Human Circulatory, Lymphatic, and Excretory Systems
The Internal Environment and Blood
Cells live in an environment that provides the nutrients they need. In the same environment, cells release waste substances resulting from their metabolism. Multicellular organisms have an internal environment formed by interstitial plasma: a liquid that fills the existing spaces between cells. This needs to be renewed continuously. Blood renews interstitial plasma given that it circulates around the whole organism, carrying nutrients and removing waste products. If the blood stops circulating, interstitial plasma isn’t renewed and the cells die.
The circulatory system connects the digestive, respiratory, and excretory systems.
- Blood provides cells with the nutrients they need.
- Blood removes waste products produced during cell metabolism.
- Tissue cells exchange substances with the interstitial plasma.
Blood Composition
Blood plasma: Water with minerals (90%), nutrients (glucose, amino acids, vitamins), waste products (urea), proteins, and hormones.
Blood cells:
- Erythrocytes/Red blood cells: Don’t have a nucleus, contain hemoglobin, a red pigment that carries oxygen from pulmonary alveoli to all the cells.
- Leukocytes/White blood cells: Have a nucleus. Granulocytes – Neutrophils, eosinophils, basophils. Monocytes, Lymphocytes.
- Thrombocytes/Platelets: Fragments of cells, substances that allow the blood to coagulate.
Functions of the Blood
- Transports nutrients and oxygen to cells and collects waste products produced by cell metabolism. It carries hormones.
- Regulation of body temperature.
- Fights against infections.
Blood Circulation
Multicellular organisms need blood to reach all of their organs. Blood needs a closed circuit.
Blood Vessels
Responsible for carrying blood around the body.
- Arteries: Carry blood from the heart to other organs. Have muscular walls that are elastic.
- Capillaries: Microscopic blood vessels in the organism’s tissues, formed by walls that have one layer of cells. Allows the entry and exit of molecules.
- Veins: Carry blood from tissues to the heart. They have valves, formed by many venules.
The Heart
Hollow muscular organ. Divided into left and right.
Subdivided into 2 chambers: atrium (upper smaller chamber) and ventricle (larger lower chamber).
Heartbeat
Constant movement. Pumps the blood from the veins into the atriums, goes into the ventricles, and is released into the arteries. Heart rate: Number of times the heart beats per unit of time.
- Atrial systole: The atriums contract and blood is propelled through the opened atrioventricular valves into the ventricles.
- Ventricular systole: Ventricles contract, the sigmoid valves open, blood exits through the pulmonary artery and the aorta, and valves that connect with ventricles close.
- Diastole: The heart relaxes and sucks blood coming from the cava and pulmonary veins into the atriums.
Blood Circuits
- Systematic circuit
- Pulmonary circuit
Circulation is:
- Double: blood has to travel twice through the heart in order to complete the whole circuit.
- Complete: oxygenated blood proceeding from the lungs never mixes with deoxygenated blood coming from the rest of the body.
- Closed: The blood, or the cells found in blood, never leaves the blood vessels.
The Lymphatic System
Is composed of lymph, lymphatic vessels, lymphatic organs, and lymph nodes.
- Lymph: Transparent liquid composed of lymphocytes and interstitial plasma.
- Lymphatic vessels: Made of beads and in their interior, they have half-moon-shaped valves. They end when they reach blood veins.
- Lymphatic organs and tissues: Produce and mature lymphocytes and purify blood.
- Lymph nodes: are made up of lymphatic tissue. They make lymphocytes and release lymph plasma.
Functions of the Lymphatic System
- It maintains a liquid balance in the internal environment.
- Protects the body.
- Collects products from fat digestion.
Excretory System
The excretory system is the group of organs that facilitate the elimination of waste produced by cells after cell metabolism.
- Carbon dioxide is formed during the metabolic reactions that produce energy.
- Urea and uric acid are formed when the body breaks down proteins and nucleic acids, respectively.
- Toxic substances: these have to be ingested. Examples of them are alcohol, drugs, and medicine.
It contributes to the adequate functioning of the body’s vital signs by regulating: the quantity of water in tissues, the concentration of minerals, the blood’s pH levels, etc. By doing this, it participates in the body’s homeostasis.
Liver: It eliminates products resulting from the destruction of hemoglobin found in old red blood cells. The liver also eliminates a small quantity of cholesterol and certain toxic substances that can affect the body.
Urinary system: This is the most important excretory system and eliminates waste products through the urine.
Respiratory system: It removes blood and releases it to the exterior.
Sweat glands: They produce sweat that, when evaporated, cools the skin when temperatures are high. Sweat also releases certain waste products to the exterior.
Kidneys
They are two organs that are behind the abdomen, one on each side of the spine. Each of them has an external section, which is called the renal cortex, an internal section known as the renal medulla, and a cavity, known as the renal pelvis. The renal pelvis collects urine, formed in the renal cortex and medulla, that contains waste products. Nephrons are the structural and functional units of the kidneys. They are responsible for filtering blood and forming urine, regulating water and mineral content in the blood.
Different parts:
- Bowman’s capsule: it’s the initial zone (spherical), inside is a capillary that folds into a ball called the glomerulus of Malpighi.
- Convoluted tube: it’s a long twisted tube that has two main parts: proximal and distal.
- Loop of Henle: this is the section located between the proximal convoluted tube and the distal one.
Nephrons end in collecting conduits that end in the renal pelvis leading to the ureter. In addition, kidneys perform other physiological functions: They actively intervene in the regulation of the internal environment. They secrete substances that stimulate the production of red blood cells (erythropoietin) and substances that regulate blood pressure (renin).
Urinary Tract
Urinary tract for urine. They connect the kidneys with the exterior.
- Ureters: they are thin tubes approximately 28 cm long, that start from the kidney and end in the bladder. When the bladder contains a large quantity of urine, it produces a nervous reflex (micturition reflex).
- Urethra: is the duct that connects with the exterior. At its end, it has a sphincter that remains closed except for when the micturition reflex occurs.
Urine Formation
The function of the kidneys is to form urine, using the nephrons, and to then release it to the exterior. Urine is a liquid obtained from blood composed of water, minerals, and diverse waste products (mostly urea and uric acid). The process of urine formation has two stages: Glomerular filtration and Tubular reabsorption.