Circulatory System

All animals must maintain a homeostatic balance in their bodies.

 

this need requires that nutrients, metabolic wastes, and respiratory gases be circulate through the animal's body.

 

Any system of moving fluids that reduce the functional diffusion distance that nutrients, wastes, and gases must traverse may be referred to as an internal transport or circulatory system

 

Sponges circulate water form the external environment through their bodies, instead of circulating an internal fluid.

 

Cnidarians, such as Hydra, have a fluid filled internal gastrovascular cavity.

 

this cavity supplies nutrients for all body sells lining the cavity, obtains oxygen from the water in the cavity, and releases carbon dioxide and other wastes into it.

 

simple body movement causes the fluid to move.

 

The gastrovascular cavity of flatworm, such as the planarian is more complex than that of Hydra.

 

In the planarian, branches penetrate to all parts of the body.

 

Diffusion distances for nutrients, gases, and wastes are not great.

 

Body movement helps distribute materials to various parts of the body.

 

However, one disadvantage of this system is that it limits these animals to relatively small sizes or to shapes that maintain low diffusion distances.

 

Pseudocoelomate invertebrates, us the coelomic fluid of their body cavity for transport.

 

Most of these animals are small, and adequate transport is accomplished by the movements of the body against the coelomic fluids, which are in direct contact with the internal tissues and organs.

 

Beginning with the mollusks, transport functions occur as separate circulatory systems.

 

A circulatory or cardiovascular system is a specialized system that move s the fluid medium called either hemolymph or blood in a specific direction determined by the presence of unidirectional blood vessels.

 

Movement is accomplished by a muscular pumping heart.

 

Some animals (echinoderms, annelids,) use coelomic fluid as a supplementary or sole circulatory system.

 

Coelomic fluid may be either identical in composition to interstitia l fluids, or differ particularly with respect to specific proteins and cells.

 

Coelomic fluid functions in the transport of gases, nutrients, and waste products.

 

it also may function in certain invertebrates as a hydrostatic skeleton.

 

Hemolymph is the circulating fluid of animals with an open circulatory system.

 

Generally, the hemolymph volume is high and the circulation slow.

 

In the process of movement, essential gases, nutrients, and wastes are transported.

 

many times hemolymph has noncirculatory functions.

 

For example, in insects, hemolymph pressure assists in molting of the old cuticle and inflation of the wings.

 

The coelomic fluid, hemolymph, or blood of most animals contains circulating cells called blood cells or hemocyte.

 

Some cells contain a respiratory pigment such as hemoglobin and are called erythrocytes or red blood cells.

 

These cells are usually present in high numbers to facilitate oxygen transport.

 

other cells to not contain respiratory pigments and have other functions such as blood clotting.

 

The number and types of blood cells vary dramatically in different invertebrates.

 

All vertebrates have a closed circulatory system.

 

Blood generally is conducted from the heart, through arteries, arterioles, capillaries, venules, veins and back to the heart.

 

It is only at the capillary level that exchange occurs between the blood and extracellular fluid.

 

overall, vertebrate blood transports oxygen, carbon dioxide, and nutrients; defends against harmful microorganisms, cells, and viruses; prevents blood loss through coagulation and helps regulate body temperature and pH.

 

As do other connective tissues, blood contains an fluid matrix called plasma and cellular elements called formed elements.

 

plasma is the straw colored liquid part of blood.

 

In mammals plasma is about 90% water and provides the solvent for dissolving and transporting nutrients.

 

A group of proteins albumins, fibrinogen, and globulins, comprise another 7% of the plasma.

 

It is the concentration of these plasma proteins that influences the distribution of water between the blood and extracellular fluid.

 

Albumin represents about 60% of the total plasma proteins, it plays important roles with respect to water movement.

 

Fibrinogen is necessary for blood coagulation and the globulins transport lipids and fat soluble vitamins.

 

Serum is plasma from which the proteins involved in blood clotting have been removed.

 

The gamma globulin portion functions in the immune response because it consists mostly of antibodies.

