Circulatory and Excretory Systems in Vertebrates
Circulatory Systems in Vertebrates
Simple Circulation
In fish, the circulatory system is simple. They have a heart with a curved shape that receives venous blood from the body. This heart consists of one atrium and a very muscular ventricle. Blood leaves the heart through arteries towards the afferent branchial arteries.
Double Circulation
In pulmonate vertebrates, the heart works as a double pump system with two circuits:
- Pulmonary Circuit: Blood leaves the heart through the pulmonary arteries and goes to the lungs, where it is oxygenated.
- Systemic Circuit: Oxygenated blood leaves the heart through the aorta, is distributed throughout the body, and returns to the heart through the vena cava.
In amphibians, the heart has two atria divided by a septum. In reptiles, the mixing of blood in the heart is reduced due to an outline of an interventricular septum. Double circulation is complete when there is no mixing of oxygenated and deoxygenated blood.
Regulation of Blood Circulation in Humans
Regulation of Heartbeat
The medulla oblongata is the center that receives heart rate information from both outside and inside the body.
Regulation of Circulation in the Vessels
Regulation occurs by increasing or decreasing the diameter of blood vessels. The vasomotor center, located in the medulla oblongata, controls the diameter of the vessels.
Structure of the Kidney
- Renal Capsule: The outer layer, a fibrous membrane.
- Cortical Zone: Has a granular appearance due to the presence of Malpighian corpuscles.
- Medullary Zone: Has a striated appearance and is divided into renal pyramids by renal columns.
- Renal Pelvis: The area that collects urine.
Physiology of the Nephron in Mammals
Each nephron consists of:
- Malpighian Corpuscle
- Proximal Convoluted Tubule: Leaves the corpuscle.
- Loop of Henle: Penetrates the medulla and consists of a descending and an ascending limb.
- Distal Convoluted Tubule: Returns to the cortex and flows into a collecting duct.
Collecting ducts terminate in the renal pelvis, a part of each ureter that carries urine to the bladder.
Formation of Urine
Urine formation involves three processes:
- Filtration: Blood circulating to the glomeruli is filtered by capillaries inside the capsule due to blood pressure. This filtering is not selective.
- Reabsorption: Initial urine contains large amounts of water. Only 1% is eliminated, and the rest is reabsorbed during its path through the tubules.
- Secretion: The passage of substances from the blood to the glomerular filtrate.
Sweat
Sweat glands are located in the skin, especially concentrated in certain parts of the body. Each sweat gland consists of a thin tube called a glomerulus that draws a liquid from capillary blood. Sweat is somewhat similar to urine.
Regulating the Concentration of Urine
Changes in the internal environment are offset by the elimination of more or less concentrated urine. The concentration of urine depends on the permeability of the collecting tubules, which is regulated by vasopressin or antidiuretic hormone. Increased blood osmotic pressure stimulates the secretion of vasopressin.
Osmoregulation
- Freshwater fish: Live in a hypotonic environment. Water continuously penetrates inside by osmosis through the gills. Kidneys reabsorb salts but very little water.
- Saltwater fish: Susceptible to constant water loss by osmosis. Bony fish reabsorb most of the water in their kidneys and produce a small amount of very concentrated urine. Cartilaginous fish have transformed their internal environment to be nearly isotonic, conserving water.
- Terrestrial vertebrates: Need to eliminate water and nitrogenous waste products. Reptiles and birds excrete uric acid, and the tubular reabsorption process is very intense, minimizing water removal. Mammals are ureotelic and produce hypertonic urine.