • Amphibians also use the skin for gas exchange (pulmocutaneous circuit).

• Maintenance of higher blood pressure in organs compared to single circulation.

Evolutionary Variation in Circulation

• Some vertebrates have adapted to intermittent breathing:

  • Amphibians and some reptiles can sustain long periods without gas exchange. (By sometimes relying on gas exchange from other tissues usually the skin)

• Frogs: three-chambered heart (two atria, one ventricle).

  • A ridge in the ventricle diverts most of the oxygen rich blood into systemic circuit and most oxygen poor blood into pulmocutaneous circuits.

  • When underwater, blood flow to lungs is shut off

• Turtles, snakes, lizards: also three-chambered, with partially divided by incomplete septum.

• Crocodilians: have a septum dividing ventricles but have connections to pulmonary and systemic circuits where arteries exit the heart

• Mammals/Birds: four-chambered heart; left side manages oxygen-rich blood, right side handles oxygen-poor blood:

  • Endothermic, necessitating higher oxygen demand than ectothermic animals.

Mammalian Circulation

• When Right ventricle contracts blood gets pumped to lungs via pulmonary arteries.

• Blood flows through capillary beds in the left and right lungs where O2 gets loaded and CO2 gets unloaded

• Oxygen-rich blood returns to the left atrium from the lungs via pulmonary veins, then enters the left ventricle to be pumped out to body tissues via systemic circuit.

• Blood leaves left ventricle via aorta, which conveys blood to arteries leading throughout the body

• First branches are the coronary arteries, supplying the heart muscle

• Further branches lead to capillary beds in the abdominal organs and hind limbs

• O2 diffuses from blood to tissues, and CO2 diffuses from tissues to blood

• Capillaries rejoin, forming venues, conveying blood to veins

• Oxygen poor blood from head, neck, and forelimbs is channeled into superior vena cava

• Inferior venae cavae drains blood from the trunk and hind limbs

• The two venue cave empty their blood into the right atrium from which the oxygen poor blood flows into the right ventricle

The Human Heart

• The heart size: approximately that of a clenched fist; composed mainly of cardiac muscle.

• The 2 Atria: thin walls, serve as collection chambers for blood returning to heart

• Ventricles: thicker walls, responsible for stronger contractions. (Especially left ventricle)

• Heart contacts and relaxes in a rhythmic cycle called Cardiac cycle:

  • Systole, pumping (contraction phase).

  • Diastole, filling (relaxation phase).

• Cardiac output: volume of blood pumped per minute; influenced by heart rate and stroke volume.

• Heart rate number of beats per minute

• Stroke volume is the amount of blood pumped ina. Single contraction

Valves Functionality

• 4 valves Ensure unidirectional blood flow, preventing backflow( which can cause murmur):

  • Atrioventricular (AV) valves separate atrium and ventricles.

  • Semilunar valves regulate blood flow to aorta and pulmonary arteries.

Heart Sounds and Rhythmicity

• Heartbeat sound:

  • "Lub-dupp" sound:

  • Lub: recoil of blood against AV valves during ventricular contraction.

  • Dup: recoil against semilunar valves during ventricular relaxation.

• Some Cardiac muscle cells are autorhythmic; can contract independently of nervous system input.

• Sinoatrial (SA) node: acts as pacemaker, regulating contraction rate. (Electrically charged event)

• Impulse that travel during cardiac cycle can be recorded through EKG

• Impulse progression through heart:

  • SA node -> AV node (conduction delay) -> Purkinje fibers (ventricular contraction).

Regulation of Heartbeat

• Pacemaker regulated by:

  • Sympathetic division: speeds up rate.

  • Parasympathetic division: slows down rate.

  • Hormonal and temperature factors also influence contraction rhythm.

