Chapter 42

Invertebrate Circulation  Hydras, flatworms, and jellies have gastrovascular cavities (nutrients reach all cells via diffusion or simple cavity branches)  Insects have open circulatory systems where ‘blood’ is called hemolymph and is not distinguished from interstitial fluid. Heart pumps hemolymph through pockets near organs called sinuses, and returns to heart through pores called ostia.  Closed circulatory systems have blood that is kept separate from interstitial fluid and is pumped through increasingly small vessels to organs.

Vertebrate Circulation  Fish have a 2-chambered heart where oxygen is pulled from water (via gills) and goes through the systemic circuit (to organs) and then returns to the heart.  Amphibians have a 3- chambered heart where blood leaving and entering the heart sometimes mix. Double circulation – blood is pumped a 2 nd time after it moves through capillary beds.

 Reptiles (except birds) have a three- chambered heart with the ventricle partially separated by cardiac muscle. A right systemic aorta also brings mixed blood to the systemic circuit.  Mammals and birds (by convergent evolution) have 4- chambered hearts for the highest efficiency, where blood leaving and entering the heart never mixes.

More on the Mammalian Heart  The contraction phase of the cardiac cycle is called a systole, and a diastole is when the muscle relaxes.  Four valves (2 in ventricles, 2 in atria) keep blood from flowing backwards.  The sinoatrial node (pacemaker) is in the right atrium and coordinates the muscle contractions of a heartbeat (with help of intercalated discs in cardiac muscle tissue).

Blood Vessels  Veins transport blood back to the heart via smooth and skeletal muscle contraction, and sometimes have valves to prevent backflow.  Arteries take blood to the pulmocutaneous and systemic circuits, and have thicker walls to withstand changing pressure from the heart.  Capillaries have thin walls so oxygen can diffuse through them. Blood flow also slows here.

More on Capillaries  Blood flow slows because capillaries have more surface area than total velocity of the blood.  Precapillary sphincters are made of smooth muscle that contracts and squeezes off blood flow in the capillary bed, except for one thoroughfare channel.

Blood Cell Types and Composition ALL BLOOD CELL TYPES COME FROM PLURIPOTENT STEM CELLS IN BONE MARROW  Platelets – Cytoplasm pinched off from bone marrow, aid in blood clotting  Leukocytes (white blood cells) – Pathogen fighters, usually in interstitial fluid 5 types: basophils, eosinophils, neutrophils, monocytyes, lymphocytes  Enthrocytes (red blood cells) – biconcave shape to hold maximum oxygen No mitochondria or nucleus (use anaerobic respiration for ATP) Hemoglobin enzymes transport oxygen for nitric oxide  Plasma – Liquid matrix surrounding blood cells Includes buffers to retain pH and protein escorts

Gas Exchange  Amphibians use can use their skin to exchange gases. They also use positive pressure breathing, where air is pushed inside its body  Animals with gills ventilate (move water over them)  Terrestrial animals have moist inner folds (lungs with alveoli) and use negative pressure breathing, where air is pulled inside the body. Respiratory pigments – proteins that load and unload oxygen into the alveoli  Insects have a tracheal system – tubes on abdomen draw in air directly to cells

Here’s what they look like:  Gills (bottom right), lungs (bottom left), tracheal system (top).

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