Circulatory systems

Circulatory systems are either open or closed.

• In arthropods and some mollusks, circulatory fluid bathes the organs directly in an open circulatory system. The circulatory fluid, called hemolymph, is also the interstitial fluid that bathes body cells.

• In closed circulatory systems the circulatory fluid called blood is confined to vessels and is distinct from interstitial fluid. These systems are found in annelids, many mollusks, and all vertebrates. One or more hearts pump blood through the vessels. Chemical exchange occurs between the blood and interstitial fluid and between interstitial fluid and body cells.

Circulatory system: a system that pumps blood through the body; can be open or closed

Open circulatory system: fluid bathes internal organs directly

Closed circulatory system: blood is confined to vessels

Hemolymph: the blood of arthropods and insects

Chemical exchange: occurs between the blood and interstitial fluid

Organization of the Vertebrate Circulatory System

• The closed circulatory system of vertebrates consists of blood, blood vessels, and a two-to-four-chambered heart.

• Blood pumped by a heart ventricle passes to arteries and then to the capillaries, sites of chemical exchange between blood and interstitial fluid. Veins return blood from capillaries to an atrium, which passes blood to a ventricle.

Vertebrate circulatory system: consists of blood, blood vessels, and a two-to-four-chambered heart; closed system

Blood from heart ventricle > arteries > capillaries > veins > ventricle

Circulation of sharks, rays and bony fish

• Sharks, rays, and bony fishes have single circulation with a two-chambered heart.

• In single circulation, blood leaving the heart passes through two capillary beds before returning. As the animal swims, contraction and relaxation of muscles help accelerate the pace of circulation.

• Meanwhile, in vertebrates, blood pumped by a heart ventricle passes to arteries and then to the capillaries, sites of chemical exchange between blood and interstitial fluid.

Single circulation: blood leaving the heart passes through two capillary beds

Double circulation and amphibians

• Amphibians, reptiles, and mammals have double circulation.

• It is called a double circulatory system because blood passes through the heart twice per circuit. The right pump sends deoxygenated blood to the lungs where it becomes oxygenated and returns back to the heart. The left pump sends the newly oxygenated blood around the body.

Gas exchange: takes place in the lungs in reptiles and mammals (pulmonary circuit) and both the lungs and skin in amphibians (pulmocutaneous circuit).

Double circulatory system: blood passes through the heart twice

Steps of the Cardiac Cycle

The heart contracts and relaxes in a rhythmic cycle. When the heart contracts, it pumps blood; when it relaxes, its chambers fill with blood. One complete sequence of pumping and filling is called the cardiac cycle.

• The contraction phase of the cardiac cycle is called systole. The relaxation phase is called diastole.

Cardiac cycle: one complete sequence of pumping and filling blood

Systole: contraction phase of cardiac cycle

Diastole: relaxation phase of cardiac cycle

In brief: Contracts (systole) > pumps blood > relaxes (diastole) > chambers fill with blood

How the Lymphatic and Circulatory systems interact

Together, the blood, heart, and blood vessels form the circulatory system.

• The lymphatic system (lymph, lymph nodes and lymph vessels) supports the circulatory system by draining excess fluids and proteins from tissues back into the bloodstream, thereby preventing tissue swelling.

Lymphatic system: lymph, lymph nodes, and lymph vessels

Strategies animals use for respiration

Breathing mechanisms vary substantially among vertebrates.

• An amphibian ventilates its lungs by positive pressure breathing, which forces air down the trachea.

• Birds use a system of air sacs as bellows to keep air flowing through the lungs in one direction only.

• Mammals ventilate their lungs by negative pressure breathing, which pulls air into the lungs. Lung volume increases when the rib muscles and diaphragm contract.

In brief: Positive pressure breathing (air forced down the trachea), negative pressure breathing (air pulled into lungs) air sacs (air flows through lungs in one direction)

Respiratory adaptations of diving mammals

Diving mammals have evolutionary adaptations that allow them to perform amazing feats. Deep-diving air breathers can store large amounts of O2. Oxygen can be stored in their muscles in myoglobin proteins.

Diving mammals also conserve oxygen by:

1. Swimming with little muscular effort and gliding passively

2. Directing most blood to vital organs rather than muscles

3. Deriving ATP in muscles from fermentation once oxygen is depleted.

In brief: Store O2 (in myoglobin proteins), swimming with little muscular effort (gliding), directing blood to vital organs (not muscles), deriving ATP from fermentation.