 

The remaining 3% of plasma is composed of electrolytes, amino acids, glucose and other nutrients, various enzymes, hormones, metabolic wastes, and traces of many inorganic and organic molecules.

 

The formed element fraction consists of erythrocytes, leukocytes, and platelets.

 

White blood cells are divided into agranulocytes and granulocytes.

 

Red blood cells vary dramatically in size, shape, and number in the different vertebrates.

 

The red blood cells of most vertebrates are nucleated, but mammalian red blood cells are enucleated.

 

Almost the entire mass of RBC consist of hemoglobin, an iron containing protein.

 

the major function of an erythrocyte is to pick up oxygen from the environment, bind it to hemoglobin to form oxyhemoglobin, and transport it to body tissues.

 

Blood rich in oxyhemoglobin is bright red; as oxygen diffuses into the tissue, blood becomes darker and appears blue when observed through the blood vessel walls.

 

However, when this deoxygenated blood is exposed to oxygen it instantaneously turns bright red.

 

hemoglobin also carries waste carbon dioxide in the form of carbaminohemoglobin from the tissues to the lungs or gills for removal from the body.

 

White blood cells serve as scavengers that destroy microorganisms at infection sites, remove foreign chemicals, and remove debris that results from dead or injured cells.

 

All WBC are derived form immature cells in bone marrow by a process called hematopoiesis.

 

Among the granulocytes, eosinophils are phagocytic, and ingest foreign proteins and immune complexes rather than bacteria.

 

Basophils are the least numerous WBC.

 

When they react with a foreign substance, their granules release histamine and heparin.

 

Histamine causes blood vessels to dilate and leak fluid at a site of inflammation, and heparin prevents blood clotting.

 

Neutrophils are the most numerous of the white blood cells.

 

They are chemically attracted to sites of inflammation and are active phagocyte.

 

The two types of agranulocytes are the monocytes and lymphocytes.

 

Two distinct types of lymphocytes are recognized: B cells and T cells, both of which are central to the immune response.

 

B cells originate in the bone marrow, and colonize the lymphoid tissue, where they mature.

 

In contrast, T cells are associated with and influenced y the thymus gland before they colonize lymphoid tissue and play their role in the immune response.

 

When B cells are activated, they divide and differentiate to produce plasma cells.

 

Platelets or thrombocytes are disk shaped cell fragments that function to initiate blood clotting.

 

When a blood vessel is injured, platelets immediately move to the site and begin to clump together, attaching themselves to the damaged area, and begin the process of blood coagulation.

 

The fluid pressure that is generated by the ventricles contracting forces blood through the pulmonary and systemic circuits; the fluid pressure is called blood pressure.

 

More specifically, blood pressure is the force exerted by the blood against the inner walls of blood vessels.

 

Arterial blood pressure rises and falls in a pattern corresponding to the phases of the cardiac cycle.

 

When the ventricles contract their walls force the blood in them into the pulmonary arteries and the aorta.

 

As a result, the pressure in these arteries rises sharply.

 

The maximum pressure achieved during ventricular contraction is called the systolic [pressure.

 

When the ventricles relax the arterial pressure drops, and the lowest pressure that remains in the arteries before the next ventricular contraction is called the diastolic pressure.

 

The vertebrate lymphatic system begins with very small closed vessels called lymphatic capillaries, which are in direct contact with the extracellular fluid surrounding tissues.

 

The system has four major functions.

 

1. to collect and drain most f the fluid that seeps from the bloodstream and accumulates in the extracellular fluid;

 

2. to return small amounts of proteins that have left the cells;

 

3. to transport lipids that have been absorbed from the small intestine;

 

4. to transport foreign particles and cellular debris to disposal centers called lymph nodes.

 

The small lymphatic capillaries merge to form larger lymphatic vessels called lymphatics.

 

Lymphatics are thin walled vessels with valves that ensure the one way flow of lymph.

 

Lymph is the extracellular fluid that accumulates in the lymph vessels.

 

These vessels pass through the lymph nodes on their way back to the heart.