Patterns of blood pressure and flow

• Vertebrate circulatory system relies on blood vessels and exhibits a close match of structure and function

Blood vessels structure and function

• All blood vessels constrain a central lumen lined with an epithelial layer that lines blood vessels

• Endothelium is mouth and minimizes resistance

• Capillaries are only slightly wider that a red blood cell

• Capillaries have thin walls, the endothelium plus its basal lamina, facilitate the exchange of materials

• Arteries and veins have an endothelium, smooth muscle, and connective tissue

• Arteries have thick wall, elastic walls to accommodate the high pressure blood pumped from heart

• Thinner walled veins, blood flows back to the heart mainly as a result of muscle action

• Veins actually contain valves to ensure unidirectional blood flow

Blood flow velocity

• Physical laws governing movement of fluids through pipes affect blood flow and blood pressure

• Velocity of blood flow is slowest in capillary beds, result of high resistance and large total cross sectional area

• Blood flow in capillaries is slow for exchange of materials

Blood pressure

• Blood flows from areas of higher pressure to areas of lower pressure

• Blood pressure is force exerted in all directions, including against the walls of blood vessels

• Recoil of elastic arterial walls plays a role in maintaining blood pressure

Change in blood pressure during the cardiac cycle

• Systolic pressure is the pressure in the arteries during ventricular systole; highest pressure in arteries

• Pulse is rhythmic bulging of artery walls

• Diastolic pressure is pressure in the arteries during diastole; it is lower than systolic pressure

Regulation of blood pressure

• Homeostatic mechanisms regulate arterial blood pressure by altering the diameter of arterioles

• Vasoconstriction. Is the contraction of smooth muscle in arteriole walls; increase blood pressure

• Vasodialation is the relaxation of smooth muscles in the arterioles, causes blood pressure to fall

• Nitric oxide (NO) major inducer of vasodilation

• Peptide endothelium, potent inducer of vasoconstriction

Blood pressure and gravity

• Blood pressure is generally measured for an artery in the arm at the same height as heart

• Healthy blood pressure is 120 mmhg at systole and 70 mm hg at diastole

• Gravity has a great effect on blood pressure

  • Fainting caused by inadequate blood flow to the head

  • Animals with long necks require very high systolic pressure to pump blood a great distance against gravity

Capillary function

• Blood flows through only 5-10% of body’s capillaries at a given time

• Capillaries in major organs are usually filled to capacity

• Blood supply varies in many other sites

• 2 mechanisms regulate distribution of blood in capillary beds

  • Constriction or dilation of arterioles that supply capillary beds

  • Precapillary sphincters control flow of blood between arterioles and venules

  • Blood flow is regulated by nerve impulses and hormones

  • Exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries

  • Blood pressure tends to drive fluid out of capillaries and blood proteins tend to pull fluid back

  • These proteins are responsible for much of the bloods osmotic pressure

  • There is a net loss of fluid from capillaries

Fluid return by the lymphatic system

• Lymphatic system returns fluid that leaks out from the capillary beds

• Fluid lost by capillaries called lymph

• Lymphatic system drains into veins in the neck

• Valves in lymph vessels prevent the backflow of fluid

• Edema: swelling caused by disruptions in the flow of lymph

• Lymph nodes; organs that filter lymph and play an important role in the body’s defense (Lymph nodes become swollen and tender when fighting infection)

• Helymph: mix of blood & intersituent fluids (in insects)

Blood components function in exchange, transport, and defense

• With open circulation, the fluid is continuous with the fluid surrounding all body cells

• Closed circulatory system of vertebrates contain more highly specialized fluid called blood

• Blood in vertebrates is a connective tissue consisting of several kinds of cells suspended in a liquid matrix called plasma

Plasma

• Contains inorganic salts as dissolved ions (electrolytes)

• Influence blood PH and help maintain osmotic balance

• 2 types of cells in blood plasma

  • Red blood cells : erythrocytes

  • White blood cells: leukocytes

• Platelets: help with clotting

  • Formed blood clot within vessel called thrombus