Blood Composition and Human Circulatory System
Composition of the Blood
Blood is a composite liquid with a plasma part and blood cells.
Plasma
Plasma is the liquid, yellowish part of blood, making up 90%. It contains a variety of substances including proteins, carbohydrates, fats, minerals, O2, CO2, and nitrogen. Plasma without some blood serum proteins is called blood serum. When solidified, it is called coagulation.
Blood Cells
- Red blood cells or erythrocytes: These are biconcave cells without nuclei. They are elastic and deformable. Their red color is due to hemoglobin, which transports oxygen and removes carbon dioxide.
- White blood cells or leukocytes: These are larger cells with nuclei. There are several types: neutrophils, eosinophils, basophils, lymphocytes, monocytes, and megakaryocytes.
- Platelets or thrombocytes: These are cell fragments without a nucleus, derived from megakaryocytes. They are involved in the blood clotting process.
The Heart
The heart is formed of muscle tissue called the myocardium. The heart is divided into two halves, left and right, each with an upper atrium and a lower ventricle. Ventricle walls are thicker and pump the blood. The atria and ventricles communicate through atrioventricular valves:
- Mitral valve: Connects the left atrium and ventricle (2 leaflets).
- Tricuspid valve: Connects the right atrium and ventricle (3 leaflets).
Routes of Entry and Exit
Veins reach the atria. Four pulmonary veins (two from each lung) reach the left atrium. The superior and inferior vena cava reach the right atrium. Arteries exit the ventricles. The pulmonary artery exits the right ventricle. The aorta, which drives blood to the rest of the body, exits the left ventricle. The output of blood from the ventricles is governed by the semilunar valves, situated at the beginning of the arteries, preventing blood return.
Operation of the Heart
The heart has two types of coordinated movements: draining blood from veins and boosting it into arteries. This cycle is called heart failure:
- Atrial Diastole: Blood enters the heart.
- Atrial Systole (Fibrillation): The atria contract, passing blood into the ventricles.
- Ventricular Diastole: Ventricles receive blood from the atria and expand.
- Ventricular Systole: Ventricles contract, and the semilunar valves open.
Double Circulation
Humans have a double and complete circulatory system. Blood passes through the heart twice, establishing two circuits:
- Pulmonary Circuit: Established between the heart and lungs. Blood carrying carbon dioxide from all organs reaches the right atrium through the superior and inferior vena cava. It flows to the right ventricle and then to the lungs via the pulmonary artery (shorter route: lesser circulation).
- General Circuit: Established between the heart and different body organs. Blood cells carry oxygen and nutrients (longest route: greater circulation).
Lymphatic Circulatory System
This system is responsible for catching excess liquid remaining between cells and blood, returning it, and transporting fat. It consists of:
- Lymph Capillaries: Small cups finely distributed throughout the body, collecting interstitial fluid.
- Lymphatics: Vein-like structures. These vessels flow into the circulatory system and release their contents.
- Lymph Nodes: Small swellings along lymphatic vessels, eliminating foreign substances and microorganisms from the lymph. They are plentiful behind the ears, underarms, neck, and groin.
The interstitial fluid within the lymphatic vessels is called lymph.
Excretion and the Urinary System
Excretion is the process of removing waste substances from blood cell metabolism. Main organs include the kidneys, lungs, sweat glands, and liver.
Urinary System
- Kidneys: Organs located in the back of the abdominal cavity on either side of the spine. They filter blood to remove waste substances, resulting in urine. The kidney has a renal cortex, medulla, and pelvis.
- Urinary Passages: Tubes that carry urine from the kidneys to the exterior: ureters, urinary bladder, and urethra.
Excretory Organs
- Lungs: Remove carbon dioxide produced by respiration.
- Liver: Expels waste from digestion and some drugs, which are excreted into bile and eliminated through feces.
- Sweat Glands: Exocrine glands located throughout the skin.
Nephron Function
Nephrons are responsible for filtering and purifying the blood, forming urine.
- Glomerulus: A cluster of capillaries through which blood flows, filtering waste.
- Bowman’s Capsule: A bag surrounding the glomerulus where plasma filtration occurs.
- Renal Tubule: A bent tube that reabsorbs water and other substances, excreting waste.
- Collecting Duct: A common tube for several nephrons, carrying formed urine to the renal pelvis.
The main processes are filtration and reabsorption.